Friday, March 13, 2020

Mediumwave Loop Efficiency For The DXer

Many DXers are aware that an external, passive air core loop antenna can be tuned and coupled inductively to a portable mediumwave radio. Signal enhancement is usually quite good.

DXers may be unaware that an external air core loop antenna can be wired directly to the current corral of DSP radios. It is spelled out right in the manufacturer's data sheet for the Silicon Labs radio chips. It replaces, and is soldered in place of the internal ferrite loop. The document suggests a loop of minimal turns connected to the circuit board through a 1:5 winding (the so-called 25x step-up ferrite core transformer) thus providing the correct coil inductance of 180-450 micro-Henries. It was apparent to me that using a full inductance loop was also possible, bypassing the need for the transformer. This would also result in greater signal gathering ability. Some time ago I did an article called A Hardwired Loop For DSP Radios on this blog.

Loop tuned by a capacitor

Using an air core loop and the signal measuring capabilities of many of these radios we can determine a number of interesting things not possible with other analog or digital superheterodyne radios off the shelf. Today I thought we'd take a look at the mathematics of these loops, both passive and directly-wired. Don't be scared off by the mathematics of it - the toughest thing you will have to wrestle with is multiplication and division, or possibly getting the logarithm of a number from a calculator.

We will answer some interesting questions:

  • How big of a signal can I expect from a passive loop antenna of a certain size?
  • How much better will it be if I increase its size?
  • How much signal voltage is generated at the loop output for a certain field strength?
  • What about the reverse of this - what is the field required to generate that voltage?
  • What is the gain of my loop over the internal ferrite loop or another loop?


The signal voltage induced in a loop is proportional - increases linearly - with the number of turns, the area of the loop, and the frequency being received. Bigger is better, within certain parameters. We can measure the output of the hard-wired loop in microvolts using the modern DSP receiver. This is indicated by the RSSI "dBµ" figure on the display. We can then use that figure in the same formula to calculate the apparent field strength of the signal.

Small loops, that is, loops where the total length of wire is less than 1/10 wavelength at the operating frequency, are called magnetic loops. They respond to the magnetic component of the passing wave. The loop is a transducer which transforms the electromagnetic wave energy into a usable voltage source. The number of turns in the winding, the physical size or area, and the frequency determines the loop's efficiency at transducing the incoming wave.


First, let's review the following S-unit chart from the article: The Ultralight dBµ Mystery, S-Meters, And Field Strength. This will give us an idea of what dBµV values we might see on our radio's DSP display.

S-unit     µV   dBµV
S9+60  50000.0   94
S9+50  15810.0   84
S9+40   5000.0   74
S9+30   1581.0   64
S9+20    500.0   54
S9+10    158.1   44
S9        50.0   34
S8        25.0   28
S7        12.5   22
S6         6.3   16
S5         3.2   10
S4         1.6    4
S3         0.8   -2
S2         0.4   -8
S1         0.2  -14

The modern DSP receivers like the Tecsun PL-380, 310, etc. which employ the Silicon Labs chips, measure and display dBµV as received at the tuned front end across a load. They call it the RSSI indicator. It is measuring the voltage output of the ferrite or air core loop at the radio's input.

dB = decibels of course, simply a way of expressing magnitudes of a value, like voltage, logarithmically.

µV = microvolts, or millionths of a volt.

dBµV is a voltage expressed in dB above (or below) one microvolt. This is measured across a specific load impedance, commonly 50 ohms.

The 'dB' or decibel measurement is a logarithmic ratio as you may know. In terms of voltage, an increase of 6 dB is a doubling of voltage. So, if our little DSP radio receives a signal at 28 dBµ and it increases to 34 dBµ, the received voltage has doubled. Coincidentally, this is also an increase of one S-unit!

Use the following formula to convert dBµV to microvolts, or millionths of a volt:

                    µV = 10 ^ (dBµV/ 20)

To convert microvolts back to its decibel representation:

                    dBµV = 20 * Log(µV)

(Log is the common logarithm, or base 10).


One more concept we must address which is rarely mentioned in technical articles: A tuned loop produces higher voltage output levels than an untuned loop. In fact, much higher. When connected to our radio the loop is effectively tuned by the DSP receiver, and the voltage output "at the tuned frequency" is greatly increased from that of the untuned, unterminated loop sitting out in the open. Think of the DSP receiver as a variable capacitor which tunes the loop's inductance. This is true for the ferrite loop as well. Combined with the loop inductance, it forms a tuned circuit that literally concentrates the signal's field causing greater current flow at the tuned frequency.


Let's talk about loop efficiency. The efficiency of the loop determines the sensitivity of the loop. How is one loop better or worse than another? We can calculate a loop's efficiency if we know its area, number of turns, and the wavelength we wish to receive on it. Once that is known we can make comparisons to other loops.

An 18 inch untuned loop

Our efficiency factor here is often called the "effective height" H, in meters.

Effective height of an 18 inch loop of 12 turns:

                    H = (2 * pi * N * A) / wavelength

                         pi = 3.14159
                         N = number of turns
                         A = area of the loop in square meters
                         wavelength = wavelength of the frequency, in meters.

Effective height, in meters, is what the popular articles call it, sometimes referred to as He. That's a bit of a misnomer. It is actually a ratio or percentage of the wavelength, 0 - 1, since its value is derived by division by wavelength of the received signal.

Referencing the formula above, wavelength is easily calculated. It is the speed of light in meters/second (299,792,458) divided by the frequency in Hertz (Hz).

Wavelength example for 640 KHz:

     299792458 / 640000 = 468.425 meters.

Next, area of the loop in square meters. In the US we commonly use imperial measure, feet. Area conversion to meters is easily done. One square meter = 10.7639 sq ft. Length of a meter is 3.28084 ft. We square this, as such:

     1 sq meter = 10.7639 sq ft. =  3.28084 ft. * 3.28084 ft.

Area, for practical example:

     Area of 48 inch loop = (4ft * 4ft) / 10.7639 = 1.486 sq. meters
     Area of 18 inch loop = (1.5ft * 1.5ft) / 10.7639 = 0.209 sq. meters
     Area of 12 inch loop = (1ft * 1ft) / 10.7639 = 0.093 sq. meters
     Area of 9 inch loop = (.75ft * .75ft) / 10.7639 = 0.052 sq. meters

So, plugging in the values for our 18 inch loop, assuming a frequency of 640 KHz:

     Effective height H = (2 * 3.14159 * 12turns * 0.209area) / 468.425wavelength = 0.03364

Our loop's effective height H is 0.03364.

If we double the number of turns to 24 the effective height is doubled to 0.06728.

If we double the area to 0.418 sq. meters the effective height is doubled to 0.06728.

If we double the received frequency to 1280 KHz the effective height is doubled to 0.06728. Aha! More signal output as we go higher in the band!

The loop's voltage output will be directly related to its efficiency, or effective height. As you can see, efficiency increases linearly with the number of turns, the area of the loop, and the frequency. Voltage output will track right along with that too.


Let's measure the voltage output of our loop. We find the loop's voltage output by converting the RSSI value right off the display of our DSP receiver, marked "dBµ" (which is actually dBµV).

63 dBµV from a station on 1040 KHz

Here's an example. In western Arizona, at mid-day we'll tune to Los Angeles station KFI-640, a 50 KW outlet. At 240 miles, it's a fairly weak signal (about 17 dBµV) using the radio's ferrite loop. However by removing the ferrite and hard-wiring an 18 inch square loop in its place it generates a commendable 42 dBµV at the receiver.

Our conversion formula again is:

                    µV = (10 ^ (dBµV/ 20))

Substituting values:

                    125.89(µV) = 10 ^ (42/ 20)

We have a loop output of 125.89 microvolts.

Now that we know the loop's effective height H and the voltage output of our loop we can calculate the received signal's field strength. Be aware, there is a slight hitch here. The calculated E field is the "apparent" E field, not the actual one. Follow along and I'll explain further.

Let's calculate the apparent electric field E required to produce that loop output. The formula becomes simple at this point:

                    Erms(V/m) = Vrms / H

Vrms is the loop output V in Volts or mV or µV
Erms is the electric field E in V/m or mV/m or µV/m (volts, millivolts, or microvolts per meter)

Be sure to use the same factors in the formula: microvolts to microvolts, millivolts to millivolts, and volts to volts. If we use microvolts in the equation we will have the answer in microvolts, as such:

Substituting values:

                    E(µV/m) = 125.89 / .03364
                    3742.27(µV/m) = 125.89 / .03364

Our E field is 3742.27 µV/m (microvolts per meter). This is equivalent to 3.74227 mV/m (in millivolts per meter).

But wait, there's more. 3.742 mV/m seems awfully much for KFI Los Angeles at 240 miles. Its groundwave field strength on computed charts is only 0.209 mV/m. What's going on here?

Recall above I said a tuned loop produces higher voltage output levels than an untuned loop. Remember that the tuned loop literally concentrates the signal's field, the same as a ferrite loop does. This concentration results in an apparent increase in the E field, or a "gain" if you will. The Erms field we just calculated includes the literal "gain" of the loop as well. The gain of the tuned 18 inch loop makes the apparent field equivalent to 3.742 mV/m! This is a ratio increase of 3.742 / .209 or 17.904.

This is a hard concept to wrap your head around. The "efficiency", or effective height formula, does not tell the whole story of how we get from actual field strength - the E field passing our loop - to signal strength - the loop's output. There is also gain involved, plus a little thing called Antenna Factor.

In the previous article Decoding Antenna Factor In Ferrite Loops on this blog, we dived into antenna factor. Antenna factor applies to all kinds of antennas, not just ferrite loops. Since we're here let's calculate the antenna factor of our 18 inch loop.

From a chart, KFI-640 will produce an actual E field here of .209 mV/m, or 46.4 dBµV/m.

You might recall from the The dBµ vs. dBu Mystery: Signal Strength vs. Field Strength? article on this blog the conversion formula to get from millivolts per meter to dBµV/m, also known as dBu, or engineer's dBu.

                    dBµV/m = 20 * Log(mV/m * 1000)        ...a.k.a. dBu (lowercase 'u')

Substituting values:

                    46.4(dBµV/m) = 20 * Log(.209 * 1000)

We are receiving KFI at 42 dBµV on the receiver's RSSI display for our 18 inch loop. Since we are dealing with decibels on both sides of the equation, we can use simple subtraction to arrive at our antenna factor. Antenna factor of our 18 inch loop is then 4.4 dB (46.4 - 42).

9 inch Helper Loop

I have a little 9 inch loop I built which I call my "Helper Loop". The side length is exactly half of the 18 inch loop, so the area is one-fourth that of the 18 inch. Thus, we should see about one-fourth the signal output. Let's compare it to the 18 inch. First we calculate the efficiency, or effective height again.

Plugging in the values for our 9 inch loop, assuming a frequency of 640 KHz again:

     Effective height H = (2 * 3.14159 * 24turns * 0.052area) / 468.425wavelength = 0.01674

Our 9 inch loop's effective height H is 0.01674.

Directly-wired to the DSP radio, we tune to KFI-640 again at mid-day and see an RSSI dBµV of 30.

Substituting values again:

                    31.62(µV) = 10 ^ (30/ 20)

We have a loop output of 31.62 microvolts.

Now we'll calculate the apparent electric field E again:

                    Erms(V/m) = Vrms / H

Substituting values:

                    E(µV/m) = 31.62 / .01674
                    1888.88(µV/m) = 31.62 / .01674

Our apparent E field is 1888.88 µV/m (microvolts per meter). This is equivalent to 1.88888 mV/m (in millivolts per meter).

Back to our measured RSSI outputs again. Our 9 inch loop's output is 31.62 µV. Our 18 inch loop's output was 125.89 µV. That's virtually four times the output of our 9 inch loop which is one-fourth its area. In dB (voltage), exactly +12 dB greater signal output is generated by the 18 inch loop which of course is 4 times the output as well. Remember, 6 dB is a doubling of the voltage, and another 6 dB doubles it again.

The gain of the tuned 9 inch loop makes the apparent field equivalent to 1.888 mV/m. This is a ratio increase of 1.888 / .209 or 9.033.

Wrapping up, using simple subtraction again to arrive at our antenna factor for the 9 inch loop, (46.4 - 30) = 16.4 dB Antenna Factor. This, again, is a 12 dB difference.

Here's a slightly different way to express our original  formula:

The induced voltage V of an untuned loop (the loop's output) is:

                    V(µV) = ((2 * pi * N * A) / wavelength) * E(µV/m) * Cos(theta)

                    Remember, our effective height, H, is this part:  ((2 * pi * N * A) / wavelength)

                    V = H * E * cos(theta)
                    E * cos(theta) = V / H

                         V is in µV (loop output)
                         H is the loop effective height
                         E is the field strength of the passing wave in µV/m
                         Cos(theta) is the cosine of the angle between the antenna and the transmitter

For untuned loops, the calculated E field is the actual passing field. Tuned, the calculated E field is the apparent passing field. Think of it this way: tuning a loop does not change the loop's core efficiency H which is determined by turns, size, and impressed wavelength, but it will indeed change the loop's output V.

A note on angle theta: Theta is the angle that the plane of the loop makes to the station's passing field. Our desired angle is almost always zero, pointed directly at the station, for max signal pickup. Since the cosine of 0 = 1, we can leave that factor out of the equation. Note that if you rotate the loop 30 degrees away from the station you have reduced the signal pickup by Cos(30), or 0.866. 60 degrees, Cos(60), or 0.5, half!

A 42 inch tuned loop

Above, a 42 inch loop tuned with a variable capacitor. Remove the capacitor and directly-wire this loop to a DSP radio after removing the radio's ferrite. You will see results! Watch out for overload!


Some interesting facts about loops, both passive and directly-wired:

1. A 48 inch loop gathers 16 times more signal than the 12 inch loop because it has 16 times the area of the 12 inch loop. A 9 foot loop gathers 81 times more signal than the 12 inch loop! Loop area is the determining factor here.

2. Halving the received frequency (let's say from 1200 KHz to 600 KHz) results in half the induced voltage given the field strengths at 1200 KHz and 600 KHz are equal at the reception point.

3. More turns are better. Pack as many turns as you can into your loop. Additionally for passive tuned loops: weigh turns over capacitance when calculating tuning parameters. For SiLabs DSP type radios, try to keep your loop inductance at the upper end of the range, 450 micro-Henries.

4. Be sure the plane of your loop is aligned at 0 degrees to the station. Rotating the loop 30 degrees off the station reduces the maximum induced voltage to 86.6%, because cosine(30) = 0.866. Rotating 60 degrees off the station reduces it to 50%, because cosine(60) = 0.5. Rotating 90 degrees to the station reduces it to 0%. The total null is a theoretical value of course, and not attainable in actual practice as there is no perfectly nulling loop.

5. Remember that tuned loops generate lots more signal than untuned loops.

Saturday, February 1, 2020

2020 US-Canadian Mediumwave Pattern Reference Is Here

The 2020 US-Canadian Mediumwave Pattern Reference for all stations is now available. Find the download link at upper right. Remember, the links change each time a new set is uploaded. Always look to this RADIO-TIMETRAVELLER site for the current link. Download is 52 MB.


Media Fire link here.

When downloading from the Media Fire link, be sure to click the DOWNLOAD button.

The Media Fire site is ad-supported and has several ad links on the page and will also issue an ad pop-under. Just ignore these.

A mirror link for the 2020 files has not been established yet.


The maps are HTML-based, so no regular install is necessary. Simply unzip the downloaded file and click on the individual map file to run. The map will open up in your web browser. They are self-contained, with image icons embedded right into the code. You must have an internet connection to view the maps.


January 21, 2020:

1. Much of the summer and fall of 2019 has been spent bug fixing and tweaking the skywave formulas for accuracy. A slight tweak to the groundwave formula has brought the predicted groundwave strengths more in line with V-Soft ( Skywave values now more reflect actual received signal strengths as measured. Consequently the mV/m threshold was upped this year to 0.1 mV/m (40 dBu). If signal overlap is a problem, simply turn off all plots and select the ones you want.

Additionally, a rather lengthy skywave overhaul now permits skywave calculations for any date and time of the year, accurate to the solar latitude of the chosen location. In order to show a median skywave calculation value, the date of November 5 has been chosen, exactly halfway between the Autumnal equinox of September 21 and the Winter solstice of December 21, the dates of the most extreme deviation from the median.

The nighttime skywave calculation is based on midnight Central Standard Time (SS+6). The daytime groundwave calculation is based on noon Central Standard Time (SR+6).

Los Angeles station KABC-790 was missed in last years maps due to an error in the FCC's database archiving its license to cover. It has been patched in this year, as the FCC has not corrected the problem.

Daytime Franklin, VA station KJZU-1250 is missing this year due to errors in its tower record.

Missing Canadian Nova Scotia and Newfoundland stations have been added. In previous years an error in RDMW's filtering had bypassed them.

Missing Canadian station CHHA-1610 (Toronto) has been added. The Industry Canada database is missing a class identifier for this station, so I have placed it in Class C which I believe to be correct. I have written to IC about this omission, but they have not responded.

New on the maps this year are the Canadian low power stations, which generally run 20-40 watts. Due to the low power, they generally will not generate a skywave pattern, but the daytime pattern should be substituted. They may be receivable at distance, however, don't give up!

Again for 2020, the following parametrics are considered in the skywave calculation:

   * Hourly transitional loss variance from sunset to sunrise.
   * Seasonal gain or loss, January - December.
   * Diurnal enhancement at the sunrise and sunset period.
   * Winter daytime skywave enhancement (only on maps created for times during the day).
   * Daily seasonal nighttime skywave enhancement.
   * Take off angle variances for stations at relatively close distances (experimental).

2. Colored plot (yellow), again, for groundwave 1.0 mV/m level.

3. Small changes made to the map's title bar heading. Signal dBu (dBµV/m) is now displayed instead of millivolts per meter. Also the map's day of year (DOY) and GMT time "z" are displayed.

4. Unlimited, Daytime, and Critical Hours plots are at the 1.0 (60 dBu) and 0.1 mV/m (40 dBu) levels. Skywave is set at the 0.1 mV/m level. Levels have been chosen to minimize pattern overlap yet still attempt to show what you can accurately hear during the day and night.


Included is a complete set of GoogleMap-based, HTML-driven maps which show the most current pattern plots of all licensed US and Canadian mediumwave broadcast stations from 530 - 1700 KHz. The set includes all frequencies for the indicated services: Unlimited, Daytime, Nighttime, and Critical Hours. Individual maps are grouped by channel frequency: 540, 550, 560, .. 1700 KHz, etc. Data for the plots in this offering is based on the current FCC and Industry Canada databases available at the time of its creation (January 21, 2020).

The daytime map series, in two parts, shows expected groundwave coverage patterns for Unlimited and Daytime (part 1), and Critical Hours (part 2) operations. Daytime signal patterns represent groundwave coverage at two levels, out to the 1.0 and 0.1 millivolts per meter contours (60 dBu and 40 dBu respectively). The choice of these levels is made in order to more closely match those which might be helpful to the mediumwave DXer. Note that daytime reception of signals out to and beyond the depicted 0.1 mV/m pattern is very possible, and in fact likely for the DXer. The contour line represents a signal strength at the station's extreme fringe distance, a level usually received on a sensitive portable radio with a low ambient local-noise level. I have chosen this signal level to give a good representation of what can be received by most DXers during sunlight hours.

The nighttime map series shows expected skywave coverage patterns for Unlimited and Nighttime operations. Nighttime signal patterns represent the standard SS+6 (sunset plus 6 hours, or approximately midnight Central Standard Time), 50% signal probability at 0.1 millivolts per meter (40 dBu). Note also that nighttime reception of signals out to and beyond the depicted pattern is very possible, and in fact quite likely for a skywave signal. The maps represent a signal strength at the fringe level. I have chosen this signal level to give a good representation of what is possibly received by most DXers on an average evening. The nighttime signal probability of 50% means that the signal will be received at this level approximately 50% of the time at Central Standard Time.


Using the actual FCC database files, Radio Data MW will auto-generate an interactive HTML pattern map, showing the pattern plots for all stations included at the discretion of the user. A complete set of mediumwave pattern maps can be generated in about eight hours of processing time. Processing time had increased by nearly two hours by 2019 due to enhanced skywave calculations and other upgrades.

For daytime signal maps, Radio Data MW generates a real pattern plot based on transmitter power, antenna array efficiency and directivity, ground conductivity and ground dielectric constant of the path to the receiver. Increased conductivity of water paths over the Great Lakes are also accounted for. It displays actual (but approximate of course) signal level boundaries for Local, Distant, Fringe, Extreme mV/m levels, or any custom mV/m level chosen by the user.

For skywave signal maps, predicted signal levels are calculated in accordance with current FCC or ITU methods of recent years (1999 onward). A number of parametrics are now analyzed and accounted for in the calculation, namely diurnal and seasonal changes, and daily sunrise and sunset enhancements to the signal. The process is rather complicated.

The online Google Maps API is used to generate and plot each station on a map of the US. An accurate flag pin is placed at each transmitter location, and in satellite view may be zoomed in to see the actual transmitter site. Map flags are color-coded to indicate Unlimited (light red), Daytime (yellow), Nighttime (black), and Critical Hours (grey) services. Each flag has a tooltip-type note, and when hovered over with the mouse will display a note on the station.

A pattern plot for each station is generated and displayed. Each pattern can be calculated using standard formulas used by the FCC or ITU to compute the base values at one kilometer, and field strength formulas at distance based on the works of many people over the years. See Field Strength Calculations: A History and Field Strength Calculator One, previously posted on RADIO-TIMETRAVELLER. See the RADIO-TIMETRAVELLER blog at:

An accurate ray path can be drawn from all transmitters to a user-specified receiving location by inputting latitude-longitude coordinates on the heading bar at the top of the map. Super-imposed on the pattern plots, the ray paths show the listener where he or she falls on each station's pattern, a handy guide to knowing where you stand.

Individual station plots can be turned on or off by a checkbox. Click the station flag and you will see the option in a pop-up balloon. Check or uncheck the box, then click the ReDraw button. The entire plot set can also be turned on or off by buttons at the top of the map.

Included in each station's flag tooltip are FCC facility ID, engineering (application) ID, and distance of the station from the home latitude-longitude. Of interest to the DXer, by setting the home location latitude-longitude to your location and redrawing the map, each flag tooltip will have the distance from your location to the station.


A varying amount of pattern overlap exists on the maps, some extreme, as it does in real life. For the Daytime and Critical Hours plots, the outer 0.1 mV/m signal level ring represents an extreme groundwave fringe distance where a station can be heard. At that level, there may be some minimal overlap with co-channel stations.

Pattern overlap is of course much more severe for skywave on the nighttime plots. A level of 0.1 mV/m was chosen to represent the fringe distance a station is heard at night about 50% of the time. This may seem low to many, why not increase it to lessen the overlap? Unfortunately, increasing it even to 0.15 mV/m results in no skywave plot at all for many stations under 1500 watts as their skywave signal never reaches the 0.15 mV/m threshold at points around the compass. This is particularly bothersome in the northern latitudes above 40 degrees north where signals are weaker.

The unusual case exists on the graveyard channels (1230, 1240, 1340, 1400, 1450, 1490 KHz). The plots are a massive overlay of signals (as it is in real life!). There is no real good way to display a graveyard channel for station-to-station comparison but to throw them all in there and then allow you to choose which ones to compare. Virtually 99% of all graveyarders run 1 KW power to a single tower. The technical reality is that a one kilowatt station does not produce a skywave signal in any direction above a level of about 3 mV/m. Raising the plot mV/m level to reduce the chaos unfortunately results in no plot at all for most stations.

The solution to the graveyard confusion (all, really) is simple, and one of the enhancements added in 2016. You can turn plots on or off individually, or all at once. Turn all plots off and simply check the plots you wish to see.


As of 2019, the skywave calculation has been totally overhauled and enhanced to more reflect actual signal expectations across the U.S. at night. The fact of life is that pattern overlay occurs on many frequencies. Simply select the plots you want to analyze. Check the No Plots checkbox then ReDraw to turn off all plots. Click any station flag and check the box to plot that station then ReDraw.

You will occasionally see a skywave plot which looks much smaller than surrounding plots. This is a case where the station's skywave signal did not meet the mV/m threshold (0.1 mV/m). The groundwave plot level is substituted in this case. The station does in fact have a skywave component, however small, it will be measurably less than the 0.1 mV/m level (very weak). It may be receivable!

The darker line defining the outer edge of the skywave plot shows the location of the 0.1 mV/m signal point at all compass points. Be aware that skywave signal strength does not decrease linearly with distance from the station. From the station outward, the signal strength will generally increase to a point usually 200-400 kilometers distant where it will peak, then decrease somewhat linearly from there.

Also note that the atmospheric background noise level on the mediumwave band is generally considered to be approximately 36 dBu (dBu in this case = dBµV/m), equivalent to 0.063 mV/m. Signals below that level will not be heard unless they fade up above the noise. A gain or directional antenna can be used to increase signal strength while limiting or even reducing the overall atmospheric background noise level.

Image below is an example of the 1040 KHz skywave map.

Hope you enjoy.

Saturday, December 21, 2019

Decoding Antenna Factor In Ferrite Loops

Mediumwave DXers and signal measuring enthusiasts, let's have a look at Antenna Factor and see if we can use it.

Q: What is Antenna Factor?

A: A correction factor translating arriving field strength to antenna output. Bonus: It can also be used in reverse, translating antenna output to arriving field strength.

Q: Can the mediumwave DXer with a modern portable DSP receiver use Antenna Factor to any advantage?

A: Yes, of course. You may not realize it, but you are holding a rudimentary field strength measuring device in your hand.


Antenna factor is often greatly mis-understood. Antenna factor is the relationship between the electric field strength "E", which is the strength of the radio wave field surrounding the antenna (usually referred to in millivolts per meter) to the voltage output "V" of the antenna itself (usually measured in microvolts).

Simply, it is the ratio of the former to the latter - antenna field to antenna output - and shows the antenna's ability to convert the surrounding received field to a usable voltage. It is literally the relationship between field strength and signal strength. In the mediumwave DXer's case, "V" is the ferrite loop's voltage output - signal strength! And another bonus for us - they are directly proportional - that is, an increase in the antenna's surrounding field results in a proportional increase in the ferrite loop's voltage output.


Wouldn't it be interesting if we knew the antenna factor of our ferrite loop? Or at least its approximate value? What might we do with that? With our modern DSP chip radios displaying an RSSI (Received Signal Strength Indicator) voltage, the output of its ferrite loop, we could easily determine an approximate value of the electric field strength of the station at our location, a value right in line with the published values gotten from V-Soft Zip Signal and radio-locator, or an expensive signal measuring device.

Let's take this apart, piece by piece, and see how we can get there.

Although the field strength and antenna output measurements we talk about in the paragraphs above are voltage, antenna factor is generally expressed in "dB" microvolts per meter, or simply "dB".

The decibel (dB) unit of measurement is used to express the ratio of one value of a power or field quantity, or a voltage, to another. It is a logarithmic measurement, in that the decibel equals 10 times the common logarithm of the power ratio or 20 times the common logarithm of the voltage ratio. Remember, dB, used alone with no reference, is a ratio. 0 dB means no change, or an equal ratio.

In our case we are dealing in voltage ratios here, not power ratios, so our formula will be 20 times the common logarithm of the voltage ratio.

So, if our voltage ratio of E/V is 2 for example, or double, our dB value is 20 times the log of 2, or 6 dB. Note that a doubling of the voltage (i.e., 2) is a 6 dB improvement.

National NC-270 S-Meter

Devopedia has a great summation of the decibel if you would like to delve into it further.

Old timers will remember the analog S-meter on the old receivers. It was calibrated S-0 to S-9, then in dB above S-9. Each S-unit was a doubling of received voltage from the previous S-unit, or a 6 dB gain. Consequently, each S-unit increase is an increase by the power of 2. S-9 is 2^9 stronger than S-1, or 512 times greater signal voltage! S-9 represented 50 microvolts of receiver input measured right at the antenna - signal strength. Thus the S-meter was commonly called a signal strength meter. Be sure not to confuse field strength with signal strength. Field strength is the electromagnetic field surrounding an antenna and signal strength is the converted voltage at the antenna output.

Now, what about ferrite loops and frequency dependence? Many will recognize that their radio might be a little more sensitive at one end of the band than the other. At one given frequency, for all stations received on that frequency, the relationship between their field surrounding the antenna and the output voltage of our field converter (our ferrite loop) is constant. This being so, the antenna factor is also a constant for all at that given frequency. It may be slightly different at a different frequency as we will see.

RSSI - 19 dBµV

So by knowing the antenna factor and the RSSI voltage we can work in reverse to determine the field strength at the antenna. The antenna factor (in dB microvolts per meter) can simply be added to the value of voltage measured (in dBµV) at the ferrite antenna output! It is a simple matter then to convert that antenna factor dB value back to a field voltage in millivolts or microvolts per meter.

What are the two values dB microvolts per meter (dBµV/m) and dBµV telling us? A ratio, that's all. It's the amount of field or voltage compared to one millionth of a volt (the "µ", or "micro", means one millionth).


The three pieces of information are:

  1. E - Field strength of the radio station's signal at the antenna (in dBµV/m).
  2. AF - Antenna factor of our antenna (our ferrite loop), in dB/m.
  3. V - Voltage output of our ferrite loop (the RSSI reading in dBµV).

Our formula for determining antenna factor AF is this:

                              AF(dB/m) = 20 * LOG(E/V)

                              Note: E and V must be in the same units - voltage!
Our formula for determining field strength E is then this:

                              E(dBµV/m) = V(in dBµV) + AF(dB/m)

Our formula for determining signal strength V (RSSI) is then this:

                              V(dBµV) = E(dBµV/m) - AF(dB/m)

Knowing any two of the three values, we can calculate the third value. We can determine a variety of useful information knowing our RSSI reading and one other parameter.

We will first need the E, or field strength values, of some nearby sample stations. We will use those values in our formula to calculate some antenna factors for various frequencies in the mediumwave band. They should prove relatively close in value. We will then average them to get an overall average antenna factor value, or AF.

Item #1, field strength E, for daytime reception out to about 200 miles of our chosen stations can easily be gotten from the V-Soft Zip Signal site. Accuracy is very good, and compares to within approximately 5% of the enhanced Norton signal strength formula I use in pattern calculation. Field strengths are published to your zip code origin, which is generally your post office latitude-longitude. Using some sample field strength values and the RSSI values from our receiver, we can calculate an approximate antenna factor for our radio.

Once we know the antenna factor, using simple addition we can calculate the field strength of any station we receive by adding the antenna factor dB right to the RSSI reading!

Note the value "A" in the pictured formula - Cable attenuation. We can ignore this in our formula as we have no feed line! We are measuring right at the antenna itself.

Before we get to our calculation examples, let me clear up one item of mis-represented terminology, the dBµV/m. In engineering parlance, field strength, or millivolts per meter (mV/m) is usually converted to dBµV/m and casually referred to as dBu (lower case "u"). It is NOT the same as the dBµ sticker (the RSSI value) on your DSP radio. "Engineer's dBu", the decibel representation of microvolts per meter, is RF field at the antenna and is short for dBµV/m. The radio's value "dBµ" is voltage output of the ferrite loop as compared to one microvolt, properly known as dBµV. Shame on Tecsun and others for printing dBµ on their radios. Inaccurate and very confusing.


The mediumwave E field at the receiver site (the antenna) is usually measured in millivolts per meter, mV/m. Though V-Soft will show the conversion of millivolts per meter to dBµV/m for you, you might want to be able to do it for yourself at some point.

If you'd like to figure it yourself, you can by using the following formula:

                              dBµV/m = 20 * Log(mV/m * 1000)

To reverse the computation, converting dBµV/m back to mV/m:

                              mV/m = 10 ^ (dBµV/m / 20) / 1000

                              ...or back to microvolts per meter:

                              µV/m = 10 ^ (dBµV/m / 20)

Note: Log is the common logarithm, or base 10.

Let's try some calculation examples to get a baseline. We'll start at the lower end of the band using our Tecsun PL-880.


Phoenix's daytime KTAR-620 is 129 miles east of me over fairly flat desert terrain. The station runs 5 KW daytime. Center tuned to KTAR on the Tecsun PL-880, the RSSI display shows a 35 dBµV signal off the ferrite loop. The V-Soft chart for my zip code shows the expected field strength of KTAR should be 0.83 mV/m (830 microvolts per meter), equivalent to 58.3 dBµV/m. We subtract our RSSI dB reading from the V-Soft dB value (58.3 - 35) = 23.3 to arrive at our antenna factor in dB. So our AF = 23.3 dB/m.


Yuma, AZ daytime KBLU-560 is 69 miles south of me over rugged, mountainous desert terrain. The station runs 1 KW daytime. Center tuned to KBLU on the PL-880, the RSSI display shows a 35.5 dBµV signal off the ferrite loop. The V-Soft chart for my zip code shows the expected field strength of KBLU should be 1.28 mV/m (1280 microvolts per meter), equivalent to 62.1 dBµV/m. We subtract our RSSI dB reading from the V-Soft dB value (62.1 - 35.5) = 26.6 to arrive at our antenna factor in dB. So our AF = 26.6 dB/m.

Another example, but this time let's use a different frequency at mid-band.


Daytime Lake Havasu, AZ KNTR-980 is 58 miles north of me over varied desert terrain. The station runs 1 KW daytime. Center tuned to KNTR on the PL-880, the RSSI display shows a 28 dBµV signal off the ferrite loop. The V-Soft chart for my zip code shows the expected field strength of KNTR should be 0.77 mV/m (770 microvolts per meter), equivalent to 57.7 dBµV/m. Our difference here between the V-Soft reading and the RSSI reading is 29.7 dB. So our AF = 29.7 dB/m.

Let's do one more example, this one a little farther up in the band.


Phoenix's daytime KPXQ-1360 is 116 miles east of me over fairly flat desert terrain. The station runs 50 KW daytime. Center tuned to KPXQ on the PL-880, the RSSI display shows a 20 dBµV signal off the ferrite loop. The V-Soft chart for my zip code shows the expected field strength of KPXQ should be 0.33 mV/m (330 microvolts per meter), equivalent to 50.3 dBµV/m. Our difference here between the V-Soft reading and the RSSI reading is 30.3 dB. So our AF = 30.3 dB/m.


So we are seeing that the antenna factor for our ferrite loop in the PL-880 is generally in the upper 20s on average. The antenna factor will vary a little from the low end of the band to the high end. This is because our ferrite may be less sensitive at one end versus the other. Not unusual. Mine tends to run in the low 30s near the high end and the middle 20s at the low end, signifying that the sensitivity of my PL-880 is a little better at the low end of the band. It's exactly the reverse of what you might think. The lower the antenna factor's dB value is, the better is the ferrite's (or any antenna's) ability to convert the field to a voltage.

I have averaged these stations above and several others and have chosen an average antenna factor value of 27 dB to work with.


Now that we have an average antenna factor of 27 dB for the PL-880 let's find a strong station at night and see what kind of field strength it's putting into my location here in Arizona. KCBS-740 (50 KW) out of San Francisco, CA is a good choice, and puts in a tremendous signal here at night. Average RSSI readings on the PL-880 are in the 48 dBµV area when the signal is peaking.

Adding our antenna factor, 27 dB, to 48 dBµV, we get 75 dBµV/m. 75 dBµV/m is 5623 µV/m (microvolts per meter) or 5.623 mV/m (millivolts per meter). Now of course being skywave, this value will hardly be published as such in a field strength chart due to the extreme number of variables involved in skywave propagation. However, with our DSP radio we can read it's instantaneous value and calculate what its instantaneous field strength is.

Remembering the radio-locator signal strength scheme for daytime strengths:

  • 2.5 mV/m (68 dBuV/m, local)
  • 0.5 mV/m (54 dBuV/m, distant)
  • 0.15 mV/m (43.5 dBuV/m, fringe)

We can see KCBS-740 at 5.623 mV/m puts a whopping signal into southwestern Arizona, at urban levels at night, stronger than any daytime station due to our remote location.


Let's check the base reception level of our PL-880 - its ability to pick up the weakest signal it can. High daytime KMZQ-670 out of Las Vegas, NV (25 KW), 198 miles distant, is right at the noise level on my PL-880, running about 15-16 dBµV. Adding our antenna factor 27 dB to 16 dBµV, we get 43 dBµV/m. Converting 43 dBµV/m back to microvolts per meter we get 141 µV/m or 0.141 mV/m (millivolts per meter). This can be confirmed by radio-locator, as the radio-locator coverage map shows KMZQ's fringe (blue line, 0.15, or 40 dBµV/m) to be very close to my location.

So we see that our PL-880 has the ability to receive signals during the daytime out to radio-locator's fringe ring, or 0.15 mV/m (40 dBµV/m).


Generally in quiet locations, the mediumwave broadcast band's atmospheric noise level runs about 0.063 mV/m, or 36 dBµV/m. What will our PL-880's ferrite loop output be for a signal right at the noise? We can calculate that easily. 36 dBµV/m - 27 dB = 9 dBµV or 2.81 microvolts. This is also -97.9 dBm into a 50 ohm load. dBm is a figure generally used in defining sensitivity of receivers, meaning the decibel ratio compared to one milliwatt (m). In this case we are -97.9 dB less than one milliwatt! Remember, -107 dBm is equivalent to 1 microvolt signal strength at the antenna output, or 1 millionth of a volt. The lower the number in dBm (the more negative), the weaker the signal is. We see that the PL-880 hears down to about 0.15 mV/m, but not quite to the 36 dBµV/m atmospheric noise level of 0.063 mV/m.


I have an Eton Traveler 3 from which I've removed the ferrite loop and replaced it with an 18 inch box loop. The loop is wired directly to the radio's input. This means the DSP chip's RSSI value will be directly reading the voltage off the loop. We can determine it's antenna factor too.

For our test station let's use Yuma's daytime KBLU-560 signal again, 69 miles distant running 1 KW. Our RSSI display shows a 58 dBµV signal off the hardwired loop. The V-Soft chart (as before) shows the expected field strength of KBLU should be 1.28 mV/m (1280 microvolts per meter), equivalent to 62.1 dBµV/m. We subtract our RSSI dB reading from the V-Soft dB value (62.1 - 58) = 4.1 to arrive at our antenna factor in dB.

So the antenna factor for our 18 inch box loop is very low at 4.1 dB. Notice also that we have a 58 dBµV signal voltage being output as compared to the 35.5 dBµV signal voltage off the PL-880's ferrite, meaning much greater signal gain. Quite an improvement from the ferrite loop of the PL-880! A quick subtraction shows we have a 58 - 35.5 or 22.5 dB voltage gain. Converting the dBuV values to actual voltage, we have jumped our output from 59.5 microvolts to 794 microvolts, a 13.3 ratio increase by using our 18 inch passive loop!


So there you have it. A new field strength measuring device awaits your testing! Be sure to remember to use the same units in your calculations. Don't mix millivolts per meter with microvolts per meter, or dB with actual voltage.

I hope I've provided for you some new and interesting things to try. Be sure to realize that the values we are calculating are ballpark values. They are not exact like might be read off a $15K dollar field strength device like the Potomac 4100. But even ballpark values can tell us many interesting things about our radio environment and the modern marvel DSP receiver.

Potomac 4100 - $15K

Friday, November 15, 2019

DSP Radio Chip Notes

A recent re-inspection of the Silicon Labs DSP radio chip datasheets left me with some new impressions of their quirks and capabilities. Additionally, a question was recently asked about observed differences in sensitivity between ultralight DSP portables. I thought I'd talk a bit about this and also some other general DSP chip notes related to chip radios.


Consumer chip-based DSP radios are really quite simple in their design. Many if not nearly all of the portable radios we DXers use are employing the Silicon Labs Si4734/35 or Si4831/35 chips. The Si47xx is first generation and the Si48xx is the second generation or newer chip. Their basic functionality is the same.

Circuit-wise, in most consumer-grade radios the chip is generally wired right to the input coil, that is, it is connected directly to the two wires of the ferrite loop. Simplicity! Largely gone are the days of superheterodyne local oscillators and ferrite loops with touchy and complicated multiple windings which are difficult to align. The entire radio is right there on the chip.

The core-sensitivity of every "radio on a chip" is identical. Manufacturing defects or minor chip-to-chip manufacturing differences should be near nil. Connected directly to the ferrite loop, the sensitivity differences in radios therefore are defined by the length and efficiency of the ferrite loopsticks and also their shielding and positioning away from noisy circuitry.


The dBµ signal strength display "19" on the front of this radio (RSSI reading, actually in dBµ/V) is reading the voltage off the ferrite loop as input to the chip. 19 dBµ represents 19 dB above one microvolt. Converted, it is 8.91 microvolts. So the dBµ signal strengths you are seeing are really a measure of antenna voltage and not differences in sensitivity of the chips, all other things being the same. Essentially you are seeing the sensitivity and efficiency of your ferrite loop, measured by the radio. How cool is that!

Another interesting fact to consider is the AM sensitivity of the 1st generation Si4734/35 chip and the 2nd Si4831/35 chip differ by 5 microvolts. Surprisingly the older Si4734/35 is the more sensitive - 25 µV vs. 30 µV (lower is better). But this difference (about 1.5 dB) will be hardly perceptible in reception. Still, a little improvement often makes the difference when signals are at the noise level.

The renowned CCRadio EP Pro uses the same 2nd generation Silicon Labs Si4831/35 chip we are discussing here. It achieves its nearly on-par sensitivity with the famed Panasonic RF-2200 because of its 200mm twin-coil, tuned ferrite loopstick. You can make a Tecsun PL-380 as sensitive as a Panasonic RF-2200 simply by removing the ferrite loopstick and substituting a 24 inch air core loop. A great trick except for the signal overload which cannot be handled adequately by the 380's DSP chip.

The second generation improvements in the Si4831/35 chips seem to be mostly added bells and whistles in control circuitry. However the sensitivity of the FM section of the newer Si48xx has been improved considerably, from 4 µV down to 2.2 µV (again, lower is better), amounting to nearly a 6 dB improvement.

It should be noticeable as it's almost a doubling of the signal sensitivity. Consequently if you are an FM Dxer, be on the lookout for newer DSP radios that use the second generation Si48xx chips.


Does your radio's sensitivity seem off? My PL-880 seemed so right from the start. An additional item to consider is channel centering. In other words, is the channel you are tuned to centered in the filter passband? One would assume that in a DSP-based radio the channel would always be centered. Not necessarily true. I bought a Tecsun PL-880 about a year ago. I was disappointed with it's sensitivity on mediumwave and found it was no better than my PL-380 or Sony SRF-59. I did the SSB centering tweak, noting that this was fairly close right out of the box. Still no improvement, but that effects SSB reception only. What to think? Did I get a bum unit?

Tecsun PL-880

The PL-880 has a wonderful fine tuning control available all the time - right under the main, or coarse, tuning control. It's a separate dial which allows tuning in finer 1 KHz steps. Now a year later, recently after tuning to a fairly weak mediumwave station, I happened to bump the fine tuning up 2 KHz and noticed something very unusual. The signal's audio got stronger and more bassy and the RSSI strength on the meter (dBµ) increased by +5 dB. The next day at mid-day I performed the same test on distant groundwave station KFI-640, Los Angeles, about 240 miles distant. KFI is one of my test stations when I test and compare radio sensitivities. I had only been able to get about 16 dbµ out of KFI at mid-day, with difficult reception just barely above noise level. Tuning up 2 KHz to 642, KFI now shows about 21-22 dBµ with clear audio and acceptable reception, well above the other two radios mentioned.

So check your channel centering in AM mode on these radios. Note that with some DSP radios this test might not be possible. Unfortunately what happens on some models is the RSSI reading will drop to zero since you are off-channel, making checking signal strength impossible. Check your radio to see.


I hear talk all the time about connecting up modern portables to longwire antennas. Some portables even have a separate antenna jack for shortwave and even mediumwave. My advice: be careful. And be even more careful if you live in a dry climate like the desert where humidities are very low.

Many years back, I blew out two Sony ICF-2010s with attached longwire antennas. Understand, the '2010 was notoriously sensitive to external antennas even though it was set up for it. The overload, whether it be signal or static pulse, would zap the input FET transistor. It was replaceable and you could even get one at Radio Shack.

My Kaito KA-1103 was very sensitive to static discharge, though technically the original is not a DSP chip radio. The PL-880 is turning out to just as sensitive to static. It's common here in the desert at certain times of the year when it's extremely dry to get a static discharge off a door knob or other metal object, or radio, grounded or not. Recent static discharges to the PL-880 are resetting it just by picking it up while I'm slightly charged. No harm so far. The '1103 got to the point where it corrupted the ROM and wouldn't work anymore. And guess what you have when you connect your little radio up to a longwire? A giant atmospheric static discharge receptor fed right to the radio's input circuitry. You don't have to be in a thunderstorm with lightning to have a static pulse.

Tecsun PL-380

So on to the Sangean ATS-909X. About a month into owning the '909X the AGC started acting up. Sensitivity was way down as observed on the field strength meter. Reception then became intermittent and within minutes would eventually  fail altogether. At first, resetting the radio - but only by removing the batteries - corrected the problem. The problem would reoccur and I went through the same routine each time. In the end it became permanent and resetting the radio had no effect. The radio was dead. What had I been doing the week before all this started? I had been messing with a longwire antenna connected to the antenna port. Coincidence? Maybe, maybe not. As I have witnessed in the previous paragraph, static pulse can not only zap the front end of the radio, it can corrupt the radio's ROM operating system. Resetting the radio does not always fix this. I sent the '909X in to Sangean for repair and they reprogrammed the ROM. The radio has been fine ever since.

I've done a lot of experimenting with directly-connected air core loops, particularly with the PL-380 and Eton Traveler III. It is common to overload or even static-zap these radios in this way because of your direct connection to the input pin on the SiLabs DSP chip. In these cases the radio has so far always recovered. The symptoms are de-sensing of the radio to zero signal for perhaps ten minutes or so. Shut the radio off and let it recover if it will.

Again, be careful. Modern transistorized or chipped portables will not stand up to longwire antennas and static pulse like the old tube radios of yesteryear did. Ground the longwire temporarily at first  before connecting to the radio.


Sangean ATS-909X
On rare occasion, a design engineer will use one of these chips in a different fashion and to greater advantage. Sangean was one of them. In what I consider one of the most highly underrated radios - the Sangean ATS-909X - engineers used the SiLabs DSP chip at the back end of the radio, in the I.F. section. This, to take advantage of the chip's superior DSP filtering in the I.F. stage, running the I.F. signal through the chip.

So, incorporating this into a traditional PLL-designed radio is a stroke of genius, combining the best of old front-end design with new technological filtering abilities at the I.F. stage. The result is a radio with premium selectivity.


I am guilty as most, I continue to buy this DSP stuff hoping for a more sensitive radio. But the truth of the matter is that they are all basically the same in their electronics when they use the same chip, unless of course they use some kind of additional input or bandpass filtering ahead of the DSP chip or pre-amplification of the signal off the ferrite loop or are engineered into a different design like the Sangean ATS-909X. Few do. Otherwise the difference lies only in the base output off the ferrite loopstick, its length, antenna factor (efficiency), or positioning in relation to surrounding circuitry (a source of birdies and display noise).

To wrap up, go for the radios which have the longest ferrite loopstick. Until newer 3rd generation DSP chips come out, hopefully with greater sensitivity and lower noise floors, the only difference is in your ferrite or external antenna.


Silicon Labs Products page:

From the Silicon Labs site. Modern DSP chip usage in commercial and consumer radios:

Si4730    FM/AM Receiver
Si4731    FM/AM Receiver with RDS
Si4734    FM/AM/SW/LW Receiver
Si4735    FM/AM/SW/LW Receiver
Si4736    FM/AM/Weather Band Receiver
Si4737    FM/AM/Weather Band Receiver with RDS
Si4738    FM/Weather Band Receiver
Si4739    FM/Weather Band Receiver with RDS
Si4820    Mono FM/AM Receiver
Si4824    Mono FM/AM/SW Receiver
Si4825    Advanced Mono FM/AM/SW Receiver
Si4831    Stereo FM/AM Receiver
Si4835    Stereo FM/AM/SW Receiver
Si4836    Advanced Stereo FM/AM/SW Receiver
Si4822    Mono FM/AM Receiver
Si4826    Mono FM/AM/SW Receiver
Si4827    Advanced Mono FM/AM/SW Receiver
Si4840    Stereo FM/AM Receiver
Si4844    Stereo FM/AM/SW Receiver