Tuesday, December 8, 2020

The RFI Menace And Reduced Noise Antennas


Having been in this game for 60 years, I can say that RFI and line noise has grown out of control, and especially since the advent of the cheap controller chip + home computers + digital communication + smart TVs + micro-electronics, et-al. And by "out of control", I mean that the crescendo of noise on the bands is becoming virtually impossible to identify and corral. Back in the 1980s when it started getting worse, it was still possible to identify sources and eliminate them using time-worn choke-suppression methods. Now, not so much. The genie is out of the bottle and it ain't going back in.

One of the best tools I have found to identify RFI is a spectrum analyzer. No, a $2000 unit isn't necessary. You already have one if you own an SDR receiver. I have an SDRPlay RSP1a, purchased at $119 U.S. and it's quite easy to take a look at any frequency from 10 KHz on up to see where the problem areas are. Spread a short wire across the floor in the house, connect it up, and you will see all kinds of mysterious RF. A pocket or portable sniffer receiver can work for this too but it's much easier to see the RFI's extent on an SDR receiver's spectrum display. The sniffer receiver is better used to locate the RFI.

I currently use my RadiWow R-108 as a sniffer receiver when walking around the house or property. This is used once the RFI "problem" frequencies are identified on the analyzer. Your Tecsun PL-380, PL-310, or other portable receiver can do the same.

RFI hash @ 500 KHz - the elevated noise floor is -85 dBm! NOT signals!


Let's go over the big RFI offenders to our DXing. The big offenders at my DXing home are:

My Hewlett-Packard 24 inch computer monitor. Huge, wideband, low frequency buzzing in a range across the VLF, longwave and lower mediumwave bands, particularly in the 300-900 KHz segment. The switching power supply creates some of this but the majority comes right off the screen's surface when the display is lit. Efforts to reduce this RFI have only been mildly successful, but luckily its range is only about 15 feet. The downside is the radios need to be within 15 feet of the monitor, particularly the SDR.

My laptop's switching power supply. I have a recent (2020) Acer Nitro 5, 15.6 inch, with AMD Ryzen 5 4600H mobile CPU. Huge, wideband, low frequency hash between 0 and 600 KHz. Virtually all of this disappears when running on battery only. You can't run on battery forever, however.

Old style fluorescent lighting, particularly the old 4 ft. shop lights. Best is to just keep them turned off.

Light dimmers. Don't use them. Keep them off or remove them.

LED light bulbs for house lighting. The bad ones create a high frequency hiss. Luckily the range is only a few feet, but the house is full of them now due to power saving measures. Use good quality LED bulbs. Philips has been highly recommended.

Low voltage lighting used in the kitchen. Lots of wiring through the walls go to a transformer box in the cellar. When the lights are on they inject an additional huge buzz at the lower end of the mediumwave band, peaking at about 550 KHz. The emissions from these range throughout the house. The condition is virtually eliminated by keeping the lights off.

A myriad of switching "chopper" style wall transformers. Some are much worse than others. Try to identify the worst offenders. I try to put all of these on power strips so I can switch them off when not in use.

Unknown sources of frequency spikes. Strong 10 KHz spaced spikes from 9 MHz to 16 MHz, peaking in the 9.5-9.9 MHz and 10.7-12.5 MHz area. This one is intermittent. It can last ten minutes or an hour or more, then disappears. I have not ruled out that this signal may be coming from the mains feed to the house.

**Note: this RFI source just above has been identified. It comes from a $2000 Fisher & Paykel kitchen refrigerator. Fisher & Paykel is a major appliance manufacturer which is a subsidiary of Chinese home appliance manufacturer Haier. It is a multinational corporation based in East Tamaki, New Zealand. In 2012, Haier, a major Chinese appliance manufacturer, purchased over 90% of Fisher & Paykel Appliance shares. Partial solution: wrapping the power line cord through two Workman RFC-1 snap ferrite cores has reduced the problem 50%. More cores have been ordered.

A new 43 inch Toshiba smart TV and DISH satellite box combo. Tremendously strong RFI, a high-pitched squeal in the LW and MW bands coming out of these boxes out to a 6-8 ft. radius, which then couples to lines. It might be possible to put these on a switchable power strip, but then you have the device reboot problem every time you want to use them. Satellite box boot time is often 5 minutes. That's a no-go.

Those are just the biggest offenders. Not mentioned is the RFI coming off the computerized de-humidifier in the cellar, the computerized water conditioning system, and the two computerized heat pumps hanging off the back of the garage.

So you can see the frustration. It's not practical to try to eliminate all of this RFI unless you'd like a lifetime career in RFI removal. I suspect this is the case almost everywhere.


Being a ham as well, I've experimented with just about every wire antenna you can imagine over the last 60 years. My days of winding power line chokes are over. Common-mode chokes, current isolators, et al, are the rage these days - these to reduce RF pickup on the feedline and to lessen the possibility of the feedline from becoming part of the antenna system. They can help, but they are a Band-Aid to the real problem. Why not lessen the noise in a different way? My solution is to build inherently quiet antennas which are resistant to noise, and feed them correctly. 

Three things are important.

1. Get the antenna well away and out of your house.

An end-fed longwire attached to your shack window fed with 15 ft. of coax across the floor isn't going to do it. If possible, on your lot, put the feed point as far away as you can. This, for starters, is one of the most important things you can do. Don't worry about cable feed length. Coax feed at mediumwave or even shortwave frequencies has minimal loss. 100 feet of the old 50 ohm RG-58 on mediumwave presents only about 0.37 dB signal loss, virtually unnoticeable. RG-6A TV coax, 75 ohm, is even less at about 0.28 dB per 100 ft. I use RG-6A here almost exclusively, as it is cheap and readily available through many suppliers.

So, get that feed point as far away from your house as possible.

2. If you can, choose an antenna that is basically a short circuit. What did you just say?

Loop antennas are essentially short circuits to high frequency impulse noise. Long wires, verticals, and dipoles are not. They are RFI magnets, and particularly so if they are not balanced antennas (the dipole is at least balanced). Much of the high frequency noise component of RFI is short circuited in the loop. Small loops are even better for noise suppression, but their drawback is they often need active amplification due to lower signal delivery. Loops work well when placed close to the ground and you don't need high supports for wires.

They can also be laid flat on the ground itself which reduces RFI even more. This is where our Loop-on-Ground antenna will come in.

3. Use an isolating transformer at the antenna feedpoint. Very important. Feed any antenna with a transformer-balun isolating device, even if it is naturally a 1:1 match. There must be no common ground connection between the coax feedline and the antenna, i.e., between the primary and secondary of the transformer-balun. The antenna should remain floating and the coax remain floating. This isolating-matching device does three things which help abate noise:

     1) Matching the antenna greatly increases received signal strength. Increasing signal strength often will raise the signal above the noise floor. Remember when receivers had preselectors to peak the antenna, which made the difference of hearing a signal or not? This is what a broadband matching transformer is actually doing - matching the antenna to the receiver across a wide range of frequencies.

     2) The transformer, at least the one we will use, totally isolates the antenna from the receiver, eliminating the direct wire connection and lessening RFI picked up by the antenna from transferring to the coax. Much of the RFI will be consumed in what I call the secondary, or load side (antenna side) of the balun, as it appears as a direct short to the high frequency component of noise.

     3) The transformer/balun reduces antenna loading because it presents a proper load impedance to the antenna. Loading down the antenna destroys bandwidth and lowers signal strength. Take a longwire for example. A longwire antenna has an inherently high feed impedance, generally 450 ohms, nothing near the usual 50 ohms of a receiver. With no matching device, the input signal delivered to the receiver is a simple resistance ratio. The signal is delivered through a 450 + 50 ohm series divider. The receiver gets 50/500ths of the available signal without the proper transformation. That's 1/10 of the signal being picked up by the antenna! No wonder my receiver can't hear!

The Balun One Nine by NooElec, a 9:1 balun

Balun One Nine on Amazon. NooElec makes a cool little 9:1 ratio balun transformer for about $15.


The Quarterwave Folded Monopole antenna. Everybody starts out in radio trying a longwire or dipole. These are huge noise magnets in RFI-prone locations. If you are an old timer you remember the folded dipole. It traditionally was a halfwave length antenna, like the dipole. It too is essentially a short-circuited antenna as it loops back on itself at the mirrored low impedance node, opposite the feed point. Another version of the folded dipole is the quarterwave folded monopole, a vertical, though it can be configured in other positions. It is half of a folded dipole. The quarterwave folded monopole is also short-circuited and is easily grounded as well. It's inherent impedance is 150 ohms at resonance (468/f-MHz), half that of the 300 ohm halfwave folded dipole, so if possible use a 3:1 matching balun to get to 50 ohms. If you don't have a balun, don't worry too much about using one on this antenna as the 3:1 matching discrepancy isn't that far off. If the antenna can't be erected as a vertical due to height restrictions it can be run as an elevated end-fed antenna of any length. Possible configurations are an end-fed inverted-V (feed end starts at ground, high in the middle) or an end-fed slanted wire (feed end starts at ground).

This antenna is essentially a transmission line antenna. Keep the wires parallel and anywhere from a quarter inch to an inch apart. Erected as a vertical, it has great low angle response for that extremely distant DX.

The LOG antenna, or Loop On Ground is another variation of the close-circuited loop only it lays flat on the ground. It is also best fed with a balun. A spool of 100 ft. of 18 gauge wire on Amazon will only cost you about $9. Lay it out in a square, 25 ft. to each side, and feed it at a corner. It is an excellent low noise performer, though with shorter lengths of wire the signal pickup is quite reduced. My 100 ft. length lying on the ground shows close to 15 db less noise than the 6x12 ft. flag antenna in the tropical band (60 meters), with about equal signal strengths. The difference is in the substantially better signal-to-noise ratio. A 15 dB reduction in noise while holding the same signal strength as the flag antenna is a 15 dB SNR improvement!

I've written an extensive article on the Loop-on-Ground antenna which might be of interest:

The Loop-on-Ground Antenna For The Noise-Challenged

The LOG antenna is somewhat directional, having a fattened hourglass pattern, with slight nulls at the feed corner and the corner opposite the feed. Both high and low angle reception are good, within its range. Best results are when the overall loop length is about 15% of a full wave for the frequency of interest. A 60 ft. total length works well for the 2-8 MHz range.

KK5JY has an excellent article on the Loop On Ground antenna, with illustrations. Be sure to check it out.

The Flag Antenna is a smallish but very efficient antenna especially for mediumwave work. It is usually configured in the shape of a rectangle and is easily ground-mounted if outside. I have a 6 ft. tall by 12 ft. long flag antenna erected indoors on the second floor, running east-west. The lower wire runs along the floor. Two 6 ft. fiberglass rods form the uprights for the ends. Although my house is very noisy with RFI, the noise pickup on this antenna is very low. The rectangle is broken at one corner on the floor nearest the radio, a vintage tabletop Allied A-2515. A 9:1 balun is used to match the antenna to a short 9 ft. length of coax feeding the receiver. Even un-amplified, this broadband flag has wonderful sensitivity from the AM broadcast band through about 6 MHz. On the mediumwave band, it is about the equivalent of a 4 ft. passive loop which is usually tuned.

The BOG antenna, or Beverage On Ground is a good choice if you have the room on your property. It is basically a very long wire laid on the ground (100 ft. or more) and may be terminated through a resistor to ground at the far end. Termination to ground gives it directional characteristics off the end. It is a variation of the classic Beverage antenna, which is usually a few feet off the ground.


For the AM broadcast band, feeding any of these low noise antennas to a pocket or portable radio is easy. I find inductive coupling best. Salvage a short ferrite rod or bar from an old pocket radio. Three inches in length is about right. Remove all the magnet wire from it. Using some solid, insulated telephone wire of about 24-26 gauge, wind about 15-20 turns close-wound around the ferrite rod. Solder or clip the two ends of wire from this coil to the coax feeder coming from the antenna, one to the center and one to the shield. Hold the ferrite close to the radio's internal ferrite which will inductively-couple the signal to the radio. The advantage over a passive loop here is you have a broadband antenna which does not have to be tuned.

Inductive pickup loop

Shortwave antenna coupling to the pocket or portable is more difficult. A simple clipped wire to the telescoping antenna can greatly increase the noise pickup. If your radio does not have an external antenna jack to safely connect the coax feeder with adaptors then you might have to perform surgery on the radio. Be sure to ground the coax shield to the radio's ground. In any event, be extremely careful if directly connecting outside wire or coax to these modern DSP radios. I cannot stress this enough. You can easily fry the inputs to them. I destroyed a $200 Sangean ATS-909X this way two years ago. Luckily it was still in warranty and I was able to get it repaired and reprogrammed.


Antennas that are not essentially short-circuited can work but be aware they will capture more noisy RFI. Above ground dipoles or end-fed longwires are two such types. Be sure to use a matching device in any case, which will help.

I hope this has been helpful to you. Please experiment!

Monday, October 12, 2020

Notes On Soft Mute And Analog Tuning In DSP Radios

On the surface, soft mute in the modern DSP-chipped receiver seems a bit mysterious. What is it? How does it work? Why do we have it? In this article we'll explore how soft mute works and explain its technical details.

As a side topic, but somewhat related to soft mute, we'll also tackle the pseudo-analog tuning of the Silabs 483x DSP chip and see what's up there. This is the chip used in many of the cheap portable and pocket radios found today. They are mechanically tuned with a dial knob and have a traditional AM band scale with dial indicator. But they aren't the pocket radio you remember from the old days.

So let's get right to it.


What is soft mute?

Soft-mute is a further lowering of the audio level of the received signal when it drops below a prescribed signal-to-noise ratio. It was implemented in consumer grade DSP radios to provide a more "comfortable listening experience" for the casual listener and not the DXer. The idea is to relieve the listener from all that nasty low level "static" and "interference", or as Silicon Labs states: " attenuate the audio outputs and minimize audible noise in compromised signal conditions."

Soft mute attenuation is available in the Si473x digitally-tuned series of chips as well as the Si483x analog-tuned series of chips. The soft mute feature is triggered by the SNR (signal-to-noise) metric. The SNR value is directly readable by the chip's software when you tune to a station. The software reads the quality of the signal through its SNR value and makes soft mute changes accordingly. The SNR threshold for activating soft mute is programmable, as are soft mute attenuation levels, attack/release rates and attenuation slope.

The Tecsun PL-380, PL-310, PL-330, and other radios all may set different soft mute values than the chip's default values shown below. Settings for soft mute are initialized during the power up sequence.

The 4 soft mute parameters: Rate, Slope, Max Attenuation, Threshold.

Rate (default): 278 dB/second (range 1-255, actual figure 278 = setting * 4.35)

Determines how quickly the soft mute is applied/released when soft mute is allowed (enabled). 

Slope (default): 2 dB (range 1-5 dB per dB below SNR threshold)

The attenuation slope for soft mute application - in dB of attenuation per dB SNR below the soft mute SNR threshold. Translated: how much audio attenuation is applied as the SNR and signal quality decreases. A setting of 2 will lower the audio by 2 dB for each 1 dB reduction of SNR below the starting threshold at which soft mute kicks in. An example: soft mute starts to kick in when the SNR decreases to 10 dB. At 10 dB, there is 0 dB of soft mute. When the SNR decreases to 9 dB, soft mute reduces the audio level by 2 dB. When the SNR decreases to 8 dB, soft mute reduces the audio level by another 2 dB (4 dB total). By the time the SNR hits 2 dB, the soft mute has reduced the audio level to a max of 16 dB. It will go no lower as the max soft mute has been applied. Note that every 6 dB of audio reduction is a halving of the audio voltage level. 12 dB of reduction is then 1/4 of the original audio voltage level. 16 dB (max soft mute) is a reduction of 84.2% (0.158).

Max Attenuation (default): 16 dB (range 0-63 dB, max attenuation of soft mute)

If set to 0, soft mute is disabled entirely.

Threshold (default): 10 dB (range 0-63 dB, SNR at which soft mute starts to engage). Silabs states, "for a tuned frequency".

Note that the Threshold setting is applicable only "for a tuned frequency". I take this to mean that soft mute is dis-engaged totally when not tuned to an exact 9 or 10 KHz channel, which is apparently why the 1 KHz off-tuning hack works.

What you're hearing when a signal's SNR lowers below the threshold and the soft mute kicks in is the Slope factor in action. The Slope factor is lowering the audio volume accordingly.


How to defeat soft mute?

Soft mute can be somewhat minimized by increasing signal strengths to the radio by using a directly-coupled loop, passive loop or other inductively coupled antenna. What happens is you are increasing signal levels, thus improving the SNR, making the signal exceed the threshold where soft mute is engaged.

The other (original) hack is to tune off the channel by 1 KHz and raise the volume on the radio. Being off-channel disables soft mute.


Two other interesting parameters effecting tuning and seeking, not related to soft mute.

AM Seek/Tune SNR Threshold.

SNR Threshold which determines if a valid channel has been found during Seek/Tune.

Specified in units of dB in 1 dB steps (0–63). Default threshold is 5 dB.

This tells us that when you do a scan, only stations with >5 dB SNR are eligible to be stored.

AM Seek/Tune Received Signal Strength Threshold (RSSI).

RSSI Threshold which determines if a valid channel has been found during Seek/Tune.

Specified in units of dBµV in 1 dBµV steps (0–63). Default threshold is 25 dBµV.

This tells us that when you do a scan, only stations with >25 dBµV RSSI are eligible to be stored.



The Silabs 483x series of chips are analog tuned and they have no digital LCD display. Tuning is accomplished through a tuning knob connected to a 100K ohm potentiometer. They attempt to mimic the analog tuning of the old traditional analog superhet radios when you "sweep" through a station's carrier. Silabs has developed a special tuning formula in software to simulate this. From the DXer's point of view it doesn't work. I've given a lot of thought to how their algorithm works in software.

Over the summer here in North America I have bought quite a few of the cheap Chinese analog-tuned DSP Ultralights. Though I have found some can be quite sensitive (like the Sangean SR-35 and the ultra cheap Dreamsky Pocket Radio), the SiLabs tuning algorithm is still wonky and masks a lot of weaker adjacent channel signals. It becomes tedious for serious DXing. Poor selectivity and overload problems can also be evident on these units, depending on the unit.

As stated, the problem with the current analog-tuned theory is that a weaker adjacent channel signal is masked deliberately if next to a more overwhelming signal.

A typical tuning scenario goes like this. Find a strong station where you know a weaker station sits right next to it on the adjacent channel. The weaker station would be strong enough to be received on a normal superhet radio. With the Silabs 483x radio, tune to the strong station's channel. Now tune to the adjacent channel (the weaker station). The strong station is still there, only at a slightly reduced volume. The radio is attempting to mimic tuning "through" a station like in the old days, increasing the strength of the station as you approach its channel center, then decreasing the strength as you depart. But where is the weaker station?

Here is what is happening in software (I think), preventing you from receiving the weaker adjacent channel.

Let's say the following numbers below, 0 | 5 | 20, represent frequencies 1020, 1030, and 1040 KHz. 1020 KHz has no signal on channel. 1030 KHz has a weak signal of SNR 5 dB. 1040 KHz has a strong signal of SNR 20 dB.

   FREQ  1020   1030   1040
   SNR =   0  |   5  |  20

In these DSP radios, hardware generates a tuning interrupt in software when changing the tuning knob. It causes the software to take over and analyze what just happened. 

Initially, tune to 1040 KHz from somewhere above in frequency and begin receiving the strong station.

Now tune to 1030 KHz. Software then does this:

1. The tuning interrupt is generated.

2. Hard mute the audio.

3. With audio off, electronically retune to the new channel (1030) and test the new channel's SNR. If valid (SNR >= 5 dB), remain on this new channel and unmute the audio. If not valid (SNR < 5 dB), electronically retune back to the original channel (1040) and reduce the audio 6 dB and unmute. The dial will point to 1030 even though we're hearing 1040.

Now tune to 1020 KHz. Software then does this:

1. The tuning interrupt is generated. Remember, though the radio dial shows 1030 KHz, the radio is still electronically tuned to 1040 KHz.

2. Hard mute the audio.

3. With audio off, electronically retune to the new channel (1020 this time) and test the new channel's SNR. If valid (SNR >= 5 dB), remain on this new channel and unmute the audio. If not valid (SNR < 5 dB), electronically retune back to the original channel (1040) and reduce the audio an additional 6 dB and unmute. The dial will point to 1020 even though we're hearing 1040.

Additionally, for each of the two scenarios above, we must also be sure in step 3 that the original 1040 channel maintains a SNR above the SNR of the newly tuned channel or we force-tune to the new channel.

Electronically retuning the DSP chip is simply a matter of electronically setting the proper internal capacitance to resonate with the ferrite coil at the desired frequency. It's done with a single software command.

It's complicated.

If you start at 1020 KHz then approach 1030 from below the situation changes, as we are comparing 1030 to 1020 now, 1020 having no signal at all. If 1030 is a valid channel (SNR >= 5 dB) then the DSP chip tuning remains at 1030, the hard audio mute is unmuted, and the station is received. Drawing from this scenario, we can conclude that if we approach a weak signal from the right tuning direction that we might be able to hear it.

Compounding the problem, these 483x chips also generally have soft mute enabled, which may mask very weak stations. The weak station will still need to overcome the soft mute threshold to some degree.

According to Silabs, this new tuning algorithm has been "audience tested" to a positive level of acceptance. The best approach for the DXer would be to have a radio where soft mute is disabled altogether and no tuning algorithm so that when you move the tuning dial it always changes the frequency.

Surprisingly, this wonky tuning algorithm can be somewhat minimized by increasing signal strengths to the radio by using a directly-coupled loop, passive loop or other inductively coupled antenna. What happens is you are increasing signal levels, thus improving the SNR, so the signal meets the threshold requirements for a valid signal. The radio then tunes to the proper signal and frequency.

A description of even weirder analog tuning anomalies can be read here:

Notes On The XHData D-219 Analog DSP Radio

I hope this analysis of soft mute and the DSP analog tuning mechanic has proven useful and interesting. All technical data has been gleaned directly from Silabs data sheets for the respective 473x and 483x DSP chips. The programming guide for these chips was particularly helpful in understanding the operation of soft mute.

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. In close proximity, within 1/10 wavelength, 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  dBm
S9+60dB  50000.0   94   -13
S9+50dB  15810.0   84   -23
S9+40dB   5000.0   74   -33
S9+30dB   1581.0   64   -43
S9+20dB    500.0   54   -53
S9+10dB    158.1   44   -63
S9          50.0   34   -73
S8          25.0   28   -79
S7          12.5   22   -85
S6           6.3   16   -91
S5+4.9dB     5.6   15   -92
S5           3.2   10   -97
S4           1.6    4  -103
S3+1.9dB     1.0    0  -107
S3           0.8   -2  -109
S2           0.4   -8  -115
S1           0.2  -14  -121

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.