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Tuesday, October 5, 2021

Magnetic Longwire Balun : The Original by RF Systems

 An oldie but a goodie. Remember this?

Magnetic Longwire Balun

Years ago (almost 40) when I was quite active in my ham career and shortwave DXing I purchased one of these for about $40. An expensive little device. But I was curious about the claims.

It was advertised as a magnetic balun, low noise, matching almost anything below 40 MHz. Receive only of course. You can search on "RF Systems Magnetic Balun" and find many sites, blogs and forums where the merits of this device have been discussed.

The RF Systems Magnetic Balun is basically a 9:1 impedance matching transformer. "Balun" means balanced-to-unbalanced transformation - a transformer device which takes a balanced impedance (like the center feed point off a dipole) and transforms it to an unbalanced impedance (like coaxial cable). The RF Systems Magnetic Balun is really not a balun but what is known as an Un-un. In this case, a 9:1 impedance transformer, transforming an unbalanced input to an unbalanced output. One such example might be to match the unbalanced end-fed longwire to a 50 ohm coaxial cable, which is also unbalanced.

In more recent times, I've had very good luck with mine using it with both a "longwire" and a 25 ft., ground-mounted, but ungrounded vertical. Longwire in this case, means anything of 25 to 100 ft. in length, which is not really long in the traditional sense.

In personal experience over the years, the low noise claim gave minimal results. Low noise might be gained by positioning your longwire a ways away from any noise source and feeding the receiver with a long run of coax. But that's just common sense. Grounding the coax's shield at the entry point to the house can help too.

Presented just below are good quality .JPG images of the original document which came with the RF Systems Magnetic Balun.

Other Matching Options

Good or even better results can sometimes be had with other matching devices like an antenna tuner which will also do the matching like a balun would. MFJ Enterprises has quite a few which can be used for simply tuning odd lengths of wire. Google will produce many results for antenna tuners. They can be made pretty simply with a variable capacitor and some hand wound coils.

For AM broadcast DXing I prefer inductive coupling right into the radio's own ferrite antenna. Matching is pretty well taken care of then and good signal transfer is also accomplished, and often without the overloading that can occur when directly connected.

A circuit example of a 9:1 impedance transformation "magnetic balun" can be found at M0UKD.

This is another "un-un", as it transforms between unbalanced input (the longwire) to unbalanced output (the coax input of the radio). This circuit is actually the same circuit as the RF Systems magnetic balun I have. It takes a high impedance (the longwire) and transforms it by a 9:1 ratio down to a lower impedance (usually the radio's input connector). Note that radios with external longwire posts like a CCRadio SW may already be set up for high impedance input. It depends on the input circuitry used in the radio.

Be careful directly connecting wire to any of these DSP radios. Modern chip electronics is not so forgiving as the old tube stuff from my generation. I have fried several radios with static discharge.

Inductive coupling to a radio's ferrite loop can easily be done.

Find an old piece of ferrite bar or rod and close-wind 15-20 turns of insulated wire on it. Ground one end to the earth and connect the other end to the longwire. Then couple the ferrite bar/rod to your radio's ferrite antenna. You can even run it through a length of coaxial cable, though better results will be obtained by using the 9:1 balun where the longwire connects to the end of the coax (outside).

My 25 ft. vertical is set up that way. Picture the vertical as a simple end fed longwire (fed at the bottom and insulated from the ground). It is attached there to the 9:1 balun and through the balun to the coaxial cable. The 50 ft. coax cable runs to the inside of the house where the center and shield of the coax is connected to the 15-20 turn winding around that spare piece of ferrite bar/rod.

Sensitivity can then be adjusted by the closeness of the coupling to the radio.

Be aware, most of these radios are easily overloaded by excessive signal. It shouldn't take much of a longwire to do that.

The old-fashioned Pi type tuners work quite well for longwires. This involves two variable capacitors with a coil between them. I used to have a homemade one which I used for years on the ham bands for both transmit and receive. That style can match about anything.

The Grove TUN-4 Antenna Tuner

Grove Electronics, now out of business, (they also published the Monitoring Times magazine years ago) used to sell quite a few receive tuners. I still have the TUN-4 which is quite nice and will tune from below the AM band to 30 MHz. It also has a preamplifier. You can often find these and the others on eBay. The American Radio History web site has many old radio catalogs which you can peruse. Here is an old Grove catalog, dating back to 1989.

Grove Catalog, 1989

The Grove TUN-4 Antenna Tuner

Why impedance matching?

Maximum signal is transferred (and I might add with minimum distortion) if the impedance is matched between input and output of a circuit. To give a simple example, in the old days of Hi-Fi the audio enthusiast always made sure his speakers were matched to the stereo's speaker output. If the stereo had an 8 ohm speaker connection, you made sure you used 8 ohm speakers, not 16 or 32 ohm speakers. Maximum signal would be transferred to an 8 ohm speaker, and also as important, minimum distortion would result.

The same is with radio and antennas. Typical longwire antennas might have an inherent impedance in the neighborhood of 450 ohms, or more. Old radios which had a single antenna post for a longwire usually made sure that the input was designed around 450 ohms. The coax inputs or mini-jack inputs of most receivers are usually designed around an input impedance of about 50 ohms. If we connect a longwire to one of these inputs we are creating a series circuit of 450+50 or 500 ohms at the input with our "tap" at the 50 ohm point above ground. Ohm's law tells us that we are only getting 50/500 or one-tenth the signal available from the longwire. This is remedied by the impedance matching transformer or by the antenna tuner. Properly matched, we get the full signal off the longwire.

In the ham radio world, good matching is even more important when transmitting to an antenna. A poor match results in the antenna reflecting some or most of the power right back at the transmitter. These reflections also exist on receiving antennas when mis-matched.

Some radios seem to be a little more tolerant of antenna mismatch, some not. I have the SDRPlay RSP1a SDR receiver here and I find it not tolerant at all of antenna mis-match. Another, the Yaesu FRG-7, dating back to the late 1970s, is particularly sensitive to proper impedance matching. This is a very sensitive radio, but you must match the antenna to it to get maximum results, and particularly on mediumwave. Many radios, however, will respond favorably by hooking up any sort of wire to them.

For receive purposes, feeding the radio using 75 ohm coax instead of 50 ohm won't matter much. I've used 75 ohm TV cable for years for receiving on the HF bands - 30 MHz and below. Transmitting or VHF/UHF work would be a different story. The 75 ohm to 50 ohm mis-match is a 1.5:1 mis-match, which coincidentally is also a 1.5:1 SWR (Standing Wave Ratio) in the ham radio transmitting world. That's usually about at the edge of acceptability for transmitting in the HF range.

The impedance transformation of 450 to 50 ohms is a 9:1 ratio of course. It is based on the windings ratio and is equivalent to the square of the windings ratio. If the input side (the 450 ohm side) has 3 times the number of turns than the output side (the 50 ohm side), then the turns (windings) ratio is 3. 3 squared = 9, so the impedance transformation is 9:1.

For 75 ohms, you'd need an impedance transformation ratio of 6:1 (450/75). The square root of 6 is approximately 2.5, so you'd need a turns ratio of 2.5 from input to output. 10 turns to 4 turns would do it.

Understand that the 450 ohm figure for the longwire may vary greatly above or below this figure depending on the frequency you are receiving. In other words, the impedance presented at the end of the wire is frequency dependent. The original concept of a longwire was a wire of several wavelengths. Casually, someone throws an odd length of wire out a window and calls it a longwire. If your end-fed "longwire" is anywhere near one-quarter wavelength of the frequency being received, you probably are looking at an impedance of 20-75 ohms, not 450 ohms. An end-fed halfwave length of wire might present itself around 1000-2000 ohms. The point I'm trying to make here is that the 9:1 balun is there just to get you in the ballpark for this higher impedance, matching-wise.

And now, the RF Systems Magnetic Balun. Click on each image for the full resolution.










Sunday, September 26, 2021

Greyline Propagation

Let's talk about greyline propagation.

Greyline propagation, or propagation along the earth's sunrise/sunset terminator, is very noticeable on the 160, 80, and 40 meter ham bands and lower shortwave bands. Mediumwave signals can and do travel that path as well.

What does the greyline look like on a map?

There's a great solar clock map I've found which I use called Simon's World Map. You can start it and leave it on your desktop.

Simon's World Map

It's freeware. The creator (Simon) is the guy involved in the HF+ Discovery SDR project, competitors to the SDRPlay and SDRUno, and others.

A screenshot of mine in use right now:

The night/day overlay over the world map shows the width of the earth's terminator and its transition from total darkness to the sunrise and sunset edge. The actual sunrise or sunset point is the edge where the overlay disappears. The lightest band of the overlay is civil twilight, where the sun is just below the horizon to a point 6 degrees below the horizon. The next darkest band is nautical twilight, where the sun is between 6 degrees and 12 degrees below the horizon. The next darkest band is astronomical twilight, where the sun is between 12 degrees and 18 degrees below the horizon. The darkest area is where total darkness exists, where the sun's position is greater than 18 degrees below the horizon. Medium wave and other signals in the lower shortwave bands, as described above, can propagate along the terminator at greatly reduced signal attenuation - often received at astonishing distances. 

Space Weather Specialists define it this way on their Real-Time Maximum MUF page, found here:

Space Weather Specialists

Their definition puts the greyline stripe exactly between the rising or setting sun and the 12 degree point of darkness below the horizon, also known as the Nautical Twilight point. However, they seem to ignore any greyline existence on the daylight side of SR/SS.

Civil twilight is halfway to Nautical Twilight, or 6 degrees below the horizon.

Published sunrise/sunset times in newspapers and charts almost always base their times on "Official" sunrise/sunset which is 90 degrees 50 minutes, or 90.833333 degrees from solar zenith, making their times several minutes different from actual SR/SS. Actual SR/SS is based on 90 degrees from solar zenith.

I would argue that greyline conditions of course exist for a time past sunrise and before sunset, straddling the actual SR/SS. How much? I would tend to split it at about 6-9 degrees on either side of SR/SS, but it depends on the frequency too. 6 degrees rotation of the earth is 24 minutes of clock time.

12 degrees below the horizon seems a bit much to me. 12 degrees of earth curvature is 828 miles or 48 minutes of sun travel which is quite a departure from actual sunrise and sunset. Again, I would tend to split it at 6-9 degrees on either side of SR/SS. 7.5 degrees is 30 minutes, which seems about right.

VOACAP has a nice calculator which gives their representation of greyline start and end times by location and date:

VOACAP Calculator

VOACAP uses the 6 degree below the horizon (darkness side) and 3 degree above the horizon (daylight side) points for their greyline calculations.

Their greyline notes can be read here.

My mediumwave broadcast pattern map set  for 2022 is almost ready for publication, coming mid-October. This year it will be available exclusively on The Medium Wave Circle. Many changes and updates have been implemented over the last 18 months. One is a greyline day/night overlay, similar to Simon's World Map. Coming soon!

Greyline is a good topic for discussion. Maybe others have thoughts.

Bill

Friday, August 27, 2021

A Radio Story

I had a conversation recently with a fellow who was interested in starting into the radio hobby, and I thought the conversation might be of interest to anyone who has ideas of radio DXing. I'll reproduce the gist of the conversation here.

Nice to meet you. Glad to see people are still getting involved in our "archaic" hobby!!

I'm 74 years old this year and have been involved since about 1960, some 60 years. I got my first ham license in 1963. Before that I used to listen on my grandfather's old Sears Silvertone wooden console as a young boy. Shortwave was fun in the days back then and in the 1970s, all during the Cold War. Fascinating and scary stuff.

Sears Silvertone 7067, about 1942

I sort of quit the ham activity in the late 1990s after some 30 years, though I've hung on to that license. My ham and radio interest was always more of a technical one, though I made a lot of contacts all over the world via my preferred CW (code) mode over the years. Since about 2008 I've been doing a lot of mediumwave DXing mostly, but recently got back into shortwave listening too over the last couple of years.

International shortwave is only a fraction of what it was years ago. Most stations have moved on to the internet or don't exist at all. But don't be discouraged as there is still a lot of stuff out there to hear. It's just a little more difficult than the big booming signals of the old days.

I'm glad you decided to look for something beyond the Tecsun PL-380. It's 12 year old technology at this point and has been much improved since 2008 or so, even for those radios that are using the same radio chip. Silicon Labs makes the radio chips, though the Chinese have cloned them now and they produce their own too.

C Crane, a US company, also makes some nice radios, and sensitive too. Their original CC Skywave is actually sort of a "super" PL-380, but at about $80. The Chinese then cloned it with the RadiWow R-108, and in my opinion, did a better job for half the price. Understand, this is a tiny radio, it will fit in the palm of your hand. It is sensitive, and has corrected many of the original consumer gripes we had for the PL-380.

https://ccrane.com/

I have a Tecsun PL-880. They are good radios with nice sound. It's more paperback book sized, where the PL-380 and R-108 are cigarette pack sized. On medium wave it's more sensitive with the longer ferrite antenna. Shortwave is a bit better too, and of course it can receive single sideband SSB as well. FM is superb of course. The only gripe I have with mine is the frequency readout is 2 KHz off on the AM band. An alignment problem I'm sure, but I don't think it's correctable at this point.


I also have a Sangean ATS-909X, a newer model to the old '909. There is also an even newer model just out, the ATS-909X2, available on Amazon. All the 909's are beautiful looking radios. I love mine and actually prefer it to the PL-880, though it performs about equally. I find that radios you buy have a certain "feel" about them, independent of their performance. I call it "fun factor". The 909X has greater fun factor for me for some reason.

https://www.amazon.com/SANGEAN-Ultimate-Multi-Band-Radio-ATS-909X2/dp/B08MSXX6LH

On the PL-660 and PL-880 comparison. I don't have a '660 but always wished I'd tried one. I do have an older Tecsun PL-600, sort of the predecessor to the '660. They are analog radios, superhetrodyne design. They are good radios, and the '660 is probably equally able as the '880. However, the '880 will have better bandwidth options since it uses the DSP chip, which is very nice to have. I suspect the speaker sound is better in the '880 too. Sensitivity may be about equal. Tuning may be preferable on the '880 as well, especially for SSB.

On shortwave, all of these radios will improve with a little wire clipped to their telescoping whip antennas. Just a word of advice - be careful when connecting outside long wires to them. Static discharge can destroy them in a hurry. I have ruined two - an old Sony ICF-2010 years ago, and the Sangean '909 I have. I was able to send the '909 in for repairs under warranty and it came back in perfect shape, luckily.

In these modern times we are plagued with RFI - radio frequency noise from all kinds of devices. Try listening outside away from your house if you have problems. Or even go to a park.

Three different web entities produce up-to-date shortwave schedules. They are:

EIBI http://www.eibispace.de/

NDXC (AOKI) http://www1.s2.starcat.ne.jp/ndxc/

HFCC http://www.hfcc.org/data/

I find EIBI and NDXC rather good. You can download them for free. Once unzipped you will find text files that you can look at.

China has a huge presence on shortwave, and I find listening to them rather fun, especially their music. Radio Romania has a nice signal into the US. Also Turkey and Greece, and I love their music as well. I often listen to Radio New Zealand at night. Sadly, Australia is not on any more. BBC still has a presence, mainly out of their Ascension Island relay, Asia and the Middle East. Also Voice of America. Lots of signals coming out of Africa. There are many others.

For best results, learn to listen at the right times. After dark and at sunrise/sunset times, check the 4-10 MHz bands. During the daytime and at sunrise/sunset times, check the 10-21 MHz bands, particularly 11500-12100 KHz, 13500-14000 KHz, and 15000-15800 KHz. Best times are actually the hours right around sunrise and sunset. Where you are on the west coast, check for Europe late in the afternoon through the evening. Asia will be dominant during sunrise hours. Listen for long path propagation, a weak, warbly signal with an echo, indicating reception from both directions. It is indeed possible and happens all the time, but you need a good antenna usually. Consult those shortwave schedules frequently. Find yourself a nice world map clock off the web, showing a world map with the light and dark areas of the world for the current time. "Simon's World Map" is a great little clock-map that is free and you can install it on Windows. It's by the guy behind the HF+Discovery SDR radio I think.

https://www.dit-dit-dit.com/world-map

The last five years or so I've gotten into SDR radios, software defined radios. I have an SDRPlay RSP1A (Ham Radio Outlet has them) and an HF+Discovery. Either is about $120. They would have been the equivalent of a $3000 military grade radio back in 1970. They are cigarette pack sized and plug into your USB slot. You tune and use them through software on your computer. They have spectrum displays and lots of bells and whistles that you'd never find on a portable. If you get deep enough into the hobby you might want to try one of these as the entry price is certainly very reasonable. They require an outside antenna for shortwave, or a loop can be used very effectively for the medium wave band.

Hope this has been helpful and has given you some ideas. Let me know how it goes. Have fun.

Bill

Tuesday, December 8, 2020

The RFI Menace And Reduced Noise Antennas

THE RFI MENACE

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 sources 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 and 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 but it's much easier to see the RFI's extent on an SDR receiver's spectrum display. We can use the sniffer receiver 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 little DSP receiver can do the same.

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

BIG OFFENDERS

The big offenders at RADIO-TIMETRAVELLER's eastern QTH are:

My HP 24 inch computer monitor. Huge low frequency buzzing in a range across the VLF, LW and lower mediumwave bands, particularly in the 300-900 KHz segment. Here I use the monitor plugged to the HDMI output of my laptop. Efforts to reduce this RFI have only been mildly successful. A small one meter coaxial loop oriented just right helps.

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

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. The condition is greatly reduced by keeping the lights off.

A myriad of "chopper" style wall transformer chargers. 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.

An unknown source generates a myriad of repeating, dual and triple frequency spikes across several ranges of the HF spectrum, separated by about 20 KHz. It starts in the 11-12 MHz band and worsens as we get to 16 MHz where it remains strong all the way to 28 MHz. It is constant.

An intermittent and unknown source is causing strong 10 KHz spikes from 9 MHz to 16 MHz, peaking in the 11-12 MHz area. It can last ten minutes or an hour or more. There could be a possible connection to the signal above, though that signal is constant. I have not ruled out that this signal is coming from the mains feed to the house.

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

A new 43 inch Toshiba smart TV and DISH satellite box combo. Tremendously strong RFI, a high-pitched squeal in the LW and mediumwave 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.

LED light bulbs. These 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.

Then there is the general RFI coming in off the mains feeding the house. We have an old farm house here and the street lines are above ground on poles 50+ years old. The telephone lines accompanying them are also 50 years old and won't even support DSL due to their advanced deterioration.

And those are just the biggest offenders. Then there's the RFI coming off the computerized de-humidifier in the cellar, the electric water heater, the computerized water conditioning system, and the two computerized heat pumps hanging off the back of the house.

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.

ANTENNA SOLUTIONS FOR NOISY ENVIRONMENTS

My solution is to build inherently quiet antennas which are resistant to noise. Two things are important.

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

     Loop antennas are essentially short circuits. Long wires or open-fed wires or dipoles are not. They are RFI magnets. 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 ground-mounted. They can also be laid flat on the ground itself which greatly reduces RFI. Use a matching or isolating device. Positioning the loop at ground level allows the coax feed's shield to be easily grounded, another plus.

2. Generally, feed any antenna with a transformer/balun matching device, even if it is naturally a 1:1 match. This matching/isolating device does three things which help abate noise:

     A) 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 on the band of interest.

     B) The transformer isolates the antenna from the receiver, eliminating the direct wire connection. Some of the RFI is consumed in the secondary, or load side of the balun, as it appears as a direct short to the high frequency component of noise. Also ground the receiver's coax feed's shield at the connection to the balun if at all possible.

     C) The transformer 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 feed high 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 antenna 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.

ANTENNAS WITH GOOD NOISE RESISTANCE

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!

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.

FEEDING A POCKET OR PORTABLE RADIO HAVING NO EXTERNAL ANTENNA

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.

NON SHORT-CIRCUITED ANTENNAS

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!