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 an 18:1 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, an 18:1 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 at the bottom of this post are good quality .JPG images of the original document which came with the RF Systems Magnetic Balun. catalog item for 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 similar, but 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 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 RF Systems balun or 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 RF Systems 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. The RF Systems balun, at 18:1, presents a turns ration of about 4.25, the square root of 18.

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 or 18:1 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.

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