Antenna Tuning Units For Shortwave Listening

Make the antenna connection to your radio a match made in heaven.

Whether receiving or transmitting, it's all about energy transfer. For us listeners, all we want to do is get whatever signal the passing wave has deposited on our aerial element, wire, beam or rod at whatever impedance transferred to an impedance our radio will like.

Do that and losses are less, reception is better and with an ATU, you get a bit of free gain and a little less background noise.

And they are passive, no electronic gain to add noise and get overloaded by the local AM station.

In fact, an ATU will pre-select the range of frequencies you want to hear and by default reject the rest. This can improve signal-to-noise ratio, so improving your perception of the signal you want to hear.

A modern radio passes a band of frequencies (band pass tuning or filtering) for processing. In that band, signals large and small appear.

The ratio between them can be a million to one or greater, the quiet stuff getting lost in the shadow of the loud.

Use the preselection of an ATU to narrow the band and the radio stands a better chance. So why aren't we all using them?

Our picture show the simplest pi-tank but they come in all sorts of configurations. In short, if you are serious about SWL, get an ATU.

Co-ax Antenna Feeders - Low Loss Or Dead Loss?

Coax-fed antennas have become law during the development of the latest generation of radio sets. In one corner, the radio hams talk of antennas as religion. In our corner, we still believe radio is fun as long as you follow the ground rules - puns being a cheap form of journalism - the most important being the efficient transfer of energy from aerial site to radio set.

Single-ended antennas, whether they be a short whip, an end-fed wire or an MF TEE, will only approximate a resistive match at odd multiples of the frequency at which the antenna achieves Quarter Wave Resonance and then only if the termination is with respect to a common ground, both for the feedline and the antenna itself.

At all other frequencies such an antenna looks like an impedance in series with a resistance.

Coast Stations used them. Ships use them. Intercept Stations use them. Casual listeners use them. They are also found on military radios and the slower aircraft being used for communications and for receiving Electronic Navaids. In fact they are almost universal and because of this they receive no more than a passing glance in the grand scheme of things. Truly, familiarity has bred contempt.

In the good old days, when receivers had real front-ends and the price of copper was reasonable, this was of little consequence. The end-fed antenna was simply brought in directly to the receiver terminals via a healthy piece of copper tubing through a hole drilled in a plate-glass window. Low loss? Virtually no loss in practice.

By understandable means to us old-timers, including the use of warmly glowing, EMP-immune valves filled with excited electrons, the flow of which was deflected somewhat by the energy from the antenna, this lot used to be converted into an intelligible signal, the translation of which would be transcribed by the Operator in the soft, comforting light emanating from the dial of the receiver.

The Three-Legged Fuse

But then Three-legged Fuses came along to replace warmly glowing, EMP-immune glass bottles; they called them Transistors. An epoch had ended. So what else was new?

Co-ax was almost new. In their haste to exhibit their understanding of co-ax and the Three-Legged Fuses, ignoring the basic antenna theory they ought to have learned at their mother's knee, engineers used it everywhere.

Antennas were designed for it. Three-legged fuses had impedances that matched it; new problems arose which were further compounded by broadband front-ends. This was the demise of Performance and Immunity. The demise of the Vale Four-gang Variable Condenser - the black art of tracking a superhet is lost!

But What About Images, IPs and Overloading?

That was what the front-end was all about, was it not? So, to eliminate the images, up-conversion was born - and Synthesisers and low-pass filters. Three-legged fuses proliferated, interbred and mutated. Now there are many-legged fuses, I gave up counting long ago!

Time was when we could afford a receiver, now you need a mortgage to get the down payment together. But we digress

All this is Progress, we are led to believe but antennas are still the same, more power to them. Engineers are not. They understand the multi-legged fuses and the up-conversion techniques and their intellects are overloaded with digits and Op-Amps and Bragg Cells and Fast Fourier Transforms. There is no room left to understand the Antenna - the only means that exist to collect the signals they need so that they can exhibit their fantastic abilities.

Luckily, there are few of us old-timers left, we know where the priorities lie. We must remind you. Co-axial cable is a low-loss conductor of RF energy only when it is terminated in something like its nominal impedance, usually 50 or 75 ohm, which is a very low value in terms of the natural impedance of a non-resonant antenna.

No matter how clever you are, you can't successfully feed an end-fed, non-resonant antenna directly into a piece of Co-ax.

Matches Made In Heaven

Regardless of Progress and Education, you still can't beat the laws of physics. Un-terminated co-ax cable is VERY capacitive. UR67 for example, has a capacity of 30pF per foot - that's about 99pF per metre. A typical MF TEE antenna may look like 200pF in series with 6 ohms - and site layout may well make it necessary to feed it via 100 Metres or more of UR67.

Would YOU feed a 50 ohm receiver input via a capacitive divider of 2/99, all but a 2% transfer?

A ten-metre whip on a building might have a capacity of 5pF and a resistance of 2 ohm, fed via 8 Metres of co-ax. This gives you a similar capacitive divider of 25/800, not quite so bad as the MF TEE, but not much better!

Yet, without thinking, you do it all the time and there is no recognition of the problem I have revealed, because nobody wants even to admit that there is a problem.

Short Antennas

When an antenna is short (less than 90 electrical degrees in length), as is normal in Coast Station use at MF, or in many Transportable and other applications at HF, the antenna looks like a small number of Ohms in series with a capacitance.

This obtains at any frequency at which it is shorter than an odd multiple of 90 degrees and longer than an even multiple of quarter-wave resonances.

At all other frequencies (except at odd multiples of Quarter Wave Resonance, where the natural termination of the antenna is, almost, purely relative) the appearance of the antenna at termination is of a capacitance in series with an inductance.

Now we must give a little credit to those clever young engineers who design modern receivers but don't understand antennas. They have pushed the thresholds of sensitivity down to levels unheard-of in the good old days, albeit in 50 ohm to match their beloved co-ax.

Thanks to their ingenuity we ought to be able to get a usable signal at a much lower threshold. And so we can! Trouble is, some of these lads have read CCIR Report 322 and decided that sensitivity below 2MHz is of little consequence because the amount of noise down there will defeat the signals anyway.

In part they are right, but if a signal can be weaned from all that noise, facility should be provided to do it.

Top Band, Top Loss

And it is below 2MHz that the non-resonant, single-ended antenna is most likely to be used, simply because the physical size of a resonant antenna at these frequencies precludes its use except in very special circumstances and at great expense, both for the physical structure and in terms of real-estate.

Is there a practical solution? Within limitations, yes, there is. In order to reduce the capacitive divider problem the co-ax must be terminated in some sort of load that is within reasonable shooting-distance of its nominal impedance.

We are not worried about VSWR or power-handling in this case, the strongest signal we are going to get will be in the order of a couple of volts or so.

Just A Few Turns For The Better

The problem of terminating a single-ended antenna into co-ax has long been recognised. We have seen a special, pretty little box to do this. It has a nice little feed-through insulator on one end and a Type N Connector on the other.

Inside, there is nothing except a small ferrite ring, sixteen turns primary, four turns secondary to get the GENERALLY HIGH impedances seen at a long-wire down to the GENERALLY LOW impedances needed by a modern receiver and it only costs the customer a few quid. Or a hell of a lot more if that customer is depending on his dealer to cover his basic lack of knowledge or interest in antennas.

Allowing for the credit we gave the youngsters for pushing down sensitivity thresholds of receivers, we can neglect a true impedance match when we terminate the antenna.

Now this, as far as the MATCH is concerned, consists of the R component of the antenna plus the Loss Resistance of the Antenna System including its associated ground system, and you can measure as many as you like, you will find the R component varies between 2000 and 20 ohm.

Put bit of capacity and some inductance in series with that and you will find that the impedance works out to be well within shooting distance of 400 ohm. If we accept that we are not worried about VSWR (although it will contribute to loss; hopefully, the increased sensitivity of these modern receivers will have compensated) we can tolerate say, 100 ohm at the top and 10 ohm at the bottom of our scale; a chunk of suitable ferrite wound with a 4:1 ratio, the low winding to the co-ax, high to the antenna feed, common earth, makes a remarkable difference.

If you use a toroid, it can be auto-wound and tapped, in practice a toroid of 20 turns Bi-filiar wound with an additional 20 turns on the Antenna side (40 on the antenna, 20 on the co-ax) seems to work pretty well anywhere. Better still, you can use a compensating RC network on the antenna side. Either solution is certainly better than leaving the co-ax open and trying to contend with the amazing losses of the capacitive divider! There is a worthwhile benefit, too.

Static Discharge

With this sort of termination on the antenna, any static build-up short of a direct lightning strike has a leakage path to ground; vulnerable solid-state front-ends and multicouplers of whatever gain some free protection (which we never really needed with valves and heavyweight tuned copper coils in the front-end).

Now we have done something about it, why don't you! It helps our customers to receive signals and to get their money's worth!

Better Now?

This was written by Bill Shearman at RACAL and shows the house style. The EMP-immune glass bottle is a radio valve. An ECC189 in cascade, an EF183, EF80, EF50 or 6K7G, depending on your generation, provided the amplification in the radios of yore.

Some of these are still kept on as they are less affected by ElectroMagnetic Pulse, an after-effect of a nuclear strike - an important consideration for reception in the professional sector.

IPs are intermodulation products, the dire consequences of strong signals on different channels mixing in the early stages of a radio to spoil our enjoyment with extra noise and reception of stations that do not exist.

All I am preaching is that to get anything useful out then try to get a reasonable signal in. The rapid growth of the accessory market brings us antenna tuning units (ATU), preselectors and matching transformers - baluns, to you - low loss cables and connectors. They WILL make a difference!