The Way We Were

Here is a classic copy of The Guide from 1999. See how much has changed, how much remains the same but above all, enjoy the nostalgia:

AERIALS, ANTENNAS AND EARTHS

We are on shaky ground here. Rain forests have been lost for paper to print the endless hallowed textbooks on the subject so we don't feel we should add to the debate. Having said that...

Your first point of reference is The Manual. The antenna stages of your radio will exhibit some kind of electrical characteristic. This is a Complex Impedance, usually edited down to "impedance" - the resistance offered to the radio signal - for the sake of common usage. If you follow the suggested designs in the instruction manual, then the burden of thought rests with the set maker and the aerial will be a good match. This has little to do with dating agencies - our "good match" is the best transfer of energy from the aerial to the radio which is all we are trying to achieve. This can also be done without the slightest knowledge of the radio's input impedance, offering more reassurance to the beginner.

You will note the writer can't make up his mind on what term to use, "antenna" or "aerial." They are interchangeable - your commentator being a dear old-fashioned thing trying to make a point. Current designers working in the white heat of new technology do seem to loose touch with the fact that the basic physics remain the same, only the "top layer" of jargon follows fashion.

And now, over to Smug Corner

Welcome to a new feature of your HF Guide. This is a chance to score points off your elders and betters who inhabit a land where so much money has gone over the counter, they have "Receivers," not "a radio" and have a bad case of the Rhombics for an antenna where we have got a bit of wire. If we have a portable radio we won't even have that.

Users of portable radios and scanners, those with a reasonable RF performance, get an early chance to visit Smug Corner. Even the small telescopic or helical antenna will deliver a signal, albeit at a changeable - usually high impedance and at a low level, the input stages designed to cope with all this. No antenna wires leave you free to listen anywhere, locations near windows giving best reception without the screening effects from any metalwork used in the building.

Portable users are strongly recommended to use a mains power supply when listening at home. This saves a fortune on dry cells and provides an earth path for unwanted signals. One of my sweeping generalisations is to state that DC battery power is up to 200 times more expensive than using the AC adapter.

Using ni-cad rechargeables is a debatable saving as the convenience of not buying dry cells is negated by the lower voltage available. A radio expecting to see 6 volts from four AA cells will only get 5 volts from a set of ni-cads, a loss of 16%. Not much in real terms, but enough to affect the RF performance of one of the better portables. If you have upgraded, the losses may degrade the new set to the level of the one just replaced. Remember; performance is such these days that each new model only brings an incremental increase in spec...

If the portable has an antenna connector, short pieces of wire can be tried, but don't go to any great lengths - pun intended - to put up big aerials for portables. Too much signal can cause more problems than too little.

For those of us who require an outdoor aerial - by far the best for general reception as we get away from electrical interference inside the house - we always recommend The Long Wire.

The Traditional Long-Wire

This, as its name implies, is a simple single length of wire of a thickness strong enough to support its own weight, insulated or not, as long and high as the local geography allows. Technocrats will call this an "Inverted L" as the longer limb of the capital letter L is the bit that runs down the garden, the shorter limb swinging down to form the downlead to the radio. Technophobes will say it is easy to put up. Simply use insulators at each of the three points of the L and you are away. If you feel this prose is labouring toward a "what the 'L" punchline, then there it is, with all the feeling of inevitability...

Try to form the aerial and downlead in a single unbroken length of wire. This will avoid making connections outside and the possible future effects of corrosion affecting reception. If you are out in the country, a long wire can be very long offering some advantages at lower frequencies.

Keep it away from any overhead powerlines as their throbbing 11,000 volts will do little for the radio or your hairstyle. They are also the transmitters of electrical noise at the very frequencies you thought you were gaining some advantage by "going for the big one."

Life is like that.

Connect all long-wires great or small to the WIRE point on the back of the radio. While you are there you may see a large coaxial connector. This is for specialist antennas that achieve resonance - that is, a maximum efficiency at a single or narrow range of frequencies - a characteristic of them being a low impedance that may be carried by coax cable.

The advantage of a coax feed is the screening effect the cable has against localised interference, no special care has to be taken in the handling of the cable and, provided some effort has been made to "match" the coax at both ends, then the antennas can be remotely sited away from noise sources. Our traditional long-wire will also be a low impedance at some frequencies so don't hesitate to experiment. You can calculate at what frequencies this will happen if you feel the need to. We prefer the "suck it and see" method as no amount of sums can argue with a higher signal meter reading...

Readers of the previous editions will note that so far we have stayed out of The Snug Bar of our village inn, The Duck and Fruitbat, a tribute to a great radio voice here in the UK who got your scribe "shambling around in the early bright" most mornings on BBC Radio Two's Early Show, the much missed Ray Moore. Radio DJ's are only special to their local audiences, Ray's name will mean little to this WWW readership. Every reader will have a radio name they grew up with...

The Snug is a place we went to hide from Management (the wife, that is), life in general, a place for the quiet contemplation of our hobby's technical issues. We can hide no longer.

Wired?

Coax-fed antennas have become law during the development of the latest generation of radio sets. The Duck and Fruitbat is quiet of a lunchtime this days. 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 use 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 glass bottles 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 internals of the receiver.

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, willy-nilly. Antennas were designed for it. Three-legged fuses had impedances that matched it; new problems arose which were further compounded by broad-band front-ends (or total lack thereof). 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 and 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 Synthesizers and low-pass filters. Three-legged fuses proliferated, interbred and mutated. Now there are 32-legged fuses, possibly even larger numbers of legs exist, 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.

Why not?

Well, regardless of Progress and Education, you still can't beat the laws of physics. Un-terminated co-ax cable is VERY capacitive. UR67 or example, has a capacity of 30 Picofarads per foot - that's about 99 Picofarads per metre for the Metricated types. A typical MF TEE antenna may look like 200 pF 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.

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.

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 which 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.

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.

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? 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. IP's 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!

There is an awful lot going on in the world of wireless, many thousands of transmissions all seeking our attention, some weak, some very strong, all likely to interfere with each other. By using an antenna that favours the frequencies we want to hear, it will go some way to discard those of less interest and reduce the chances of IPs - intermodulation products - spoiling the fun.