Listener's Guide - 1999 - The Listeners Guide

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so bad as the MF TEE, but not much better! Yet, without thinking, you do it all the timeand there is no recognition of the problem I have revealed, because nobody wants evento admit that there is a problem.When an antenna is "short" (less than 90 electrical degrees in length), as is normal inCoast Station use at MF, or in many Transportable and other applications at HF, theantenna "looks" like a small number of Ohms in series with a capacitance. This obtains atany frequency at which it is shorter than an odd multiple of 90 degrees and longer thanan even multiple of quarter-wave resonances.At all other frequencies (except at odd multiples of Quarter Wave Resonance, where thenatural termination of the antenna is, almost, purely relative) the appearance of theantenna at termination is of a capacitance in series with an inductance. Now we mustgive a little credit to those clever young engineers who design modern receivers but don'tunderstand antennas. They have pushed the thresholds of sensitivity down to levelsunheard-of in the good old days, albeit in 50 ohm to match their beloved co-ax. Thanksto 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 decidedthat sensitivity below 2MHz is of little consequence because the amount of noise downthere will defeat the signals anyway. In part they are right, but if a signal can be weanedfrom 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 beused, simply because the physical size of a resonant antenna at these frequenciesprecludes its use except in very special circumstances and at great expense, both for thephysical structure and in terms of real-estate.Is there a practical solution? Within limitations, yes, there is. In order to reduce thecapacitive divider problem the co-ax must be terminated in some sort of load which iswithin reasonable shooting-distance of its nominal impedance. We are not worried aboutVSWR or power-handling in this case, the strongest signal we are going to get will be inthe 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-throughinsulator on one end and a Type "N" Connector on the other. Inside, there is nothingexcept a small ferrite ring, sixteen turns primary, four turns secondary to get theGENERALLY HIGH impedances seen at a long-wire down to the GENERALLY LOWimpedances needed by a modern receiver and it only costs the customer a few quid. Or ahell of a lot more if that customer is depending on his dealer to cover his basic lack ofknowledge or interest in antennas...Allowing for the credit we gave the youngsters for pushing down sensitivity thresholds ofreceivers, we can neglect a true impedance match when we terminate the antenna. Nowthis, as far as the MATCH is concerned, consists of the R component of the antenna plusthe Loss Resistance of the Antenna System including its associated ground system, andyou can measure as many as you like, you will find the R component varies between2000 and 20 ohm. Put bit of capacity and some inductance in series with that and youwill find that the impedance works out to be well within shooting distance of 400 ohm. Ifwe 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) wecan tolerate say, 100 ohm at the top and 10 ohm at the bottom of our scale; a chunk ofsuitable ferrite wound with a 4:1 ratio, the low winding to the co-ax, high to theantenna 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 Bifiliarwound with an additional 20 turns on the Antenna side (40 on the antenna, 20 onthe co-ax) seems to work pretty well anywhere. Better still, you can use a compensatingRC network on the antenna side. Either solution is certainly better than leaving the co-axopen 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 directlightning strike has a leakage path to ground; vulnerable solid-state front-ends andmulticouplers of whatever gain some free protection (which we never really needed withvalves and heavyweight tuned copper coils in the front-end). Now we have donesomething about it, why don't you! It helps our customers to receive signals and to gettheir money's worth!Better now? The "EMP-immune glass bottle" is a radio valve. An ECC189 in cascade, anEF183, EF80, EF50 or 6K7G, depending on your generation, provided the amplification inthe 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 forreception in the professional sector. IP's are intermodulation products, the direconsequences of strong signals on different channels mixing in the early stages of a radioto spoil our enjoyment with extra noise and reception of stations that do not exist. All Iam 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 andconnectors.They WILL make a difference!There is an awful lot going on in the world of wireless, many thousands of transmissionsall seeking our attention, some weak, some very strong, all likely to interfere with eachother. By using an antenna that favours the frequencies we want to hear, it will go someway to discard those of less interest and reduce the chances of IPs - intermodulationproducts - spoiling the fun.So where do all the rejected stations go?Down the GROUND wire into God's good earth. In many instances, this is via the earthlead in the mains cable and the plugs third pin.THIS LEAD IS FOR ELECTRICAL SAFETY AND SHOULD NEVER BE REMOVED.The problem is this path is shared by every other electrical appliance in the house, someof which will be fitted with suppressors which will now be using the same path to earthelectrical noise. Nobody has ever taken the time to have a word with this interference,suggesting it should only go to earth without seeing your radios ground lead as a wayinto the set where it now finds itself in series with the aerial signal and so a part of it.Hence, more noise...The answer is to add extra earthing as per the suggestions in the instruction manual.This will often shorten the earth path and make it more effective. After a wet Saturdayafternoon doing manic Dracula impressions with large earthing stakes, you may feel yourattempts at getting a good earth are better than the bloke who wired your house and astrong desire to cut the earth lead in the plug.NEVER DO THIS. THERE IS A LEGAL REQUIREMENT FOR SAFETY EARTHING.Some have found using a piece of coax left over from the antenna installation as anearth lead has some advantages.A Co-Axial EarthThe wire core and the outer braid are connected together at the earth spike outside. Inthe radio room, only the inner core is connected to the GROUND point on the radio. Aswe have come to the sorry conclusion our earth lead is actually a part of our aerial, anyinterference picked up in the earth path is conducted to ground by the outer braid,leaving the centre core path "in the clear".If we have problems with mains-borne interference, one answer is to allow our signalearth and our safety earth to ground in separate paths. If we are to maintain theintegrity of our safety earth we can build in a high impedance "barrier" at signalfrequencies in this path and take our station earth to ground from the radio side. This isnothing new. They were using isolating transformers as an end to common-mode noiseback in the Thirties. These devices are now back in the accessory market. Forgive thewry smile of the old timer - the more things change to improve, perhaps, the more theystay the same.Actively Seeking Signals...Over in Smug Corner sits the owner of a portable receiver. He knows - and we have toadmit - that the performance of these sets is on the up. Flat-dwellers can forget all thathas gone before and not bother about the politics of outdoor antennas and buy aportable happy in the knowledge that it will perform very well. They can also see if this isthe hobby for them or check out local interference by getting one of the many entry-levelsets coming out of China and still have FM stereo to fall back on if the bands are quiet.Don't you just hate it when that happens!Yet they still enjoy their listening without all the discussion and installation of any specialaerial array.How do they do it? The telescopic rod antenna on the portable is all they are using...
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Page 3 and 4: hobby, we hope this Guide will give
Page 5: Wired?Coax-fed antennas have become
Page 9 and 10: NOTES TO THE FREQUENCY LISTSThe fre
Page 11 and 12: 2300 2495 Tropical Band2495 2850 Fi
Page 13 and 14: The writer remembers a Practical Wi
Page 15 and 16: This will cause interference in all
Page 17 and 18: Time0133GMT0233GMT0303GMT0533GMT063
Page 19 and 20: station he resolved, for the new ge
Page 21 and 22: system, but a playout of real voice
Page 23 and 24: 9830KHz: Croatian Radio. News heard
Page 25 and 26: the carrier, if only it had one, th
Page 27 and 28: 25) Just because you know the dicti
Page 29 and 30: Those who have a noise blanker - th
Page 31 and 32: So, a "five and nine" report is an

Listener's Guide - 1999 - The Listeners Guide

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