Connecting several radios to the one external aerial.

[brymac]

I finally got fed up with restoring sets which then were limited by a poor quality external aerial (usually a couple of feet of wire) and strung a wire between two trees and ran it into the house, where I have, amongst others, a 1950s Pye radiogram which has now been transformed by a decent aerial connection!

But - if I link that external aerial to another set in another part of the house, the performance of both sets diminishes. How do I effectively provide several connections to the same external aerial (as you do with a TV roof aerial) without degrading the strength of the incoming signal, so that we can listen to two or more valve radios in different parts of the house?

Bryan

[Peter16]

To be honest I don't see how you can get the same results when several radios are connected to the same aerial unless you have a distribution amplifier as you would have for TVs

I think you need more wires/trees or fewer radios or just connect them to the aerial one at a time.

[Ian - G4JQT]

For signal reception one needs the cleanest possible path between aerial and receiver. Even a slightly corroded joint (which will act like a bit like a diode) will introduce intermediation products - sounding like noise and a myriad of extra signals.

There are various ways of feeding one aerial to more than one receiver:

For domestic sets with just a wire aerial input socket, a star configuration of resistors is probably the simplest way, (but a well designed FET amp will also work). Ideally, each resistor should be the same as the aerial input impedances, but this is probably unknown, will vary from set to set and vary with frequency. Connect the aerial to the centre and each resistor by as short a wire as possible to each set’s aerial input. Try, say, 470 ohm resistors. Higher values will reduce interaction but also reduce signals. It also will depend on how much signal you have to start with, but the AGC should hide that a bit.

There will be some loss, and it’s possible that the signal strength in one receiver may vary slightly with the tuning of another as its RF input impedance varies.

Although impractical for home use, the real option for professional radios is to have a matching (usually 50 ohms) system with an extremely linear distribution amp with enough gain to overcome losses. But this depends on each receiver having the same (and a reasonably constant) input impedance across its receiving range.

Good luck.

Ian

[YC-156]

Originally Posted by paulsherwin

You could build a 1 transistor untuned preamp and feed the signal to the various radios via small caps from the output. Most general purpose silicon transistors should work, so you should have all the bits in the junk box.

Might be worth trying since it is so easy to do, but I wouldn't be terribly surprised if it didn't work all that well when assembled from random components.

First both the end of the longwire and the input to the radios are probably all either high imperance or highly reactive. There might thus be a severe attenuation of the signal at both input and output of the amplifier due to a mismatch with the low impedances of a class A transistor amp. Perhaps even enough to offset the amplification available from the preamp in the first place.

Secondly - and perhaps worse - is that if you actually manage to make a wideband kind of match between the antenna and the preamp, you are essentially feeding a fair fraction of all signals on MW in a given location to the preamp - at once. This will probably cause ridiculous levels of intermodulation distortion in your average small signal transistor, which will then drown out the stations you want to listen to.

The 'official' way of making such a wideband beast is to use something like a 2N3866 UHF power transistor in a heavy feedback circuit, thus making a 'strong' yet low noise preamplifier with predictable input/output impedances. Those can take in a fair fraction of the HF spectrum at once without causing undue disturbances themselves.

Would be interesting to hear how well a BC547B works though.

Frank.

[paulsherwin]

Originally Posted by YC-156

Might be worth trying [a 1 transistor untuned amp] since it is so easy to do, but I wouldn't be terribly surprised if it didn't work all that well when assembled from random components.

[]

Would be interesting to hear how well a BC547B works though.

I have built such things in the past and they have 'worked' but they weren't connected to a huge longwire antenna. The impedence mismatch was certainly compensated for by the transistor gain. I used one built into a tin box in my car boot to amplify the boot mounted aerial.

As you say, you can build one in minutes on a bit of tagstrip so it's easy enough to try out. We are only talking about AM broadcast reception here. There are much better amplified solutions but they cost time and money

Paul

[YC-156]

Paul, did you connect more than one radio to the output of the preamp at once?

I just thought of something, which can more adequately explain Brymac's problems. It didn't quite ring true to me that paralleling a few radios should cause undue attenuation.

But.

The antenna input of most valve radios consists of some form of narrow bandwidth parallel tuned tank circuit. When the tank is tuned to the receiving frequency, the antenna input impedance is some particular value.

However at the same time the tank circuit will perform like a more or less direct short circuit across the antenna terminals when away from resonance. That is of course its intended function!

So the input impedance of a valve set only has a reasonable value very close to the tuned frequency. Thus the 1/N distribution only works if all sets are tuned to exactly the same frequency at all times, which is a bit impractical.

I can see no easy way around that problem, except excessive signal levels from the preamp, or providing each set with its own dedicated preamp.


Frank.

[Station X]

The easiest method is to put up more than one external aerial. I have four because I use radios in several rooms and in my outside workshop. The fact they may be quite close together doesn't seem to matter for reception purposes.

The other easy way is to use a plugboard or switchboard to switch the aerial between receivers.

I've never tried the distribution amp approach. The mere fact it needs a battery or PSU puts me off.

[YC-156]

Originally Posted by paulsherwin

I have built such things in the past and they have 'worked' but they weren't connected to a huge longwire antenna.

Using a large external antenna is usually when the manure hits the ventilation device when it comes to preamps and receivers.

The combined signals from an effective longwire covering the lower shortwave segment can in some circumstances exceed 1V or more at the antenna terminals. That is a fairly serious torture test of a receiver's strong signal performance if ever there was one.

Frank.

[Biggles]

I would be against using a preamp of any description for the reasons already mentioned above. I would experiment with small value capacitors to share the signal, and maybe a series resistive splitter network i.e. branching off the main aerial "bus" with a series capacitor/resistor of say 100pF/100ohms in each lead fed from the bus to each receiver. Straight off the top of my head of course, and it will need a bit of playing about with. Why use a transistor preamp when you could use a valve...better overload/intercept point performance!

Biggles

[Ray Cooper]

No, I've never had to do anything like this, before - so the following are just a few wild suggestions, launched from the clouds of my imagination merely to be shot down pronto...

Biggles:

Why use a transistor preamp when you could use a valve...better overload/intercept point performance!

Why not, indeed. And it probably doesn't have to have much in the way of gain - in fact, it might be better to arrange it as a cathode follower (low output Z) and feed the output signal around on co-ax, to minimise unwanted radiation. If you don't do this, and have a pre-amp with gain, there's the prospect of the entire setup becoming unstable if your aerial picks up stuff radiated from the pre-amp output and interconnecting cables.

The reason that receivers are killing each other here is possibly because each input circuit on each receiver is tuned to a different frequency. Any RX tuned to a frequency which is not the same as the one that you're trying to listen to, therefore, is placing a low-value reactance across the whole aerial system, which will shunt away most of your wanted signal. So:-

a) tune all rxs to the same frequency, and see what happens: or

b) (and this one is a little wild) 'float' all of your input circuits above ground, and connect them in series, not in parallel. The unwanted reactances across the inputs of the other sets will then be helping you rather than hindering you.

No, I've never even attempted anything like b), so I've no idea of what might happen... However, a little experimentation like this adds spice to life, and is unlikely to be lethal.

[johnlees]

I've got a long wire aerial parallel with the back of the house and it comes in at BOTH ends, one end to a radio in one room and the other end to another radio in another room. Having read this thread I am just wondering if I have a dangerous or dodgy set-up, though I have had no problem so far.

[Biggles]

You could always connect an insulator mid way between the antenna suspension points, giving two isolated antennas. I have seen this method used where it was easier to string one wire between two rows of terraced houses, across the back lane, with an insulated section in the middle to electrically isolate the two sets, which used the two ends as feed ins.

Biggles.

[G8DLG]

As you have a good signal from outside aerial,the best idea is to use a step down transformer to 50 ohms,use 50 ohm coaxial,you will have a signal at 50 ohms,and since most sets are 400 ohms input you can connect a few sets until the load is down to 50 ohms,or you could reclaim the aerial impedance by a step up transformer at the receiver.I have used this set up for several years and gives good results,interference is down because of the coaxial feeder,you also get some degree of static protection.The transformers can be made easliy from 1" toroids,these were sold as "long wire baluns" often at stupid prices,but you can make your own in ten minuites and give a fairly flat response from 100kcs up to about 20Mcs

[paulsherwin]

Originally Posted by mikej56

Please correct me if I've got the 'wrong end of the stick' here! should we not look at this poblem as if we were 'listeners' in the '50s? How did they solve it?

In the 50s most people were relatively poor and radios were relatively expensive. Typically they only owned one, which would be in the living room and attached to a longwire aerial. If they were rich enough to afford another radio for the kitchen or bedroom they demanded, and got, an internal aerial - first a frame, then later a ferrite rod. The vast majority of small wooden and bakelite sets in the 50s have internal aerials of some sort.

Manufacturers carried on making 'main sets' without internal aerials for quite a long time because everybody had a suitable longwire aerial.

Paul

[wireful3]

Hello again Bryan,

I just retried my test with two sets. The one I used as a standard had a magic eye and was tuned to a modest signal on MW. The results show that coupled tuned circuits are complicated.
I had different results with different "second " sets. One produced a general but slight loss in signal with both joined to the aerial. Switching to LW gave a slightly smaller reduction. With a different second set though, one tuning position gave a big increase in the signal when it was tuned very close to the first set. About the same as the first set connected to the aerial by itself.

It kept me entertained for half an hour but does not add much to what has been said. It confirms that the line you are taking using a changeover switch will probably cover your needs simply and cheaply.

Incidentally in the 30s and 40s there were changeover knife switches used for lightning protection. I wonder if these were ever used to change the aerial from one set to another.

[G8DLG]

Wireful3 john,
Depending on band and tuning point on the set in use,(or even if switched off)signal on the other sets will be impaired.
If you use a balun,or to be more correct a step down transformer,so you could have a 50 ohm line,this problem does not happen,since any one set will not heavily load the aerial.I have a 50ohm feed to 5 sets,and another aerial (a loft loop) which feeds another 5 via a balun and amplifier.The balun changes impedance from 400 unbalanced to 50 bal which feeds a line powered amp.
This is rather like a distribution network used on the old piped televion,where each house was tapped into the line with a stand off resistor.

[brymac]

What have I started!!!!! Well I fixed it - it was a very technical solution; I've simply run two separate long wires into the house!! But you're all clearly enjoying the challenge of coming up with amazing solutions, so don't let me stop you.. Many thanks for all your help with this one!! Bryan

[johnlees]

And I've disconnected my system of using both end of the aerial as I thought there might be unseen (e.g. live chassis or different potential chassis)problems. I've just put an insulator in the middle of the long wire so the two ends are isolated. It's made no apparent difference to reception on either set.

[Skywave]

Just had a thought - picking up on the idea of using a valve-based pre-amp.

Following on from the notion that a suitably-designed valve per-amp will (generally) have better cross-modulation characteristics that a solid-state design, why not go the whole hog and use a small transmitting valve? These are certainly designed to handle big signals. As for S/N ratio aspects, as far as the freq. of interest goes, most of the S/N is determined by interference, static and all the other crud sloshing around anyway (certainly up to about 20 Mhz or thereabouts - possibly more) - and assuming your aerial is "in the clear", I wouldn't expect a problem on this consideration.

Oh, and if it's a LW you're using, don't forget to pay attention to getting a really good earth - the other "half" of such an aerial.

Al / Skywave.

[ppppenguin]

In another thread a fellow forum member kindly referred to the BBC Engineering Monographs. A wonderful resource indeed. Here's one that addresses aerial distribution. Perhaps a bit overdone for our requirements but an authoritative treatment of the subject with some practical circuits.

http://www.bbc.co.uk/rd/pubs/archive...nograph_53.pdf