AM outdoor aerial - feed into different rooms?

[stevehertz]

For better reception, I am going to erect an outdoor AM aerial between two chimney stacks about 30m apart (separate buildings). However, inside the house, I have say three rooms with sets that I will want to receive feeds from the aerial. How do I do this? Can I solder (and insulate) extra wires on to the 'downlead' at suitable points so as to be able to take the signal into these separate rooms? Can you 'parallel up' sets' input signals in this way? what is the effect? Does it screw impedances up, effectively lowering the signal to each set? I mean I've never heard of distribution amps for AM - are they available? would I need one?

[Radio Wrangler]

Usually the antenna input goes into a tuned circuit. Depending on how the coupling is done, some sets will act to shunt signals away from the frequency to which they are tuned. If the coupling is light then they will each affect the alignment of the others, though this is a small effect.

The worst thing is that the local oscillator of a set tuned low in the band will appear on sets tuned higher in the band because the band is wider than the usual IF frequencies.

Ideally a splitter with buffer amplifiers is needed, and the buffers need to have good attenuation in the reverse direction.

Just connecting everything together will work, but you'll run into limitations from time to time.

David

[Phil G4SPZ]

I have found that loose capacitive coupling - simply twisting an insulated fly-lead to each set around the main incoming downlead over 4-6 inches - provides sufficient signal strength and minimises loading effects.

[turretslug]

How did they do this sort of thing in the case of , let's say, an appreciable number of ship's cabin receivers where the well-heeled would have expected good reception of the World Service, yet compromise would have had to be made as regards aerial size/number and that also had to play second-fiddle to the ship's main RX/TX requirements?

[stevehertz]

Originally Posted by Phil G4SPZ

I have found that loose capacitive coupling - simply twisting an insulated fly-lead to each set around the main incoming downlead over 4-6 inches - provides sufficient signal strength and minimises loading effects.

Makes sense to me Phil. Maybe I'm mithering about nothing, but how do you stop that 'wrap' slipping down? Bear in mind it'll be outside in the rain and stuff. What about tightening a small tie wrap onto the downlead just below the wrap? I guess that'd do it. Answered my own question!

I use a Wellbrook loop and simply connect all radios in parallel using coax, OK I know it is a mismatch, but at LW/MW it doesn't matter.

http://www.wellbrook.uk.com/Loop_Antennas/LFL1010-1 a bit expensive, works a treat. Or you could just get the amplifier bit... http://www.wellbrook.uk.com/Components/LoopAmplifier and do the rest yourself.

[stevehertz]

That looks a nice unit..

A couple of questions, do you use a rotator with it? Otherwise - to an extent - you're actually limiting reception from stations that are coming in from the sides.
There looks like there are bare connection on the one in the manufacturer's blurb? on yours, not - that right?
So what type of lead comes out of the unit? You say you're using coax? presumably inner to set aerial, screen to set earth connection?
How does it receive power - what do I need to provide it with? mains?
It says it has to be mounted five metres from a building? I would have thought mounting it up high - as on a roof - would have made its job easier/better? But if you do that, it ain't five metres from roof!

Thanks.

[Dave757]

Hi,

If the 30m wire has spurs to each of the rooms where the radios are located,
why not terminate each in a small box with banana sockets for aerial and
earth. Each radio could have a flying lead from the aerial/earth connections,
and you would only then plug them in whilst in use. You could just use a switch
instead if preferred

Kind regards

Dave

[stevehertz]

Good idea Dave, then they'd be disconnected when not in use and hence have no effect on impedance and stuff.

[G8HQP Dave]

A cathode follower might do the trick. Antenna to grid. Each radio connects to the cathode via a capacitor - say 100pF each?

[Ian - G4JQT]

Normally, when distributing signals to multiple radios (ships, receiving stations, etc.) all the radios have a standard (usually 50 ohm) unbalanced RF input and all are fed via screened coaxial cable from a distribution amplifier from the aerial.

Since most domestic-quality vintage sets don't have a coaxial RF input for LW/MW/SW, feeding a number from just one antenna is a problem, not least the interactions between sets as already described.

You could try feeding them with coax connected between aerial and earth sockets from a distribution amp, but the radios' input impedances are all varying from set to set and frequency to frequency (even when switched off) - this isn't how they were designed to be used. Nevertheless it may work better than it should!

If you don't feed each radio with coax, then one problem I can see is that your outside aerial will be in the clear and hopefully not picking up too much noise. But then you start trailing the unscreened aerial wires indoors from room to room where it will almost inevitably pick up noise from your and your neighbours' devices.

I suspect your best bet is to just connect the aerial to the radio you want to use, and keep the indoor section of aerial as short as possible, but that defeats the object of your original question!

I've installed Wellbrook loops in 'professional' listening sites and they are very good, though quite expensive. If you Google 'active loop antenna amplifier' there are various designs to build yourself fairly cheaply. Loops are only directional with respect to ground-wave signals, but unfortunately that's most LW/MW stations you're likely to listen to.

The problem with aerial amps (and why they are so expensive) regardless of what type of aerial is feeding them, is that they have to be electrically very quiet (particularly if feed with only a small loop or whip), and have an extremely wide dynamic range, otherwise they overload and generate all types of noise and phantom signals.

Ian

[OscarFoxtrot]

Originally Posted by turretslug

How did they do this sort of thing in the case of , let's say, an appreciable number of ship's cabin receivers where the well-heeled would have expected good reception of the World Service

Almost always by using one radio receiver per received station, and distributing at audio frequency using multipair cables and 100V line speakers (as was done until recently for 'hospital radio' type applications). For ships, hotels etc there might also be a feed from the live band in the ballroom or an in-house programme from gramophone records. A public address book I have from the 1950s suggests distributing clock chimes at low volume too.

[Synchrodyne]

Originally Posted by turretslug

How did they do this sort of thing in the case of , let's say, an appreciable number of ship's cabin receivers where the well-heeled would have expected good reception of the World Service

I’d guess that shipboard practice would have followed professional practice, with coaxial cable LF/MF/HF distribution using active multicouplers. I dare say that if one could find a Marconi Marine catalogue, it would include distribution equipment as well as receivers and transmitters, etc. In the cabins, there may well have been a step-up transformer to better match typical cabin receiver aerial inputs. Marconi sold rebranded Eddystone 670-series receivers as cabin receivers, and I don’t think that these had coaxial aerial inputs, so if nothing else, it would have needed to cater for its own product line. Such RF distribution would have been in addition to AF PA and entertainment distribution in passenger vessels.

Active multicouplers, as primarily professional devices, probably do have much overlap with domestic installations, but for example some of those shown here might be suitable: http://www.stridsberg.com/prod01.htm. Also shown there, and here: http://www.rf-systems.nl/Shortform-catalog-complete.pdf are passive multicouplers/splitters.

So perhaps a possible, albeit elaborate solution using available components would be: longwire aerial connected to coax via a “longwire balun” (LWB); coax feed split as required using passive splitters; LWB in reverse at each outlet to feed high impedance domestic receiver aerial inputs.

In the past, with domestic antistatic aerial systems, I think that feeding multiple receivers was done simply by paralleling at the twin-screened feeder low-impedance level, with each outlet having its own stepup transformer as something of a barrier to interaction. Perhaps this could be done with the suggested coaxial system, thereby dispensing with the passive splitters.

A good, concise treatment of LF/MF/HF receiving aerials (that goes well beyond the customary “as long as possible” approach) is provided in Radio and Television Engineers’ Reference Book. See p.21-2ff in both the 3rd and 4th editions. In particular it is noted that at MF, vertical height is a key parameter, both in terms of signal capture and resistance to fading. That would suggest for example that with an “L” aerial, one would want a longish downlead before connecting to an LWB, at least if MF reception was a concern.

Cheers,

A couple of questions, do you use a rotator with it?

Yes, only for 'serious' reception, it's a home made job that gets serial data from the PC with a knob attached (via USB) to said PC.

So what type of lead comes out of the unit? You say you're using coax?

Take a look at Wellbrooks site and download the instruction leaflet. DC is fed down to coax via a choke. All other questions are also answered in the leaflet above.

One good thing about running coax round for distribution is that 'house noise' is nigh on eliminated.

[stevehertz]

Many thanks, will check it out, much appreciated.

[G6Tanuki]

Medium/short-wave "Distribution amplifiers" used in monitpring stations to feed multiple receivers often used either a single big valve run in class-A as a broadband amplifier/cathode-follower [a high standing-current means low cross-modulation when fed with a wide range of frequencies - we're talking valves more-usually found in transmitter PA-stages here - I recall one that had a 2E26] or sometimes a "distributed" amplifier using several receiving-type RF valves (6AK5) in parallel with a L-C matching network between each valve. Similar things were used as wideband IF-amps in Radar sets and in early cable-TV networks.

For domestic use I'd suggest a broadband transistor amplifier using something like a 2N5109 or 2N3866 passing a few tens of milliamps, then passive/resistive splitters on the output to feed the various rooms.

[stevehertz]

I'm sure you're right, but I need to either buy something or make it simply (spit and string), not design it! Thanks though.

[Ian - G4JQT]

At BBC Monitoring when I joined in 1079 we used two types of valve RF distribution amps. Neither of our types as I recall had unusually large valves.

I remember one of the designs being more problematic than the other, the valves being in some balanced bridge arrangement and needing frequent rebalancing to maintain the dynamic range. They had small valves with a blue tinge - EF91s?

Anyway, drifting off topic for the original post.

Ian

My loop does the job as a distribution amplifier for strong (broadcast) signals, I do have a 'proper' 3dB splitter to feed my serious radio (Perseus SDR) before anything goes to the vintage sets. Still loads of signal for them. I does take a couple a watts to merrily amplify a few microwatts of signal to keep intermodulation (very) low.

[Skywave]

Originally Posted by stevehertz

I'm sure you're right, but I need to either buy something or make it simply (spit and string), not design it! Thanks though.

I understand your dilemma. Your need is unlikely to be met by a commercially-manufactured item that was specifically made for your need: there simply isn't sufficient demand. OTOH, making something yourself on the basis of 'cheap-and-cheerful' might solve the problem - if you're prepared to accept the inevitable compromises that such an approach will bring. Sometimes, in our niche hobby, a need can only be met by a bespoke design - and in this case, that is the attitude I would take. The last post by G6Tanuki (last paragraph) is the line of investigation that I would follow.

Al.