Multiple radios running off one aerial (MW LW)
 

I've just witnessed an interesting thing.
I have quite a few radios situated down the end of the garden in various out-buildings away from the house. I enjoy "most of the time" very good reception and not much interference from quite a modest aerial of about 20 feet strung between two well established trees.
As I enjoy swapping to different radios now and again I have a "daisy chain" set up so I don't have to have a separate aerial for each radio or keep unplugging and plugging in again as I change sets.
This works really well until the other day when I switched on a radio and got really bad interference (LW).
I'd been working on another set earlier and had the chassis/speaker on the bench and was testing/listening to it with a makeshift aerial attached to my "daisy chain". This set at the time wasn't even plugged in to the mains, when I pulled out the aerial from the said radio the other radio I was listening too improved dramatically with no interference.
I'm just wondering what's going on here.

Re: Multiple radios running off one aerial (MW LW)
 

You need a proper distribution system using wideband hybrid splitters like used to feed 100s to 1000s of satellite receivers, cable receivers, Terrestrial TV and FM radio from one head end in a street or tower block.

Re: Multiple radios running off one aerial (MW LW)
 

My radios a just pre-war valve sets, it's not a problem I'm, just curious to know why it should behave like this.

Re: Multiple radios running off one aerial (MW LW)
 

The aerial input of the radio under test is detuning in some way the working receiver. Think of it as parallel tuned circuits interaction with each other. Appears you have got away with it in the past but not this time.

Has Mike states a proper distribution system would remove most of that interaction but probably not worth the effort for your system now you know it can cause problems in certain circumstances.

Re: Multiple radios running off one aerial (MW LW)
 

The attached Belling-Lee advertisement shows one way in which the single aerial, multiple receiver issue was addressed at the “domestic” level back in the day. Each receiver had its own step-up transformer, and these were connected in parallel across the screened feeder. Presumably the transformers provided sufficient inter-receiver isolation.

Attachment 187677Attachment 187678

Here is a description of a Marconi shipboard distribution system:

Attachment 187679

This could be described as a professional system designed to feed cabin receivers with domestic-type high impedance aerial inputs. Each receiver was fed from the 75-ohm coaxial distribution line via a 1 k resistor. One assumes that this was sufficient to provide adequate inter-receiver isolation, and to ensure that the receiver aerial inputs were looking back into a sufficiently high impedance that there was no significant detuning of their input tuned circuits, something that might well happen if they were connected directly to a low impedance coaxial cable. (That kind of arrangement would not be required with professional receivers fitted with 75 ohm (or 50 ohm) coaxial aerial inputs.)

Domestic quality passive (hybrid) aerial splitters intended to work at LF/MF/HF have been, perhaps still are available, for example:

Attachment 187680

These were designed to work with 50-75 ohm coaxial cables, so would a require step-down transformer between the long-wire aerial and the splitter, and then step-up transformers to provide a suitable match to high impedance receiver inputs without detuning problems. I imagine that so-called longwire baluns, used normally at the aerial end and reversed at the receiver end, would do the transformer jobs adequately. So one could probably cobble together a passive system for a small number of receivers using available non-professional components. Perhaps instead of step-up transformers at each receiver, 1 k series resistors, as in the Marconi shipboard system, might work, but with a passive system there might not be enough signal.


Cheers,

Re: Multiple radios running off one aerial (MW LW)
 

Thanks for that excellent post, I knew Belling -Lee produced equipment to do the job but couldn’t find it, I had forgotten about the domestic radios in ships, did KB advertise their radios were used on the two Queens liners?

Re: Multiple radios running off one aerial (MW LW)
 

The KB Rejectostat system had the capability of being used to supply multiple receivers.
Attachment 187684
In the diagram above KB advocate using something like 3 pin mains sockets to act as aerial sockets to plug in a single radio into multiple rooms, or to plug in multiple radios onto the aerial distribution system. A matching 3 pin plug would be inserted into each socket on the distribution system and that would be connected to the radio rejectostat unit with screened cable then into the aerial and earth sockets of the radio

KB had the contract for the supply of radio apparatus to the 3 liners Queen Mary, Queen Elizabeth, and the Caronia.

I cant be sure but they may have used the Rejectostat system for the aerial distribution. Or they may have used a system of "piped" radio stations on a 100v line speaker distribution system with a station selection switch, I am not sure.

Some of the advertising posters can be seen here. http://www.kbmuseum.org.uk/kb_posters.htm
Together with a copy of the souvenir brochure for the Queen Mary. Which I bought in the hope it might give an in-site to the radio systems, but it didn't.

Mike

Re: Multiple radios running off one aerial (MW LW)
 

Those KB outlets appear to have no matching unit inside them, was there another matching transformer required for the radio, or a special input circuit for radios using the rejectostat?

Re: Multiple radios running off one aerial (MW LW)
 

Hi Frank
The radio rejectostat transformers are not shown on that diagram, but as I mentioned the 3 pin plug would connect to the radio rejectostat unit with screened cable then into the aerial and earth sockets of the radio. On some radios there would be a 3rd socket to connect the screen of the receiver rejectostat to.
There is a link to the full details of the rejectostat system in post #41 of this recent thread on aerials https://www.vintage-radio.net/forum/...d.php?t=157926

Mike

Re: Multiple radios running off one aerial (MW LW)
 

Thanks for the confirmation Mike, I remember the Queens adverts for KB, very grand looking ads.

Re: Multiple radios running off one aerial (MW LW)
 

That’s an interesting question. Unfortunately the WW article on the Queen Mary’s radio system did not mention broadcast receiving arrangements.

Attachment 187721

Possibly in 1936 only a minority of cabins (perhaps just the staterooms and suites) might have been equipped with aerial outlets for personal cabin radio receivers. In that case something like a land-based radio relay system (65-volt line?) might have been used for the majority. The attached WW 1950 February article on shipboard communal aerial systems suggests that it was a fairly new idea at the time. The later WW article on the Marconi systems (attached to post #5) indicated that the widespread use of cabin receivers was essentially a post-WWII phenomenon. (For example the Eddystone 670 marine cabin receiver was one of its early post-WWII products.)

Attachment 187722

The Caronia was evidently the first vessel to be equipped with an HF SSB radiotelephone system for passenger telephone calls, although by 1954 the practice was said to be widespread. (The Caronia installation was 20-something years before HF SSB was adopted for “official” marine radiotelephony.)


Attachment 187723
Attachment 187724
Attachment 187725



Cheers,

Re: Multiple radios running off one aerial (MW LW)
 

Hi
thanks for those links, they are most interesting.

Mike

Re: Multiple radios running off one aerial (MW LW)
 

This short item on the Antiference Exstat aerial systems mentions the multiple receiver case:

Attachment 187960

Specifically: “Special receiver transformers are available to provide for multi-point reception where a number of receivers are required to operate from one aerial”.

No details were given, but it could have been as simple as equipping the receiver transformers with both inputs and outputs for the screened feeder to enable easy daisy-chaining.

Another professional system, this time from EMI and intended for use in apartment blocks, is described in this article:

Attachment 187961

Whilst the main topic addressed was Band III television, it may be noted that the distribution system included “all-wave radio”, presumably 150 kHz to 26 MHz or so, as well as VHF TV and FM. At the distribution boxes the signals were separated by low-pass (all-wave radio), bandpass (FM) and high-pass (VHF TV) filters. Presumably these were arranged to present an appropriate source impedance in each case, 75 ohms for FM and TV and a high impedance for all-wave radio, to match typical domestic receivers.


Cheers,

Re: Multiple radios running off one aerial (MW LW)
 

Another domestic example of an AM aerial system with multi-receiver capability was the RCA Magic Wave Antenna of c.1937. This appears to have been an anti-noise system, with matching transformers at the aerial and receiver ends and screened twin feeder in between. For use with multiple receivers a distribution transformer was available, one in, four out, from which feeders ran to individual receiver matching transformers.


Cheers,

Re: Multiple radios running off one aerial (MW LW)
 

Here is another example of a professional system for use in apartment blocks, in this case from Philips.

Attachment 189018

It is evident that at least in the distant past, quite a bit of engineering effort was put into the issue of feeding multiple AM receivers from a single aerial. More complex professional systems, such as those for apartment blocks, used distribution amplifiers, whilst simple systems for a small number of domestic receivers used passive methods. It is evident that in the latter case, the “splitting” was done at low impedance, say around 75 ohms nominal, with step-up transformers used to match the high impedance of typical domestic receiver aerial inputs, these transformers also providing the required inter-receiver isolation. The transformers would also ensure that the receiver aerial inputs were looking back into impedances that were somewhere near to what was expected and so did not suffer from undue detuning and/or desensitization effects.

Perhaps splitting could be done at high impedance from a longwire aerial by including a resistor, say 1k, in each receiver feed in order to provide some inter-receiver isolation. There would be some signal loss, but perhaps not unacceptably so if the aerial is “big” and provides a relatively high signal strength, at least on local transmitters, to start with. It might be worth a try.


Cheers,

Re: Multiple radios running off one aerial (MW LW)
 

Here is a more detailed description of the RCA aerial system mentioned in post #14:

Attachment 192187

The distribution (splitting) transformer details are provided on the final page:

Attachment 192188



Cheers,

Re: Multiple radios running off one aerial (MW LW)
 

This might form the basis of a multi AM aerial system but I don't think they market them anymore. John.

Re: Multiple radios running off one aerial (MW LW)
 

Five years ago I asked a very similar question, and four pages of responses resulted:

https://www.vintage-radio.net/forum/...d.php?t=108157