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 Post subject: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 2:54 am 
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Location: Plymouth, MI
Can two or more radios share the same ground and antenna? I'm pretty sure the answer is no, but wanted to confirm.


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 3:13 am 
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You can connect any number of TRF sets to a single antenna.
There might be some loading or other issue, but it would be minor

Superhets pose a different problem since they have oscillators that can radiate out through the antenna connection. So the oscillator of one might cause harmful interference to another on the same wire.

- Leigh

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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 3:47 am 
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Same for ground Leigh?


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 4:01 am 
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Yup.

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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 4:09 am 
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Thanks Leigh!

Dan


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 5:48 am 
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What if the Superhets are turned off, would that then no longer pose a problem to a radio on the same antenna that is on?


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 9:26 am 
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There's an easy solution, and that's to use a switching circuit. My random length long wire goes to an A/B switch in my radio room. One side connects to a Zenith console, the other goes to my den, to another switch. One side of that goes to a Philco farm set, the other to a 3 tube regen I built, a Motorola farm set, and another Zenith console. I normally only have one of those on at a time.

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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 12:00 pm 
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I have a long wire between the house and shed at the end of the garden - about 100 foot. There's a downlead from the shed end, but I am wondering if the aerial could be used from both ends - would having a downlead at both ends detract? I wouldn't have it connected at both ends simultaneously, but for convenience would need to have the downleads attached.
Tony


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 12:00 pm 
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Seem to have duplicated this but can't delete it! :roll:


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 12:33 pm 
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I'm not sure if I ever tried having two radios connected to an antenna at the same time.

After seeing what happens with atmospherics causing charges to form on long wire antennas outdoors, I never leave them hooked to any radio when not in use.
I disconnect and ground them.

Many times I have seen antennas energized by an approaching storm even snow storms will produce significant charges.
I don't exactly trust lightning arrestors to help, but a neon bulb would probably bleed off the charges.

I always took the path of unhook and ground it when not in use.

Granted this is my method, everyone else has their own.

-Steve

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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 2:46 pm 
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greenstar wrote:
I have a long wire between the house and shed at the end of the garden - about 100 foot. There's a downlead from the shed end, but I am wondering if the aerial could be used from both ends - would having a downlead at both ends detract? I wouldn't have it connected at both ends simultaneously, but for convenience would need to have the downleads attached.
Tony


Tony it will work just fine adding additional wire to your antenna on the other end. On a long wire there is no such thing as a lead in or feedline. When you add wire to the other end you are just making the long wire longer. And NO I wouldn't recommend leaving either end connected to an old radio. See Steve's comments above.

John k9uwa

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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 3:02 pm 
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Location: Near Brandon, Iowa
Pre-1940 radios use an antenna transformer to couple the antenna signals into the tuned "front end" of the receiver. This transformer has a primary winding, one end of which connects to the external antenna wire and the other to earth ground. Being a physical device, this transformer presents an electrical load to any antenna to which it is connected.

Connecting multiple radio antenna ports in parallel reduces the impedance that the aggregate places on a single antenna; i.e. it increases the electrical loading on the antenna. This will reduce the amount of signal that can be coupled into any one of the radios hanging on the antenna, resulting in reduced sensitivity compared with a single-radio connection between an antenna and ground.

Because the signals coming in over an antenna are a jumble of frequencies and electrical strengths, the degree to which a given signal will be attenuated is hard to predict. Generally speaking, strong local stations will be less affected than weak distant ones; and the station frequency should affect reception as well, since impedance varies with frequency.

Connecting a single antenna to just two radios may produce acceptable results; more than that and signal loss is likely to be noticeable for weak stations. This will occur even if all radios except one on the common antenna are switched off.


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 4:16 pm 
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The input impedance of a radio with a tuned input will vary wildly with frequency.
This changes with the frequency the radio is set for.

Connecting two radios to the same antenna can cause cause both to detune
even if they are set to the same frequency.

If you connect two radios to opposite ends of the same long antenna tuning
one can change the directivity of the antenna, as seen by the other radfio,
a lot.

What all this means is that if you do connect two or more radios ....
at any given tuning it can work great on one radio, or badly, and the
same on on another radio ... or the reverse!

In other words ... try it.


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 6:32 pm 
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lorenz200w wrote:
Pre-1940 radios use an antenna transformer to couple the antenna signals into the tuned "front end" of the receiver. This transformer has a primary winding, one end of which connects to the external antenna wire and the other to earth ground. Being a physical device, this transformer presents an electrical load to any antenna to which it is connected.

But transformers transform the load impedance back to the primary, factored by the impedance ratio.

Given that the load impedance is a tube grid circuit, its impedance is multiple megohms.

- Leigh

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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 7:32 pm 
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Leigh wrote:

But transformers transform the load impedance back to the primary, factored by the impedance ratio.

Given that the load impedance is a tube grid circuit, its impedance is multiple megohms.

- Leigh

In a typical AM radio, the antenna transformer secondary not only feeds an rf amp or converter tube grid, it also is paralleled with a tuning capacitor making an LC tank circuit. This gives (notionally) infinite impedance only at the resonant frequency of the LC pair, ideally much less at any other frequency. So the secondary load is not simply the several megohms of the tube grid; the load impedance is also a function of where the tuning cap is set.


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 10:14 pm 
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1) Is this a question that's never been asked before? Breaking new ground in radio? 8)

2) To test the performance of his 100' antenna on two opposite radios, does this means running back and forth from the shop to the house? Good luck with that. :lol:

3) If the antenna is disconnected from the shop radio, it will have zero affect on the reception in the house, correct?

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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 10:57 pm 
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Joined: Mar Sat 15, 2014 9:22 pm
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Hi, it's great to know I can use an aerial from both ends without loss! I have sets at both ends, usually testing in the shed and for listening in the house. But no, not connected at the same time.
I spend enough time running back and forth for tools that always seem to be in the wrong place, even when i have two of them - and an increasing tendency to forget what I have gone to the other end for. This can go on for some time.


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Sun 26, 2017 11:18 pm 
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My comments are based on experience rather than theory.

I repaired vintage radios full-time for a couple of years, along with a few other techs.
We had a modest outdoor anenna, with a drop at each of several benches.
It also had a drop at the burn-in rack, although those sets were only connected occasionally.

There was no observed effect with TRFs being connected to or disconnected from the antenna.

If we worked on a superhet, we connected it to a different antenna (in the attic).

- Leigh

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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Mon 27, 2017 2:21 am 
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They have made boxes to connect two or more TVs to the same antenna, I assume they must have included some sort of isolation between sets in the design. I think I have seen the same for radio receivers, it is quite plausible that at a listening post for short waves that military, FCC etc. would possibly have more than one radio connected to an antenna at one time and functioning. Perhaps there is some sort of tube or transistor circuit that they used to isolate the sets. And if it was an amplifier circuit for each output, I would assume that it would probably have to be broad banded and also low noise and low gain, you would not want alot of extra gain to pump up the signals and noise enough to cause overloading. Or it could be totally passive, if it did not introduce significant losses, I would assume. Further research is warranted. Or if not operating more than one radio at a time, a cheap switch circuit.


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 Post subject: Re: Can you have more than one Radio on an antenna?
PostPosted: Nov Mon 27, 2017 4:18 am 
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wazz wrote:
They have made boxes to connect two or more TVs to the same antenna, I assume they must have included some sort of isolation between sets in the design.


Yes they call them Splitters and a 2 way splitter loses 3 dB to each of the outputs. That is 50% of the signal. Split to more than two and the signal level is cut nearly the same. From memory a 4 way is 5.2 dB loss to each of the 4. Then to not have the loss one needs a Line Amp to jack the level before it goes through the splitter so what comes out is the same as what went into each of the cables. Somewhat the same applies to old radios hooked up at the same time.
Yes a simple switch to each one is much better.
John k9uwa

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