VHF Radios sharing an antenna

[pelllagic]

Hi All
I would like to find out whether it is practical to use two VHF Radios through a shared antenna. I have limited space on the mast of the yacht I work on and need 5 or 6 VHF units (It's a BIG yacht!)
My question is this...
If we receive a call and it is heard on the Bridge and in the Crew Mess will the Crew Mess radio be able to hear the reply made from the Bridge and therefore ignore the need to reply... Or will they not hear it, reply and confuse the calling station with a second reply...?
Any Help gratefully received
Thanks
Richard

 

[W5LZ]

Richard,
As proposed, it's not something you want to do. Yes, two radios can share an antenna. But if those two radios are on the same frequency or channel at the same time, one of them will certainly get a 'shock' when the other transmits, which is not good, and is sometimes 'fatal' for the one getting that 'shock'. There are ways around that situation which can be quite expensive radio wise. Probably the simplest is the proper use of the crew. The 'snipe' ain't the navigator and shouldn't try to act like one. Same for the radio operator, etc, etc. Sounds like you may be short crewed.
- 'Doc

 

[C2]

I've wondered about sharing an antenna too.

I have an OK antenna switch (forget which brand, maybe MFJ). It looks like it grounds the unused port, but I'm still concerned about about overloading the radios receive when the other transmits.

 

[C2]

Another idea I just had for the OP, is there a "smart" switch that will ground the input quick enough if another radio keys?

Or, perhaps more likely, a fast switching attenuator connected to each radio? Then all radios would be able to hear both the transmission without overload and any other received signal from abroad.

 

[Moleculo]

C2:

Shakespeare AS-2 VHF automatic switch

Shakespeare Marine Electronics and Data Systems - StyleAS-2

This automatic switch toggles two radios to one antenna. When you key the microphone on one of the radios, it switches the antenna to it… automatically. The radio stays locked onto the antenna until the automatic switch detects a transmit from the other radio. Easy!

• Specifications:
• Frequency Range: up to 600 MHz
• VSR: Below 1.2:1
• Power Rating: 30 W
• Impedance: 50 ohms

Pellagic, this isn't the solution to your problem. When this switch is set on one radio, the other radio isn't going to hear anything, either. I don't see how you're going to get around installing another antenna.

 

[bit_pit]

Just to throw something into the ring, and I don't know (or want to know) the legalities of such a setup...

But what about a crossband repeat setup?

i.e., Single main VHF/UHF unit that does wideband and cross repeater activated(I know some of the Alinco range did this, and were easily modifiable to do VHF marine, likely other brands as well, or a custom setup with 2 radios).

Unit recieves VHF transmission, repeats it onto UHF (PMR frequencies?) and you carry round UHF Hand helds. Respond on UHF via the hand held, it gets repeated back to the VHF frequency out the main unit. All crew with UHF hand helds would hear the conversation....

I've done similar, albeit on Ham bands for field days etc.

 

[park]

I've wondered about sharing an antenna too.

I have an OK antenna switch (forget which brand, maybe MFJ). It looks like it grounds the unused port, but I'm still concerned about about overloading the radios receive when the other transmits.

You need a duplexer.

 

[Captain Kilowatt]

You need a duplexer.

That won't work when both radios are tuned to the same frequency or anything close to that ie VHF channel 16.

 

[W5LZ]

The cross banding thingy will work. There are commercial systems of that type. The catch is that they certainly aren't free. Considering the use, I'd find a commercial shop to do the installing, this really isn't something to 'DIY'.
- 'Doc

 

[Captain Kilowatt]

I would consider remoting a separate microphone and speaker to control the one radio. At the radio station where I worked we had about a dozen places in the building with microphones and speakers that all connected to a base station located in an equipment rack. Since we had so many speakers we used a distribution amp with 70 volt transformers but the microphones were basically in parallel. For a two or three speaker system you could just parallel them all. That would allow remote monitoring and eliminate the problem of two radios into one antenna.

 

[Radio Tech]

I would consider remoting a separate microphone and speaker to control the one radio. At the radio station where I worked we had about a dozen places in the building with microphones and speakers that all connected to a base station located in an equipment rack. Since we had so many speakers we used a distribution amp with 70 volt transformers but the microphones were basically in parallel. For a two or three speaker system you could just parallel them all. That would allow remote monitoring and eliminate the problem of two radios into one antenna.

That is what I would do. One radio with several remote stations. While working in the commercial radio repair field for many years that is how we did it. Very simple. Takes no fancy stuff.

 

[park]

That won't work when both radios are tuned to the same frequency or anything close to that ie VHF channel 16.

Your right, I could not find a same freq duplexer.

 

[Beetle]

Don't confuse "duplexer" and "diplexer".

Duplexers for the same frequency, or very close bands of frequencies, are not common. At least these days. Back in the '60s, Navy ships used duplexers to put up to five 1KW transmitters on the same antenna at the same time, with less than half a dB loss on any circuit.

A "diplexer" is what you'd use to have simultaneous 2 meter and 70 cm capability with one antenna. Frequencies are widely spaced, so you don't need a duplexer.

 

[hertz]

There is duplexers to use with the same frequencies. Have a look at this for example:
VHF Duplexer - 2 transmitters to 1 antenna