One Antenna, Two Radios

[KC2QWK]

Hello all,

I've done a lot of searching online and can't seem to find a similar scenario:

I have a commercial VHF 136-174 MHz (Icom) radio and a standard VHF marine radio... and only one antenna. I need to be able to transmit on the Icom radio and receive on the marine radio. The Icom is the radio that will be used to communicate. The marine radio will be receive only, just to monitor the selected channel, without the possibility to transmit.

How can I go about setting this up?

Matt

 

[N_Jay]

How can I go about setting this up?

Not easily.

Since both are in the same band, and you want to TX, it gets complicated.

It is probably best to use separate antennas.

 

[SCPD]

you'll still have bleed over even on separate antenna's. but that is going to still work better than both radio's on one antenna. The other idea would be to program the Icom for the marine RX and put them on Priority scan. Eliminating the marine radio completely.

 

[KC2QWK]

I figured it wasn't going to be easy. It's a bit of a pain finding space on our 90-something foot tower and the capability to actually get up there. When we mounted this antenna, the fire dept. was nice enough to bring one of the guys up in the ladder truck.

The option of eliminating the marine radio is out of the question. Our policy states we have to have two radios... a separate radio has to continuously monitor marine channel 16.

Signal bleed wouldn't be good considering the marine radio will be monitoring marine channel 16.

If we have to get another antenna, that's no problem. The problem is getting back up on that tower! That's why we're trying to find any alternative option first.

 

[N_Jay]

Are there any VHF RX only antennas that you can split?

 

[KC2QWK]

I doubt it. If there are any decent looking antennas left up there, I wouldn't trust the coax. We ripped down an old 6-meter ham antenna to put the VHF marine antenna up... and installed new coax. Everything else is pretty much shot.

 

[WouffHong]

Smoke test

Hello all,

I've done a lot of searching online and can't seem to find a similar scenario:

I have a commercial VHF 136-174 MHz (Icom) radio and a standard VHF marine radio... and only one antenna. I need to be able to transmit on the Icom radio and receive on the marine radio. The Icom is the radio that will be used to communicate. The marine radio will be receive only, just to monitor the selected channel, without the possibility to transmit.

How can I go about setting this up?

Matt

One antenna and 2 radios? - How do you intend to prevent the transmitting one from blowing the front end of the receiving one? - One would need a commercial-grade powered coaxial switch with high isolation and automatically switched from Receive Radio to Transmit radio at least 500 Ms before the RF hits the switch or some way to protect the receive radio's input. :roll: :roll: Mac

 

[N_Jay]

What frequencies do you use on the ICOM?

 

[KC2QWK]

WouffHong - That's what I'm trying to figure out.

N_Jay - Mainly VHF marine. There are also some local public safety frequencies around 150 MHz and local amateur radio repeaters around the same frequencies. It's an ICOM IC-F121 radio.

 

[N_Jay]

Any type of one antenna solution will be a kluge.

What is the application? Are you a dispatch center, Races desk, or what?

 

[gewecke]

Re; antenna...

The most efficient way and safest,would be a separate antenna for each radio. If space is the problem,then you can mount two antennas on one mast?
N9ZAS.

 

[KC2QWK]

Coast Guard Auxiliary. I'm sure we could get the fire dept to help us out again, if needed. I just wanted to find out if there was a way we could do it without having to go back up on the tower. I just got back from the tower... right next to our antenna, there's a useless antenna that can get ripped off where another one could go. We'd use that spot if we had to go back up there. But again, we'd rather not go back up if there's another option.

 

[WouffHong]

Oh, well

Coast Guard Auxiliary. I'm sure we could get the fire dept to help us out again, if needed. I just wanted to find out if there was a way we could do it without having to go back up on the tower. I just got back from the tower... right next to our antenna, there's a useless antenna that can get ripped off where another one could go. We'd use that spot if we had to go back up there. But again, we'd rather not go back up if there's another option.

I dunno - this whole scenario is weird.. As a past FL 2-way/Marine Electronics company owner, I can see no reason why you have to use 2 radios for one job. Auxiliary units have no special dispensation to run higher than legal power, if that's the reason for a separate transmitter. :roll:

Mac

 

[rvictor]

Even if you put up two separate antennas, it would seem that you'd need some space between the two antennas to keep from blowing the front end of the receive radio when you transmit on the other one, particularly since they are relatively close in the frequencies that they cover.

Can you put the Marine channel 16 antenna somewhere on the building and not put it on the tower at all?

Dick

 

[KC2QWK]

We're running at the legal power. The reason for the separate radio is because we HAVE to monitor Channel 16... even when transmitting on another.

Mounting anywhere on the building isn't possible. If we have to go with the second antenna, the tower is our only choice.

 

[Don_Burke]

The reason for the separate radio is because we HAVE to monitor Channel 16... even when transmitting on another.

That may be a show stopper.

If you want to transmit on a given channel and receive on another given channel at the same time, a duplexer that would normally be used for a repeater is a possibility, assuming the two frequencies are reasonably far apart. 600kHz would be pretty easy. It gets harder as the frequencies get closer.

However, that goes out the window if you need to transmit on various frequencies.

If that is the case, it looks like you are into another trip up that tower to put up another antenna and it also looks like you will be in the market for a cavity resonator or two or three and probably some very well shielded cable.

 

[SCPD]

respectively I see your problem. What about the option of putting up a VHF Yagi off the side of the tower at about 45 to 50 feet. point the yagi out to sea since its RX only it should receive wide enough to cover your target location. While the transmitting antenna for the 121 (Great radio by the way) is at 90 feet and I'm guessing an Omni. you would have the necessary separation to avoid bleed-over. other than that I can't see a solution. If not a yagi, at least put a Marine whip half way up on an extension bracket.

 

[N_Jay]

Not to be picky, but the ICOM is not a legal Marine Band radio.

 

[KC2QWK]

I don't think a yagi would be a good idea. We have the bay and a CG Station we communicate with in one direction and a complete dead spot in the river another direction. Part of the reason we're setting up this radio is coverage of the dead spots up the river.

As for the legal issue of the 121 on the marine band, I'm not even going to comment because I don't know. Others in the Aux who are setting up these stations like ours (at least in the First District) were told to buy these radios because they can handle the marine band, the "CG" channels, various local public safety frequencies, and Auxiliary frequencies.

 

[N_Jay]

http://edocket.access.gpo.gov/cfr_2008/octqtr/47cfr80.43.htm

You should probably check into the license you operate under and the requirements for that license.