radio transmision issue -multiple radios

[DannB]

Hope im posting this in the right forum, well here goes.

I am having a issue with my VHF radios, when i key the mic on one radio it cuts out the transmission on the other this happens on low power or high power. also its not just one particular pair of radios, it happens on the many vessels i work on. how do i stop this ?

radios in question are ICOM marine VHF 1watt 25watt
antennas are shakespeare 8ft marine vhf
antennas are spaced approx. 8ft apart from each other

any suggestions?? can i add something to the radio or antenna to make this stop? im sure it is damaging the radios

thanks

 

[cabletech]

Couple of questions before we get into answers.

How big are these vessels? If they are commerical then you should have more then 8 feet to install between antennas.

Are these radios trying to work on the same frequency?

Are they both trying to transmit at the same time or is one recieving and the other transmitting?

As a rule, antennas in the same frequency band should be at a min 20ft horiz or 10ft vert seperation.

You will also find that if two radios are on frequencys close to each other that even with the seperation of antenna, you will get some degrade on the second radio when the first is transmitting.

 

[DannB]

cabletech to answer your questions

1-yes these are commercial vessels 80 to just over 100 ft. the antennas are roughly 8 ft apart maybe 10 on some of them . antennas are not gonna be moved

2- No they are not working (transmitting)on the same ch/frequncy at once

3- they are NOT transmitting at the same time. Both are always powered on though

4-this issue is not specific to a certain channel..happens on which ever 2 marine channels i am monitoring( 156.0-162.0 freq. range )

 

[KA1NTG]

They are both resonant at the same frequencies, so there's likely enough 'bleed' from the TX antenna to blank out the RX antenna. I'd highly suggest having someone with knows what they're doing and has the right tools (Antenna Analyzer, Wattmeter, Oscilloscope, etc) take a look. If you can't move the Antennas, and there are no other issues (bad ground, bad coax/connector) there may be very little you can do to remedy this.

 

[krokus]

Wirelessly posted (Opera/9.80 (Android 2.3.4; Linux; Opera Mobi/ADR-1309251116) Presto/2.11.355 Version/12.10)

You are getting desense. The transmit power from one radio is deafeaning the receive on the other radio.

Space between then antennas is key, as already stated. Second to that, use expensive filters.

 

[DannB]

fellas..thanks for all the input..last question,is this damaging the radios?? and one of the post suggests expensive filters could solve this, what are they and where could i get them..so i would need to place one on each radio?

 

[zz0468]

...last question,is this damaging the radios??

Probably not.

and one of the post suggests expensive filters could solve this, what are they and where could i get them..so i would need to place one on each radio?

The filters work by restricting the signal that will pass to a very narrow range. Narrow enough so that if you put a filter on one radio used for a CH16 watch, you wouldn't be able to use that radio on, say, CH 22A if you had to talk to the Coast Guard, or on one of the working channels. That's probably no more acceptable than moving antennas around on the boat.

The filters sufficient to do with would be physically large. Picture several cylindrical "cans", almost 2 feet long and anywhere from 4" to 10" in diameter. It could easily take 4 or 6 cans to be effective, and you would be restricted to a single channel per radio.

Easier to live with the problem.