aerial positioning

[narooma]

Following a similar thread on aerials I have been thinking about a small problem I have with VHF.
I now carry two VHF radios but the signal strength seems low. (lower than before on the original and no better on the new unit)
Before I start looking at connections, I have the two aerials within 8in. of each other on the masthead. Am I right in assuming this should not be the problem?

 

[EDM]

I am assuming that both have seperate coax cable runs....

using two antenna's next to each other itself is not a problem, however if both your radio's are powered on and you are transmitting on high power, you will be damaging the other radio, by overloading its RX circuits with too much RF power.

Did you use quality VHF spec coax ?

 

[wotayottie]

How have you measured the signal strength? Or do you mean the sound level from the speaker? Or are you talking about the strength of signal received from you by another boat?

 

[philip_stevens]

If you are going to use two aerials in close proximity to each-other, then one should be vertically above the other with about two feet spacing.

Duplex aerials on commercial ships (as on the containerships I served on) had the duplex aerials in this configuration.

As has been said, it prevents the overloading of the receiving section, while the transmitting section is working.

Alternatively, as a safety issue (mast coming down), you could have one aerial at the mast-head and the other on a pole at deck level. The only thing then, is to remember to switch off other receiving units if transmitting on the deck level aerial.

I fitted an emergency aerial on a pole at deck level on our charter Konsort - for just such an emergency of mast down.

 

[Oldhand]

[ QUOTE ]
Following a similar thread on aerials I have been thinking about a small problem I have with VHF.
I now carry two VHF radios but the signal strength seems low. (lower than before on the original and no better on the new unit)
Before I start looking at connections, I have the two aerials within 8in. of each other on the masthead. Am I right in assuming this should not be the problem?

[/ QUOTE ]

Two antennas designed to operate over the same frequency range at VHF only 8" apart will severely interfere with each others' radiation patterns and could cause weak performance in some directions and strong performance in other directions. I would on that basis alone say you have an installation problem and as already said, one of the two transmitting will pump an awful lot of power into the other and possibly damage a receiver.

 

[narooma]

Thanks to all for the replys. Will shift one aerial asap.