VHF - Transmission issues

[fjarvis]

Hi,

I've got an issue with my VHF. I replaced the aerial over winter after discovering that the previous owner had a piece of coax run to the top of the mast which did not connect to the aerial! So, a brand new aerial, cable and deck plug were procured and installed. However, I was told the other day by a marina that my transmission was garbled and unreadable - I was within 500m of the receiving station. I was transmitting on ch 80 @ 1W. I later tried a radio check with the coast guard (I think approx 5Nm from their closes aerial) on Ch16 @ 25W - weak but readable was the response.... Reception is great, I can pick up Ch 16 transmissions from a long way from aerials and AIS gets pings from boats 20 or 30 miles away. The aerial goes through a deck plug, into the AIS (Raymarine one with built in splitter) then into the VHF. All plugs are proper marine grade purchased from a marine electronics shop and the cable is all marine VHF cable.

I'm thinking that the problem may be that it was used for a while without a proper aerial (when the coax was not connected to the aerial at the top of the mast) and this has damaged the VHF (which is only 18 months old) - is this likely??

Thanks,
FJ

 

[Cliveshep]

Don't know much about RT's in general except that if you've been trannsmitting whatever power you've been using has to get radiated out of the tip of the antenna and if it can't it can often frazzle the set. Suggest you get the set out and checked over first, or installed temporarily in a mate's boat perhaps assuming he has a good working set-up with a known good antenna array?

 

[VicS]

I agree a sensible step would be to try the radio on a known good aerial.
Before doing that though do be certain that your power supply is good. When transmitting the radio draws a substantial current (esp on full power) so any poor connections will result in volts loss and adversely affect the performance.

An alternative is to buy an emergency aerial (you'll be wanting one anyway) and try the set connected directly to that.

 

[monkey_trousers]

absolutely! I'd check the power connections to the radio first and foremost. they draw a lot of current when transmitting, and an iffy connection could cause a big voltage drop when you key the mike, something that won't manifest itself when just 'recieving'

 

[tim_ber]

Agree with all of above.

Worth noting however:

I had some radio trouble, thought I'd fried the TX module.
Someone on here said it was possible I'd fried the TX module and offered to take it off my hands to fix and pass on to a mate.

Bit of Googling taught me this however:

A common reaction with radios is thus:
If power feed cable is of too high a resistance, radio will often change channel on its own (kick down to 16 and only Trans. on 5W).

This led me to think about resistance which led me to think about my power supply, cables were good, but on checking my battery, it was poor.

Sure enough, when battery fully charged, radio fine, battery drained, kick down.

Turned out my TX module etc all ok, and not fried the set. Got a good aerial, battery, cable, emergency aerial too (ebay) discovered I had an SWR meter on board, learnt how to use it, all ok now.

May help you self diagnose?
Good luck

 

[William_H]

Radiotransmission

Firstly if you do fry the transmit module it will likely blow fuses and certainly will not transmit in any form. You will have no doubt it is dead. Comparable to a broken crank shaft in an engine.

Garbled transmission on FM radio (VHF marine) usually means a bad microphone or mic too close to your mouth a shouting or other extraneous noise in the area. An experienced operator will know when transmission is garbled as opposed to just weak and hard to copy.
It would be unusual but garbled when over very short range is simply an overload problem. Usually fixed by using low power. However as suggested do check power wires especially.
And get an emergency antenna. olewill

 

[fjarvis]

Thanks for the suggestions guys. I'll look for volt drop using a meter at the radio end of the cable. I'll also look at an emergency aerial as I had been planning on getting one anyway for when using the forth & clyde canal with the mast unstepped.

Cheers,
FJ