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Old 23-04-2013, 17:17   #1
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VHF Transmission Poor

Our VHF transmission has been poor, though receiving seems to be OK.

How do I debug the faulty piece of the puzzle?

I'm thinking it's the masthead antenna, because its hardest to get to.
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Old 23-04-2013, 17:26   #2
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Our VHF transmission has been poor, though receiving seems to be OK.

How do I debug the faulty piece of the puzzle?

I'm thinking it's the masthead antenna, because its hardest to get to.
One of the most common problems is the coax between the unit and the antenna. This cable is very commonly undersized and has unsealed connectors in bad places...like the bilge.

If you have access to another known good radio then hook it up and test transmit. If you have the same issues then it is the cable or antenna.

Also does this happen when transmitting from multiple locations? Could be local RF noise if not (not an uncommon problem when connected to shore power in a marina)
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Old 23-04-2013, 17:49   #3
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Re: VHF transmission poor

also consider your power wiring. i had undersized wiring and used to get a 'low battery' message on the display whenever i keyed the mike in 'high' power (25 watts). eventually figured out the battery bank wasn't low, the wiring was too small.
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Old 23-04-2013, 17:51   #4
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Re: VHF transmission poor

Ditto Belizesailor. Try a different transmitter- thats an easy test. But first, try changing the microphone. If the element inside is damaged, it may be sending a weak signal to the transmitter.
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Old 24-04-2013, 00:04   #5
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Re: VHF transmission poor

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Ditto Belizesailor. Try a different transmitter- thats an easy test. But first, try changing the microphone. If the element inside is damaged, it may be sending a weak signal to the transmitter.
Yes! This happened to me, and I spent a lot of time checking the antenna cable, connectors, etc. It was the microphone cable.
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Old 24-04-2013, 02:14   #6
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Re: VHF transmission poor

Something like this happened to me over Easter. I was out front of the marina calling them to let them know I was after a berth. The couldn't hear me, but I could hear them perfectly. No problem, I thought. I'll use my old faithful handheld. Exactly the same problem!

To cut a long story short, I have an old 150 watt inverter that came with my boat that I was using to run the laptop computer. It was so noisy in the VHF band when running that it not only trounced my transmission signal from my wired VHF, but also the battery powered hand held. Weird thing was, reception on both was fine. Once the inverter was unplugged, both radio's transmitted perfectly. Got me beat how that works, but that's how it is!
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Old 24-04-2013, 02:24   #7
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Reef magnet. That's an interesting observation, I've been unhappy with interference on a newly installed VHF, aerial and cabling, but had not considered interference from a nearby step up transformer used to run a 24v digital instrument cluster of an otherwise 12v system. I will turn of the transformer and test.
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Old 24-04-2013, 02:38   #8
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Re: VHF Transmission Poor

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Originally Posted by Avayak View Post
Reef magnet. That's an interesting observation, I've been unhappy with interference on a newly installed VHF, aerial and cabling, but had not considered interference from a nearby step up transformer used to run a 24v digital instrument cluster of an otherwise 12v system. I will turn of the transformer and test.
Yes, it's a strange one. My first thought was a bad cable or connection on my Icom and - I presumed - a coincidently bad hand held. Fortunately the staff at the marina offered to help me test my radios by responding to my radio checks once berthed. I had my suspicion it was either the laptop or the inverter so it didn't take too long to find out who the culprit was. Another really weird thing is even at 25 watts transmit power on the Icom, the transmission was still garbled so I can only guess the interference was occurring inside the sets at the pre-amp stage.
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Old 24-04-2013, 05:07   #9
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Re: VHF Transmission Poor

Before you go aloft, take a look for a connector at the base of the mast. Does not take much corrosion to cause problems, sure easier to get to if that is the problem... ...worth a look.

Good luck,
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Old 24-04-2013, 05:52   #10
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I have a similar issue (interference when running laptop off inverter) on SSB but it is very noticeable on the receive side. Can still receive and transmit just fine but there is a low level static which makes it hard to hear weak signals.

Some additional filters would probably resolve the issue...but easy just to flip off the inverter too.
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Old 24-04-2013, 06:31   #11
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Re: VHF Transmission Poor

I've had coax problems, the connectors are fiddly.
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Old 24-04-2013, 19:16   #12
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Re: VHF Transmission Poor

All of the above are candidates.

If you can borrow a known good antenna, a short piece of good antenna cable, and a radio-equipped friend about a half-mile away, you can try the following:

Converse with your friend using your normal setup, then repeat but with the second antenna and good cable.

- if your friend notices no difference in your transmission, then there's a problem in/near the radio itself and your antenna is likely OK

- if the second antenna provided better transmission than yours, then you have a problem in antenna or line. A good marine tech should have radio test equipment like a SWR meter to help diagnose further, if you want to pinpoint the problem.
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Old 24-04-2013, 21:56   #13
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Re: VHF Transmission Poor

All the above is good info.
IM(somewhat extensive)E, good Rx and poor Tx would suggest the following in order of preference and ease of "checkability".

1. poor microphone (check by transmitting and listening on you handheld - your handheld will still pickup the main radio even if the antenna is crook. This will confirm if your Tx modulation is good i.e. your microphone is good.

2. poor antenna and/or coax (& connectors). Easiest way to check this if you don't have access to a SWR meter is simply is to use a temporary antenna system. You should have one on board already. Easy to make, just a short length of RG58 coax - say 6 feet - radio connector on one end and on the other just strip back the outer PVC cover for 20 inches (IIRC), pull the centre conductor out through the braid and lay the braid back alongside the RG58 outer cover. Hold the centre conductor and exposed braid vertical and you have a workable temporary vertical dipole. If your radio Tx improves noticeably, then trouble shoot your antenna system. Some good ideas are already in the above posts.

3. poor DC supply to radio - check by measuring the voltage at the radio while transmitting (just hold down the the Push to Talk button, no need to talk . Should have at least 11.5 volts but better if >12 volts.

This should solve most poor TX / good Rx problems but if it doesn't then try looking at local RFI issues as suggested in previous posts or start looking for a RF techie type friend
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Old 25-04-2013, 10:48   #14
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Re: VHF Transmission Poor

Thanks for all of the ideas everyone.

A great wealth of ideas here on this forum. Maybe the bosun's chair will stay packed away after all
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Old 25-04-2013, 10:54   #15
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Re: VHF Transmission Poor

I fixed problems with my VHF (lousy reception) by running new coax from the mast base to the radio, and installing gold-plated, soldered connectors - including re-terminating the coax that goes up the mast. Don't forget to seal the connector at the mast base with self-amalgamating tape.

I was lucky the coax up the mast was fine, just need re-terminating.
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