VHF poor reception?

Eeyore

Eeyore

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www.yachtrhumbdo.co.uk
I had assumed that my main VHF set 'non reception' from a nearby marina was because of a mountain blocking the direct line of sight. It's a new Icom M421 and works fine otherwise. However, my handheld was inadvertently left on this morning and picked up the transmissions from the other marina whereas the main set didn't. So I assume there is a fault with the mast-top antenna? Anyone else experienced something like this - and where do I start looking/testing?

Ta!
 
Your antenna should have a rated impedance (Ohms). Get a meter & check it. (Switch off vhf, unplug antenna measure impedance from inner to outer.)

Erm... forum let me know if I got that wrong.

There are a million things that can affect radio comms... there is a straight line multipath effect that means an antenna at one height will not receive, where an antenna at a different height will. Can't remember the formula to work it out but it depends on wavelength & distance from base station. Any other cause of multipath could affect one particular antenna.. often you only need to move a small distance (eg to the other end of the marina) for the problem to go away.

Someone with proper knowledge & understanding will come on in a minute to tell me I'm talking utter tosh, but I do have long experience using VHF & UHF and many times find a glitch clears itself as soon as you move.

You could move the mountain o'course!
 
There are several current and recent threads on SWR meters. A cheap and cheerful one is not perfect for marine VHF transmission/reception but that's what most people use successfully. Not something you need every day so you may be able to borrow one. Tells you if your antenna is working correctly.
 
If is only marinas suspect a US vs INT mis-setting. Not sure of the details but its to do with 80 being duplex INT and simplex US (or something)

Just a thought
 
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Erm... forum let me know if I got that wrong.

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You got that wrong /forums/images/graemlins/wink.gif

Resistance from inner to outer measured with a multimeter is likely to be approx 0 ohms. Or it might be infinite - depends on antenna.

Impedance at VHF is a completely different beast and cannot be measured with a standard multimeter.

Receive is much less demanding of the antenna system than transmit, so if it 'works fine otherwise', I would agree with shmoo that it is probably a simple channel-programming problem.
 
Your antenna should have a rated impedance (Ohms). Get a meter & check it. (Switch off vhf, unplug antenna measure impedance from inner to outer.)

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BOOOOM BOOOOM DUH !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


Nop..... doesn't work like that unless you have the right test equipment.

Impedance is not the same as DC resistance and is a measure used with alternating frequencies and also is dependant of the frequency at which it is being measured.

A VHF antenna when tested with a DC resistance meter will often show a direct short as there will be a matching coil in the base shorting the conductor to the braid.

The easiest way to check for a sailor is to beg steal or borrow an SWR meter which shows what is happening when the ahtenna is exposed to a transmitted signal. This shows forward and reflected currents and you want lots of forward and not a lot of reverse.

Problems usually occur at the deck joint or the antenna joint. The radio itself can be checked by direct connecting your emergency antenna to the back of the radio. You do have one??????

The antenna joints vary in design from make to make and many are solderless and rely on the end of the coax being correctly prepared and assembled with a flanged bullet which clamps the braid to the antenna body as well as making the earth connection while a pin inside the antenna pierces the centre of the conductor. You must use the type of coax where the centre conductor is multibple strands and not a single strand for it to be sure to work.

The deck connection may have water in it and this is shown by a darkening to black discolouration of the conductor and or braid. Water trapped in the cable will affect the performance and the cable must be changed out. Let us know what you find!
 
Have you switched to the USA channels setting by mistake? We did this last year entering Brixham. The marina office could hear us but we couldn't hear them on our Icom 421. We had accidentally pressed a combination of buttons which selected non-UK channel settings.

The boat (and radio) are now sold but I vaguely recall having to press and hold the "up" and "down" keys simultaneously to switch between USA and UK settings.

If I remember rightly the manual wasn't particularly helpful on the subject but there was some reference.

Good luck.
 
If reception is otherwise Ok and especially if your transmissions are ok the aerial is very unlikely to be the trouble.

Shmoo and NJMcK have probably hit the nail squarely on the head.

Check that you have not selected US channels.
Some channels that are 2 frequency (duplex) in the UK are single frequency channels (simplex) in the US.

Common problem, comes up on these forums regularly

Channels that are affected by switching to US operation are 07, 18, 19, 21, 22, 23, 65, 66, 78, 79, 80, 83, and 88. All simplex in the US, duplex on the international system.
 
Thanks for all the replies. I will check in the mornning that I have not inadvertently switched to US Channels, though Vic does not mention channel 69 - which is the channel in use when the problem occurs.
 
Ch 69 is simplex in both the International and USA channels. (see pages 52 & 53 of the manual)
So that explanation does not sound likely then

Over 50 pages of Instructions! Glad my VHF is not that complex to use!
 
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