VHF poor - change to 2nd Aerial??

[Richard10002]

I am pretty sure that I have been sailing around with a VHF with a poor connection somewhere between the set and the aerial...... people have told me that they cant hear me from pretty nearby, so the chances of getting a call to a ship 30 miles away in a distress situation seems slim.

It just so happens that I fitted a 2nd aerial at the masthead in 2006 for AIS which I didnt use, (too close to the VHF aerial!!), so I could break the connection to the existing aerial at the base of the mast, (under the deck), and connect the 2nd aerial to the wire coming from the set.

However - given that I wont be sailing anywhere for a while, how will I be able to tell that the new situation will have fixed the problem i.e. that the problem isnt between the set and the base of the mast connection.

Problem is that I can hear the marina VHF loud and clear, aswell as our handheld, and I wont be able to test at further distances - can I do anything with an electrical meter thing - it's got volts, amps, ohms and hfe, (whatever that is?).

Any help appreciated

Richard

 

[Boatman]

Try and borrow a SWR Meter from a Ham, to check the transmit issues, it will measure how much of the power you are trying to put out is being reflected back indicating various problems. You will also be able to check the radio is transmitting at the correct power, ie 5 or 25watts.

Multimeter can be used to check no shorts in the Coax as well as all the plugs, its quite possible you have a connector or cable short.

Best of luck

 

[Salty John]

But to check for a shorted cable or connector you need to disconnect at the antenna end, or you'll see a dead short anyway.
A radio check from a reasonable distance with each of the antennas would be the best way of identifying a fault in one or other system.

 

[Boatman]

Quite right, I was making an assumption always a bad move.

Do test comparisons and see which works and which doesn't. However if the output stage of the radio has been damaged as a result of a short you will still have a problem and notice no difference

 

[st599]

[ QUOTE ]
Try and borrow a SWR Meter from a Ham, to check the transmit issues, it will measure how much of the power you are trying to put out is being reflected back indicating various problems. You will also be able to check the radio is transmitting at the correct power, ie 5 or 25watts.

Multimeter can be used to check no shorts in the Coax as well as all the plugs, its quite possible you have a connector or cable short.

Best of luck

[/ QUOTE ]

You would really need to measure VSWR at the antenna for this, not at the receiver. There's no way you can tell that the co-ax isn't acting as a giant dummy load (e.g. if it's full of water) without checking further through the circuit.

hFE is the small-signal current gain of a bipolar transistor.

 

[Billjratt]

Is there an ammeter on board? or can you measure the current drawn by the radio using a multimeter?
If so, it will draw about 5amps at 25watts of transmit power.
If it draws less, it is probable that the set is sensing an aerial mismatch and shutting down the transmit power automatically to protect itself.
The only way to resistively measure the cable is to disconnect the aerial and radio first. (and even then it won't show blackwire/water problems.)

 

[matelot]

An swr meter would be fine - but only if you knew how to read the results. Best approach for the non technical is to try a different aerial (pereferably a known good aerial) and get reports (maybe by mobile phone) from a station say 10 miles away. Now if the AIS aerial is a standard VHF aerial, that will do, but I use my emergency (mast has fallen down) aerial. If you dont have an emergency aerial, nows the time to buy one.

Beware the observer at the other end thinking he hears a difference - its a bit like the dinner time "does this taste funny to you" question. It always does, and he will always hear a difference. What matters really is does he hear both signals.

Ignore your reception. You will happily receive decent signals using an aerial thats no good at all for transmitting.

 

[nanofkyle]

Had a similar problem last season, and no fancy equipment available. Raised a spare aerial up the main halyard and all was OK when connected to the set. Decided to replace the masthead unit and found that the genoa halyard had been rubbing against the downlead inside the mast and had almost severed it about 10" from the masthead. Problem solved and all back to normal now. Good luck.

 

[Scotty_Tradewind]

Hi Richard
my radio played up and I had my pal who is a tv professional bring his aerial testing gizmo and we tested each part of the cable proving that I only had the cable in the boat as an aerial. From the connector inside at the base of the mast, to the top of the mast aerial proper, it was 'dead'. I soldered each joint and cleaned up the connectors and all is fine
good luck.

 

[Anonymous]

Richard, you really need an SWR and power meter to make sure that the RF signal is actually going out into 'free space' and not just reflecting back down your cable where it can burn-out your power transistors. It happened to me /forums/images/graemlins/frown.gif

By coincidence, I decided to order one today and have found a firm who will sell one for around £25 -- covering the entire marine VHF band (some don't). I need to speak to the shopkeeper tomorrow and am going to get him to send one to the UK for me. I'm told they will send to Europe.

If you remind me tomorrow, I will give you the details of which unit I chose, the cables, etc. that are needed. I will probably forget so do remind me!