Testing a VHF with no mast...

[Captain Crisp]

Hi - I had a few issues last year with my VHF seemingly working fine when doing a radio check in a marina, but failing when a few miles out... it was still hearing but apparently not broadcasting.

I think the issue may have been that I had to create a join in the aerial cable that may have failed... So effectively my aerial was just the short length of cable up to the join. Does that sound plausible?

I'm planning to by a new complete length of cable to run up the mast.

At the moment the boat is on land with no mast - what's the best way of checking that the VHF itself isn't to blame?

Thanks,

Crisp

 

[Martin_J]

A spare antenna with a known good feed cable/plug (and preferably an SWR/Power meter).

Where abouts are you (or more importantly the VHF)? Someone from here might be nearby with a spare antenna and the right test kit.

A spare antenna is always useful on board in case of a lost mast or the masthead antenna just becoming suspect.

 

[Martin_J]

The SWR meter, if it has a power function built in will be able to confirm the VHF is outputting the right carrier power on each channel.

 

[William_H]

Really the likilhood is that your problms are in the antenna and cable. Radios are surprisingly reliable and will perform as expected if they seem to work at all.
A temporary antenna can be made of 17 inches of wire the bare end pushed in to the antenna socket of the radio. Or google for various designs. I have a home made antenna on stern rail works well. Well worth you having as an emergency antenna or even AIS. ol'will

 

[st599]

One of the odd things about the VHF course is that it doesn't give information on how to check the install.

Depending on your antenna, changing the cable may be an option. Checking the SWR is useful, but may mislead if there's something absorbing the power, like a water filled co-ax.

 

[snowbird30ds]

A radio check in a marina would probably work without the antenna connected at all (but does the radio no good) and shows the radio is ok so has to be coax/antenna issue.

 

[TSB240]

You suspect the antenna cable.

Try your radio on a known good one. This can either be a mates boat or buy a spare antenna that could be mounted on the Pushpit with a lead taken straight to your radio.

You then have fitted a back up emergency antenna that is way more effective than a hand held radio.

Joins in antenna cables should only be made with well soldered in line connectors in a dry location.

 

[wonkywinch]

There is a lot of well meaning but totally inaccurate advice on this thread. I'm sure Paul will be along in a moment to explain.

 

[Beneteau381]

Hi - I had a few issues last year with my VHF seemingly working fine when doing a radio check in a marina, but failing when a few miles out... it was still hearing but apparently not broadcasting.

I think the issue may have been that I had to create a join in the aerial cable that may have failed... So effectively my aerial was just the short length of cable up to the join. Does that sound plausible?

I'm planning to by a new complete length of cable to run up the mast.

At the moment the boat is on land with no mast - what's the best way of checking that the VHF itself isn't to blame?

Thanks,

Crisp

I had issues when I first had my boat. Tried all sorts, a lot of it wishful thinking. I changed the cable, waste of time. The bottom line, it was the ground plane in the antenna, it was corroded. Bought a cheap Shakespeare? With a cable attached and fitted it, perfect ever since. Can vouch for the home made antenna to test it. I stripped a piece of coax, the internal bent one way and the braid the other way, tacked to a piece of wood. It worked as the other post says. I cant remember my dimensions.

 

[Ian_Edwards]

The only way to make a reliable connection in a coaxial cable is with a good coaxial connector with the correct impedance. Even then there will be a reflection at the connector.
One of the best connectors to use to match the cable is a BNC, 50 ohm for most VHF set. You can get male and female versions. They are cheap and readily available, but NOT waterproof. You can make them reasonably waterproof with self amalgamating tape, taken well back up the cable and then covered with a wrap of PVC tape. Unfortunately this makes quite a bulky joint.
Since your mast is down why not put a new good quality 50 ohm cable in the mast, and if you can take that all the way to the VHF. If that's not practical put a male and female BNC connector in the cable, below deck if you can, and protect it with self amalgamating tape.

 

[Skylark]

……. I cant remember my dimensions.

Formula is frequency x wavelength = constant (velocity of propagation, 3x 10**8)

For example, use Ch16, 156.8 MHz.

Wavelength is therefore 1.91m.

A half-wave dipole is known as the fundamental. This is a balanced device but we use a coax feed which is unbalanced so core is connected to a quarter wave length of coat hanger, 480mm (ignoring the velocity factor) and the braid is connected to an earth / ground plane.