VHF aerial or cable problem

[ste7ve]

Realised I was getting bad VHF reception regarding range, so investigated with an SWR meter. SWR was about 1.65 from the radio set. Disconnected cable at connector under the mast and fitted a good temporary aerial and found much improved reception. A friend went up the mast to look at the aerial and found it is a Glomex unit with a standard VHF socket on its base and connected to the cable with a VHF plug. There was no protection on them and when unscrewed there looked like some corrosion.
Also measured d.c. resistance from the cable termination under the mast and was 16 ohms (cable + aerial). I understand some aerials present an open circuit to d.c. so this might indicate a bad cable.

My quandary is what to do about it. The problem might be the cable, the connectors or the aerial, or a combination of them.

The expensive option would be have a new cable and aerial installed.

Any views on more tests I can do?
Also any recommendations for good marine electronics people near the Hamble if I have to bite the bullet and get the professionals in?

 

[VicS]

Check the insulation of the cable with an ohms meter while disconnected from the aerial. It should be infinite.

Check the cable for continuity by shorting inner and outer together at the top and checking with an meter with a continuity buzzer or with a bulb and battery.

The aerial will probably read a dead short across its connector . Your reading of 16 ohms may indicate a bad connection on a connector due to corrosion. Only the inner core is likely to be soldered.

 

[andy01842]

I would say 16ohms is defiantly a problem, should be zero or infinite. As already stated should be infinite with aerial disconnected. Any corrosion is bad news. I would take plug off, cut cable back a few inches if cable is bright and shiny refit if it is black and corroded replace all the cable. If any water has got into coax cable it will never dry out or work again.
Wrap the connection with self amalgamating tape to keep water out.

 

[lenseman]

[ QUOTE ]
Also any recommendations for good marine electronics people near the Hamble if I have to bite the bullet and get the professionals in?

[/ QUOTE ]

Where on the Hamble are you, I am regularly down on the Hamble if I can be of any use Steve? I could be at either Hamble Marine or Deacons.

PM me if you like?

 

[William_H]

"fitted a good tempory aerial and recption was much improved" indicates that the antenna is bad and the cable is ok.

If it is convenient or you want to test the cable as previously described by disconnecting from the aerial and checking with an ohmeter from the bottom. less than 2 ohms with cenre to outer shorted and high resistance with no short.

However 16 ohms sounds likea bad antenna. You could try opening up the antenna. On the basis that you can't do any more harm. or just ave a new one.

Incidentally the VSWR test can be very good but a long cable of the cheap type will absorb power in itself. This absorbtion looks just like aperfect antenna. So loss in the cable reduces the indication of a bad aerial. The answer is to fit the VSWR meter just under the antenna for a true reading. The usual thin 1/4 inch RG58 cable has quite large losses (typically 20 decibel loss for 100 metres.) so that means that over 100 metres of cable you get only 1/10 of the power out. (for 10 metre cable you lose about 20% of power)

The fatter RG8 cable has a typical loss of about 9 decibels per 100 metres or you get about 1/3 of the power out of 100 metre cable. (for 10 metre cable you lose about 5%)

The VSWR meter measures the amount of power reflected back from the bad antenna so in 20 metres round trip it can lose 40% of the reflected power so indicating a good antenna when it is not.

The losses in the cable are directly proportional to length and somewhat proportional to frequency. So losses become a concern at VHF. Of course you must bear in mind that actual received signal stregth is logarythmic regarding transmitted power so a loss of 20% is barely perceptable.

So in practice for a 10 metre masts you might find the diligent installers have used the bigger expensive cable while many have used the smaller cheap cable. And nobody notices the difference.

Anyway good luck with the mast climbing olewill

 

[st599]

If you're on teh Hamble you could pop to Nevada Radio in Farlington and get some proper low loss cable, rather than the RG58 - that should improve VSWR somewhat.

 

[Billjratt]

If you found corrosion, the outer braid was probably black - in which case it is no use. The RF current is supposed to go straight up the surface of the cable,skipping from strand to strand across the "weave". that means it will have the same length to travel as the current in the centre core. Black means it has to follow the strands and take a longer path causing loss. The Glomex connections don't sound a good idea, I'd buy a new ss whip and use the Glomex for the tranny.Check that the radio draws about 5 amps on transmit, that indicates it's using full power.

 

[ste7ve]

To follow up I got a pro in to look at it. It turned out the cable was OK. The problem was that the aerial had water in it. On the Glomex you can unscrew the cover over the coil and it was full of water all over the internal connections and coil.
So new aerial is the answer - If I had known I could have done it with a friend but everything else pointed towards the cable and/or connector.

C'est la vie