Mast arial

[Clyde_Wanderer]

I was checking my main vhf arial today with multi metre, should I get a reading on the 200ohm scale if I put one probe on antenna (stainless steel) and other probe on the base mounting threads, and what value should I get?
I had intermittent problems and loss of signal last season, and failed to connect with the cg while requesting a radio check on a number of occasions, but worked fine at other times without checking or repairing anything.
I am going to fit a new cable which will be continues from arial to radio, no joints.
Thanks.

 

[steverow]

It is almost certain that your antenna is "loaded" ie has a coil in the base of it.
This coil is tapped to ground to give a 50 ohm match only at the operating frequency.
However your multimeter can only measure at DC not radio frequency so will measure a direct short circuit at DC.
This is quite normal and doesnt imply that the antenna is faulty.

 

[davierobb]

There are loads of dead spots on the Clyde so I would not always blame your vhf when you cant reach Clyde cg. I have been able to hear Liverpool cg outside Rothesay but was unable to raise Clyde cg for routine traffic. The single piece coax is 10% recommended, no additional joints = greater reliability.

 

[VicS]

Re: Mast aerial

Steverow is right. You are likely get a vitually zero ohms reading with a multimeter between any two points you care to pick on a VHF aerial but you may not depending on the design of it. I would perhaps be suspicious of any reading that was neither infinity nor almost zero.

What you can check is that there is no circuit between the inner core and the outer braid on your down lead once it is disconnected from the aerial. Then you can also check the continuity of both. Because it is difficult to reach both ends at once short the two together at one end then check that you have a dead short reading between the two from the other end.

 

[wooslehunter]

Re: Mast aerial

This question comes up regularly. The simplest way to check an antenna is to use a VSWR meter. You can get cheap CB radio meters but these may easily give a wrong result. A ham radio type made for the 2m band (144-146MHz) will work fine.

This is a meter that goes between the radio & the antenna, preferably close to the radio. Effectively, what the meter will give you is a ratio between the power that goes up the antenna lead and the power that is reflected back. Ideally none should come back but a small amount is quite acceptable.