VHF Aerials

[moonshine]

Here's one for the radio technical experts on the forum ! I have checked out 2 emergency VHF aerials.

Why do some aerials measure zero resistance, i.e. 'short circuit', between cable centre core and screen whilst others show open circuit, i.e. infinite resistance ?

Are they designed that way and if so how can both types work ?

Alternatively has the aerial with the 'short' gone AWOL ? BTW I measured the 'short' at the aerial and I know it is not due a fault in the PL259 plug.

Confused !!!

A

 

[starfire]

two different types of aerial.

the one showing open circuit will be a whip aerial of some sort, either 1/4 wave or 5/8 wave.

The short circuit one will either be a whip with a matching network on it or J match aerial.

what you measure at DC has no relation to the impedance at the working frequency.

If you have access to a SWR meter covering around 156 Mhz, test them with that.

Sometimes on the open circuit type, a trick was to solder a small resistor (10k) across the connector at the aerial, it will make no difference at RF, but with DC meter you can check for open or short circuits on the feeder.

 

[moonshine]

Thanks for the info, I was about to ditch the 'shorted' aerial.

They are both 'whips' the 'open' a Metz the other a Glomex. The glomex has a coil under the cover with a screw that connects the end of the coil wire to the core of the cable.

Glomex now to be kept as an emergency aerial

A

 

[Salty John]

Most marine vhf antennas are internally grounded as protection against interference from static discharges such as lightning.

A multimeter will show a dc short circuit.

Your Metz antenna has a lifetime warranty. If you have any concerns regarding the functioning of this antenna send it to me and we'll check it.