VHF Ariel

[Glyn_Fox1953]

I have recently purchased a new fold down VHF ariel to fit to a RIB and during fitting I was checking the co-axial down lead connector for a short circuit between the screen and centre conductor and discovered that even with no connector fitted the downlead has a full short between screen and centre conductor-0.5 ohms read on digital meter. This is either normal or a manufacturing fault and I wonder whether anyone can enlighten me as to which and if normal explain the reason. Here's hoping, cheers,Glyn.

 

[ShipsWoofy]

This is quite normal, you are measuring DC resistance, RF does not react in the same way.

 

[Salty John]

DogWatch is correct, what you are seeing is quite normal. Disconnect the cable from both aerial and radio before checking continuity.

 

[steverow]

Entirely normal.
The antenna has a tapped coil within the antenna, which at DC resistance will be at short circuit.
However, at 156Mhz the "impedance", (that is the term used for oppsition to AC current flow where there is both reactive and resistive components) will be much higher.
The coil is used to match the uneven length of the antenna...usually 5/8 or 7/8 wavelength long back to what it would be if it were a 1/4 wave vertical.
This is done to both to achieve gain and a flat topped radiation pattern ie to stop too much energy radiating directly upwards.

Hope that makes some sense.

Steve.