DC resistance of vhf antenna

[oldvarnish]

I've got two VHF antennas- one for VHF and the other for AIS.

VHF works fine and shows a very low DC resistance across the antenna.

My AIS does not appear to work. A DC resistance measurement across the antenna shows it to be open circuit.

When I connect the working VHF antenna to the AIS (receive only) I don't get any contacts, but I am a bit off the beaten track.

Is it the case that different types of antenna would show either a near short, or open circuit, when measured with a DC meter, but still be working?

 

[prv]

Is it the case that different types of antenna would show either a near short, or open circuit, when measured with a DC meter, but still be working?

Yes.

Its RF function has nothing to do with its DC resistance, so some are open-circuit, some look like a short, and some are somewhere in between.

Not helpful, I know.

Pete

 

[RichardS]

I've got two VHF antennas- one for VHF and the other for AIS.

VHF works fine and shows a very low DC resistance across the antenna.

My AIS does not appear to work. A DC resistance measurement across the antenna shows it to be open circuit.

When I connect the working VHF antenna to the AIS (receive only) I don't get any contacts, but I am a bit off the beaten track.

Is it the case that different types of antenna would show either a near short, or open circuit, when measured with a DC meter, but still be working?

Can you connect the VHF to the AIS antenna and see if the VHF will transmit to a handheld or another boat? That may show that it is the AIS receiver which is at fault rather than the antenna.

Richard

 

[MM5AHO]

There are many designs of antenna. Some are a DC short circuit and some an open circuit. Measuring the DC resistance between the centre pin and outer of the connector doesn't tell much unless you know the design of the antenna.
The only thing you migh learn is if you get an intermediate reading, such as say 1000 ohm, it might suggest a fault, but short or open circuit doesn't say much.

You have more information by doing your receive test. The one that receives an identical signal better is probably the better antenna, but that too might be a matter of design. Some antennas have what's called "gain". It means the signal that goes down the coax to the transceiver is of a higher voltage (even though we're talking microvolts), so the transceiver perceives it as a stronger signal. Gain is a matter of design, and for a boat application high gain doesn't mean good, as gain comes from "directionality". And you don't want your antenna to only listen to a part of the directions that it could do, when its waving about on a mast in every direction, you want it to "hear" almost universally from every direction.
To test, find a cooperating other boat. use a suitable channel and conduct tests both ways using both antennas. If there's any difference, then this should tell you. The one where you can't her him and he can't hear you is the dud one!
An SWR meter put in the line between transmitter and antenna will give some info too. High reading? -> dud antenna.

 

[oldvarnish]

Can you connect the VHF to the AIS antenna and see if the VHF will transmit to a handheld or another boat? That may show that it is the AIS receiver which is at fault rather than the antenna.

Richard

V good idea. I like to think I would have thought of that eventually!

 

[BERT T]

V good idea. I like to think I would have thought of that eventually!

Can you connect the VHF to the AIS antenna and see if the VHF will transmit to a handheld or another boat? That may show that it is the AIS receiver which is at fault rather than the antenna.

Richard

Do it on low power otherwise you run the risk of frying the RF output transistors.

 

[William_H]

Yes as said some antenna design have a coil or part of the antenna which provides DC conductivity centre to outer while others do not.
The moral of this is that all boat owners should know what the DC resistance centre to outer is for their antenna checked when it is new or known to be working OK. All for future fault finding. olewill