COMM antenna almost no resistance - broken?

[Oliver]

I helped a buddy troubleshoot his intermittent radio problems.
He has a blade type antenna installed, but doesn't recall the model. It is a pain to get to, we therefore haven't removed it yet. No markings are on the outside. I guess it is an older RAMI antenna but don't know for sure.

Anyways, I at first suspected corroded connectors, which however turned out to look like new. I then went on to measure the cable and the antenne with a multimeter: The cable checked out ok, but measured only around 0.6 Ohm between the center pin of the antenna's BNC connector center pin and the outer conductor.

Based on my knowledge of antennas I don't think this is is right but have to admit that I don't have much aircraft specific antenna experience. Two other antennas I measured for reference however had a resistance in the high MOhm range.

So - are we dealing with a dead antenna or might some circuit inside the antenna cause such a result?

Oliver

 

[Brantel]

Depends on the antenna design. Some will appear shorted at the terminals using a regular ohm meter. At RF frequencies close to their resonant frequency they should have an impedance somewhere close to 50 ohms.

If you can find a local HAM with a MFJ-259 antenna analyzer you can check it out.

 

[N804RV]

There are several types of simple power/swr meters you can get fairly inexpensively.

The biggest hassle is getting the adapters to fit on the more commonly available ones that come with a female UHF style connnectors on either side. Typically you end up with 2 PL-259 to BNC adapters a BNC female to female union.

A VSWR of 1:1.2 is good; higher than about 1:1.5 is not acceptable. Usual culprits are poorly terminated BNC connectors and oil/water soaked 40 year old RG-58 coax. The newer RG-400 coax (while not top of the line $$$) is double shielded and much sturdier than the old RG-58.

 

[dpansier]

To add to what Brian mentioned above, the common types of 1/4 wave VHF Com antennas used in general aviation can be identified into two types, simple rod type antennas and the antennas with network matching devices enclosed the base.

The simple rod type has a Porcelain type insulator and the coax connections are made by attaching the coax center conductor to a nut on the rod end.

The network types, Rami and Comant will show an open across the BNC connector when measured with a VOM and aprox 50 ohms impedance at the operating frequency.
D&M (Dorne & Margolin) and Delta Pop will show a short across the BNC connector when measured with a VOM and aprox 5o ohms impedance at the operating frequency.

We all strive for a low VSWR but keep in mind the spec's of all the general aviation antennas are listed as less than 3:1 on the bent element style and less than 2.5:1 on the straight element antennas when measured from 118 to 136 MHz.

 

[Oliver]

Thank you for your responses.
We did some more testing today and overall the system seems to work just fine. I therefore don't think that the antenna is shortened out as otherwise neither receiving not transmitting would be possible. It makes sense that a matching network is probably built into the antenna.

My buddy needs to get a SWR meter so that we can dig deeper into this issue. I hope that we find out that this is simply a problem with the cable and not his 430.