Need help finding older DME antenna

[Atlcenter]

Hi,

Hope some of you with experience in avionics can help me. The DME on our 1975 Archer began to degrade over the last year. The dash unit checked out good at a local avionics shop. As a additional test I connected the transponders antenna to the DME and it was like night and day. It really worked well. So it is either cable or antenna. I cleaned both ends with non oily contact cleaner but no difference.

I shorted one end of the coax and read the other end with a vom., It showed about .2 ohms which is what I would expect. I can't determine the capacitance though.

So, now I am trying to find a replacement for the existing antenna and here is my problem. The existing shark blade style antenna looks like the KA-60 from Bendix, or a Comant CI105 BUT mine does not have studs as they do. It has 2 phillips screws that recess into the molded plastic from the outside at each edge of the antenna. These screws are 4 inches apart. It also has a PL259 style screw antenna connector which requires an adapter. The KA-60 and others have studs that are 1.75 inches apart.

I looked on line for a replacement but I have not seen one picture or description of the style I have. It says Bendix King on the antenna.

If a direct replacement cannot be found, would it be better to just leave the existing antenna in place and install another one. This leads to another question. It seems that if I add another one it would be easier and cheaper to install a stub style antenna. Is there any difference in their performance?

Thanks for any help!

 

[Palmpilot]

Did you check for a partial short circuit on the cable?

When you checked the continuity of the cable, did you flex it to check for an intermittent connection?

 

[JHW]

just put a new spike style antenna in the center hole and put machine screws or rivets in the old mounting holes to plug them

 

[denverpilot]

You need to check the cable separate from the antenna before you know you even need an antenna.

 

[weirdjim]

Hi, Hope some of you with experience in avionics can help me. The DME on our 1975 Archer began to degrade over the last year.

That's what they made GPS for, but since you didn't ask ...

The dash unit checked out good at a local avionics shop. As a additional test I connected the transponders antenna to the DME and it was like night and day. It really worked well. So it is either cable or antenna. I cleaned both ends with non oily contact cleaner but no difference.

Cheap test ... make up a cheap RG-58 short cable to run from the end of your current DME cable to the transponder antenna. That proves the cable without taking it out of the harness. Antenna is the only thing left.

I shorted one end of the coax and read the other end with a vom., It showed about .2 ohms which is what I would expect. I can't determine the capacitance though.

What would the capacitance show you? What happens when you OPEN the shorted end?

So, now I am trying to find a replacement for the existing antenna and here is my problem. The existing shark blade style antenna looks like the KA-60 from Bendix, or a Comant CI105 BUT mine does not have studs as they do. It has 2 phillips screws that recess into the molded plastic from the outside at each edge of the antenna. These screws are 4 inches apart. It also has a PL259 style screw antenna connector which requires an adapter. The KA-60 and others have studs that are 1.75 inches apart.

I cannot believe anybody post-1950 that would use a PL-259 antenna connector in L-band. Are you sure it isn't TNC (which is what I would suspect.)

I looked on line for a replacement but I have not seen one picture or description of the style I have. It says Bendix King on the antenna.

If a direct replacement cannot be found, would it be better to just leave the existing antenna in place and install another one.

NOOOOOOOOOO!! You are already load limited. Why add more weight when you don[t have to?

This leads to another question. It seems that if I add another one it would be easier and cheaper to install a stub style antenna. Is there any difference in their performance?

Absolutely. Normally I recommend the cheapest antenna (stub or spike) you can find, but the DME frequency span is WAY too wide for that little stub with a BB on it. I recommend a small aluminum plate painted to fit the hole(s) left by the bad antenna and then the new antenna mounted to the cover plate/fuselage in the same location.

NOW, a bit if fatherly advice. Search WAY down deep into your soul and see if you wouldn't like to save 20# of useful load, deep-six the whole DME package and put in a decent GPS.

Jim

Thanks for any help!

Five characters.

Jim

 

[Atlcenter]

Great advice from everyone!

First of all, if I had and extra $10,000 plus I would be installing a G430W but that isn't going to happen.

So, I ran a test yesterday by using an new cable and connecting it to the antenna's C adapter (thought it was a PL239) and to the back of the DME using a BNC to tray mount adapter. So no old cable was in use.

No difference, no mileage showed at all even though I was flying in the same area a the previous test. I even flew toward the station but nothing.

So, it has to be the antenna. There is nothing left to test except the C adapter on the antenna.

Since I was in the belly so to speak of the plane I tested the cable with a high quality vom (fluke 45). No resistance (open) from center to shield or to airframe. Shorted one end and read the other end and saw .2 ohms or so. But, since the new cable made no difference it has to be antenna.

I found a new replacement (Mine turns out to be a KA32) for $300 or used for 125. I'm a little concerned about buying used but as this point it will work or it won't. If it shows any thing on my display it's better than what I have.

 

[alfadog]

What I did in one instance was start searching on Google images to find the make and model of antenna (assuming it is not stamped on the antenna already). Then you can go to the scrapyards or eBay if not still available.