J

John Courte

Well Known Member
After much gnashing of teeth trying to figure out why my radio is dumping RF into my EFIS, I tested my RAMI AV17 antenna with a multimeter.

When I put a probe of the ohmmeter on the center conductor of the BNC fitting and another on the antenna rod, I get no beep. I know very little about how these things are built, but shouldn't the inner BNC conductor be attached to the antenna rod somehow?

On the standard belly whip antenna, the center wire connects to the rod directly and the coax shield connects to the groundplane. Is there some element in the AV17's construction that would cause a null continuity reading?

The other stuff, the relay clicking noises, went away after I re-did my audio panel connectors with proper grounding pigtails on the shields and ARINC connections.

Still no mic audio, but I'll figure that out after I get rid of a couple of unknowns.
 
Antennas can show a dead short or open on a multimeter and not be defective. It depends on how the elements are coupled to the input connector. Your test is inconclusive unless you know what the antenna design is.

If you are having trouble with transmit RF getting into avionics, etc., a bad antenna is a distinct possibility. What's more likely is a bad coaxial cable - shorted or open - connected to the antenna.


Mark Olson RV-7A F1-EVO Rocket
 
Yep, I'd check your coax for a short. leave the antenna end un plugged, check your centre conductor to the barrel at the radio end. There should be no continuity.
 
The only real way to check an antenna is with a Bird Wattmeter. I tells you how much power is going to the antenna and how much is being reflected back. Ideally none should be reflected back.

The other option is to find a known good antenna and try it out.

Jay
 
Just had another thought...What are you using for a power source? A comm radio xmitting uses about 10 amps. Are you sure your volts aren't dropping enough to cause the efis problem?

Jay
 
Just tested the center conductor of the wire to the barrel at the radio. There is conductivity. This is a GNS430W. I pulled the antenna off and tested conductivity between the center conductor of the radio's bnc connector and the radio chassis and there is conductivity there as well. I don't know if that's normal for GNS430W units, but if there's a path there, it makes sense that a lot of RF could get dumped into places it's not supposed to go. But then again, I'm no avionics tech, so I can't say.

I was hoping to avoid this, but it looks like I'll need to take a trip up to Ed Berlin avionics and have him check the unit out.
 
Is your 430 connected to the EFIS via a ground connection (perhaps shield of a cable or something ?).
This could cause a lot of the 430's operating current to flow via the EFIS to your battery negative if the 430's own negative connection is not of low impedance. During transmit in particular this current is highly contaminated with RF and can reach a few amps - enough to destroy the EFIS in some cases.
Try and remove every single wire from your EFIS except the power supply itself. Do you still have issues if you do that ?
If yes, then two possibilities: Either the power to the EFIS is severely contaminated with RF during transmit (I have see RF up to 50Vpp on one installation - so this does happen) - or the antenna is so close to the EFIS that the field is strong enough to cause havok inside the EFIS. This often happens with ultralight aircraft and newer, powerful radios and the antenna is three feet or less from the EFIS.

Have you seperated power supply between radios and EFIS ? It's a good idea to keep this completely seperate including the negative - supply and negative will only be joined right at the battery terminals. This provides the best isolation. Make sure the cables are not bundled togther - this just allows the RF to "jump" right over to the other cables.

Should not be an issue if your 430 output measures low resistance to negative. There would be at least an RF choke to negative and this is what you are seeing. At least that is the way we have it in our radios.

In some cases you can use special ferrites (types suitable for use at over 100Mhz) over the cables to and from the EFIS. These can provide some measure of RF blocking in difficult installations where there is no luxury of space.

Rainier
CEO MGL Avionics
 
John Courte said:
...When I put a probe of the ohmmeter on the center conductor of the BNC fitting and another on the antenna rod, I get no beep...
Click to expand...

I have had an antenna go bad in the manner you describe; it had an internal open in the path between the BNC and the antenna whip. However, it was an antenna that I had made, so I had nobody to blame but myself. Actually, it was a combination radio antenna and Nicks total energy compensation probe, but that's a story for a different forum.

Looking at the picture of the antenna in the ACS catalog, it looks like a typical quarter-wave whip. In which case there will be a conductive element inside the whip that will probably be directly connected to the center pole of the BNC, and you should probably read very close to 0 ohms between the two.

The thing that I wonder is, how are you sure you've made an electrical connection between the ohmmeter probe and the conductor in the antenna rod? Did you remove the white coating to get down to bare metal? Not that I'd recommend that, though, I'm just curious.

Regardless, the radio shop should have a Standing Wave Ratio (SWR) meter that will quickly test the whole system, and a good radio guy will know how to test the antenna for functionality.

Thanks, Bob K.
 
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