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antenna question

Jrskygod

Well Known Member
Avionics experts - I have a ci-122 com antenna that I suspect may have an issue. My question is should there be continuity between the center conductor of the bnc connector and the antenna rod? If someone has a known good antenna and can test it I would be most appreciative.
 
If I Understand Correctly...

If I understand your post correctly, yes the center conductor must have continuity with the antenna rod - that's how the RF gets radiated into space. If there's no continuity, there's a problem and if you key the mike too long or too often, you may well "blow the finals" in the transmitter.

The shield connects to ground / ground plane. There must be continuity there too for the radio to work properly.
There must NOT be continuity between the shield and center conductor. if there is, the transmitted signal gets shorted to ground and goes "nowhere." Again, not good for the transmitter

If there's no continuity in either the center conductor to antenna Or shield to ground you will have severely reduced transmitting capability - and if its bad enough, the transmitter may be damaged.
 
not necessarily

You need to get data from someone who has this specific antenna. Some are capacitively coupled, and a typical ohmmeter will show infinite resistance between the BNC and the radiating element. Other designs have an inductor to ground, and a DC ohmmeter will show a "short to ground" when none exists at RF. It all depends.
 
Carl - my avionics shop already used a vswr meter on the antenna then on the antenna and cable together from the tray connector forward. All was excellent per the shop and if I remember correctly he said less than 2:1 vswr. Also tested the radio and antenna system as well, again less than 2:1 vswr and a bit more than 10 Watts. I am using an avidyne 440 and still get a Com TX Fault popup on the screen most of the time, however the reception is very good. I'm just trying to understand how the Comant antenna works as I have no continuity between the center of the bnc connector and the antenna element. Both antennas I have are the same. I have tried two radios with same problem.
 
I'm just trying to understand how the Comant antenna works as I have no continuity between the center of the bnc connector and the antenna element.

A theoretically ideal vertical quarter wave over a perfect, infinitely large ground plane, is a match for 36 ohm coax -not 50 ohm. Some low cost, simple antennas just live with the mismatch. Or count on non-ideal ground planes to add another 15 ohms or so. Better ones will have a ?matching network?, to match the antenna impedance to 50 ohms. These networks sometimes employ certain fixed lengths of some specific type of coax. More often, they use capacitors and inductors (?coils?) hidden in the antenna base. If there?s a series capacitor it will appear as some finite impedance to RF, but look like an open circuit to a DC ohmmeter. Like wise, an inductor can look like a DC short circuit, but have finite impedance to RF. If you?ve got less than 2:1 swr on the coax you?re probably okay, although lower (1:1 is ideal) is better.
As an example, the Archer nav antenna employs a series capacitor as part of its matching network, and it?s in plain sight, where you can see it. Put an ohmmeter between the center coax connection and the antenna, it will read ?infinite?.
I really have no idea about your fault message, but you could double check the coax connection at the back of the radio, see if it could be intermittent??
 
Bob thanks for the input. I have checked the antenna every which way I can think of and have reason to believe it is ok. I was trying to understand why my antennas did not show continuity, and other brands do, and now I understand why. Without contacting Comant I am assuming that I have the capacitance matching style antennas as both of my antennas show the same results. The connector in the back of the tray appears good as well. I can ohlm the coax from the pin at the tray to the antenna end and while wiggling and putting a bit of tension on it it maintains continuity. I am now leaning towards a tray mismatch problem. These Avidyne units are supposed to be plug and play with the Garmin 430, but utilizing the Garmin tray may be the root of my issue. Funny thing tho the Garmin radio I had in there never had a com issue and when I replaced it with the Avidyne it worked perfectly for many months before I would occasionally get, after an hour flight or so, a "Com TX Fail" warning and ATC would confirm that communication was garbled and unreadable. I received an exchange radio from Avidyne and now I get the "Com TX Fail" message most of the time and the radio will not transmit or transmits poorly. The receive side always is excellent and clear. Its kinda hard for me to believe that both radios have the same fault issue and that's why I'm leaning towards a potential tray problem. I'm basically just grasping at straws here as the Avionics shop has no clue.
 
Avionics experts - I have a ci-122 com antenna that I suspect may have an issue. My question is should there be continuity between the center conductor of the bnc connector and the antenna rod? If someone has a known good antenna and can test it I would be most appreciative.

My experience with multiple new CI-122’s is that you will not be able to test continuity with a multi-meter between the base center pin and the antenna element; it will read infinity.
 
Jrskygod;1389730SNIP.....Funny thing tho the Garmin radio I had in there never had a com issue and when I replaced it with the Avidyne it worked perfectly for many months before I would occasionally get said:
Ok - so this is the real problem statement - not that you have an antenna problem.

A common (and simple) cause is a loose wire on the radio breaker(s). The radio will work fine on reception but drop out on transmit. A less common cause could be a bad breaker. Beyond these simple problems you may want to just try swapping out the radio with a buddies that has a known good one. This is a fast way to bracket the issue. Considering however your avionics shop tested the radio on the bench, this leaves wiring as the culprit. Take a close look at the tray pins to see if you have bent any, then start ringing out the wiring. Here is example of having a fully removable panel proves very valuable.

Carl
 
Take a good look at the tray/mating coax connector. The center pin mating spring fingers may be stretched oversize. It is possible the Garmin and Avidyne are slightly different.
Might eve be worth it to just replace it.
 
Out of the Box....

The avionics shop checked the antennas, 2:1, avidyne 440 units , two of them installed.
What I know about Avidyne is not a lot, but, they get real hot.
At OSH and Sun/Fun I could not keep my hand on the side of a 440.
I think that is about 140 degrees, electronic hardware does not like those temps....and your in Texas ( yes, it's November).

Even Garmin 327 transponders have temp sensors internal that will lower
the power output if pinged beyond limits.

My 430 does not run with a fan, but, If I did install an Avidyne 440 in it's place
a HUGE FAN would also be installed.

OK, enuff John
 
A less common cause could be a bad breaker.

Carl

Carl,

Good advice. I experienced this very issue with a bad breaker on a GPS/COM. I was ready to buy a replacement unit but decided to bypass the breaker as a test and..... bingo. Everything was okay. So after a new breaker was installed I was back in business. I have never experienced a bad breaker before or since.

Jim
 
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