LettersFromFlyoverCountry

LettersFromFlyoverCountry

Well Known Member
Has anyone done an analysis on the differences between various transponder antennae?

I use a B&C rod antenna for my Garmin 327, and locate it just under the pilot seat.

The signal from the antenna has never been particularly strong -- it barely passed the transponder test and that might've been graded on the curve.

On the trip East, I was at 9500 over Cleveland and approach was reporting intermittent transponder. Same thing north of Rockford. But no other reports of intermittent problems on the rest of the trip, except for flying near the Allegheny state forest and I think that's because I was pretty low.

I'm thinking maybe I'll rewire the antenna lede but I'm also wondering whether there's a performance to be had by changing to, say, a particular blade style?
 
I believe that the blades are nothing more than a rod antenna embedded in a resin or plastic. What I'd do is see if you can get an antenna analyzer on the feedline and antenna. I wonder what your SWR (standing wave ratio) is.
 
LettersFromFlyoverCountry said:
Part of the problem is it's $125 every time I have someone analyze the thing. I can rip through a lot of cash while probing possible fixes.
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Have you tried seeing if there's a local ham club in you area? A lot of those guys have SWR meters and might do it for free (before the snow starts, though).
 
XOverZero said:
Have you tried seeing if there's a local ham club in you area? A lot of those guys have SWR meters and might do it for free (before the snow starts, though).
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The issue is going to be a ham having an antenna analyzer that will work with high frequencies. I have one, but it only goes up to 400mhz or thereabouts.
 
Have you had the 327 bench tested for output power, they are not known for their high power output.

Make sure you are running a good antenna feed, RG400 (max 8 ft) with good connectors are mandatory for max power.

I have seen 1 or 2 bad rod antennas in my time, but not the norm.
 
If you assume it is mechanical in nature, you'll be right 90% of the time.
  • Check the cable (crimps/terminations, kink, etc.)
  • Run another known-good cable if possible (don't dress it in, just get it there however).
  • Check the antenna bonding.
  • Check for anything loose.
 
Walt said:
Have you had the 327 bench tested for output power, they are not known for their high power output.

Make sure you are running a good antenna feed, RG400 (max 8 ft) with good connectors are mandatory for max power.

I have seen 1 or 2 bad rod antennas in my time, but not the norm.
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Best advice on transponder signal problems yet.

:cool:
 
Mark Dickens said:
I believe that the blades are nothing more than a rod antenna embedded in a resin or plastic.
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This is not always the case. The Delta Pop antenna is not just a rod encased in plastic.

http://www.vansairforce.com/community/showpost.php?p=821769&postcount=8

That said, there's probably not a huge difference with a quarter wave rod and ball antenna. If the antenna is well bonded to the airframe to make a good ground plane, then the problem most likely lies elsewhere.
 
I switch from a rod and ball to the delta pop blade style. I have had no issues. The blade is not a rod and ball encased in plastic, it is a triangular shape and very thin.
 
Walt said:
Have you had the 327 bench tested for output power, they are not known for their high power output.

Make sure you are running a good antenna feed, RG400 (max 8 ft) with good connectors are mandatory for max power.

I have seen 1 or 2 bad rod antennas in my time, but not the norm.
Click to expand...

I used RG400, <8 feet and B&C connectors that tested fine. I figured I'd rebuild them to eliminate that as a problem.
 
C-FAH Q said:
I switch from a rod and ball to the delta pop blade style. I have had no issues. The blade is not a rod and ball encased in plastic, it is a triangular shape and very thin.
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You should learn something new everyday and this is my new learning for today. Thanks for clarifying
 
Bob,
Next time you are up flying and they tell you that the transponder is intermittent stay in that location and turn the aircraft through 360 degrees in both directions using shallow banked turns and see if it starts working. If it does then you are possibly getting some shielding effect from the gear legs.
 
B & C

LettersFromFlyoverCountry said:
I used RG400, <8 feet and B&C connectors that tested fine. I figured I'd rebuild them to eliminate that as a problem.
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I would look carefully at the B&C connectors. When I originally started wiring my -8, I used B & C purchased connectors that their website indicated were for RG400. I used a genuine AMP crimper too (older W. German made version). I was not real happy with the way the connectors "felt". I found that with just a few wiggles, I could pull the connector right off the cable. The outer conductor of the connector and the crimp ferrule were just not very secure. I then purchased some genuine AMP connectors specifically for RG400 per the AMP catalog from Mouser. I found that the outer conductor post of the connector was much longer than the B&C connectors, and when the ferrule was crimped over the braid of the cable it was much more secure. My bet is that your cable is making poor ground connections with the connector bodies.

Skylor

p.s. My GTX330 transponder has always tested with very high signal strength during the biannual transponder checks, and I've never had any indications from ATC of signal problems
 
The 327 is rated at 125 watts minimum, 150 watts nominal.
The 330 is rated at 125 watts minimum, 250 watts nominal

Bottom line, the 330 has quite a bit more power than the 327 and can tolerate more losses in the transmission system than the 327 and still pass the test.
 
skylor said:
I would look carefully at the B&C connectors.
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I agree with this. I bought a few BNC connectors from B&C (that's a lot of B's and C's) because they're nearby. Totally different material and general quality than the genuine AMP connectors. I scrapped them and purchased AMP brand.
 
skylor said:
According to Garmin's spec sheet the 327 is 200 Watts nominal, not 150:

https://static.garmincdn.com//pdfs/specs/gtx327_spec.pdf
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My bad, for the Garmin 327 only, the manual specifies 150W nominal with a 1.5db cable loss which would equate to about 200W.

They don't spec power that way in any other manual except for the 327.

From field testing I can tell that the 330 (which includes the 23/33 models) normally runs 50-100w over the normal 327.
 
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