REFLECTOR: Transponder Antennas

[John Tvedte]

Craig,

Attached are the cable spec requirements for the GTX-330.  Note that max. loss in the cable is not to exceed 1.5db.

Typical RG400 attenuation @ 1000Mhz is .169438 db/ft.  Garmin specifies 8.8 ft max = 1.491057.  (The transponder is actually 1090Mhz).

Typical RG58c/u attenuation @ 1000Mhz is .222748 db/ft.  One can divide this by 1.5db = 6.734 ft.

I would check your cable lengths - and make sure you are within length limitations for the cable that you used based upon attenuation at the transponder freq.

You may be getting signal blockage - the area the factory suggests placing the transponder antenna is a good location - but with a GTX-330 one will need to use a low loss coax.

John

Please check with the radio shop regarding the antenna.
I understand that that the Garmin 330 requires a special antenna; using a high quality transponder cable.
I don't think that a home-made or Archer type transponder is going to work with the 330.
SB
----- Original Message -----
From: Craig and Denise Woolston<mailto:cdwoolston at sbcglobal.net>
To: 'Velocity Aircraft Owners and Builders list'<mailto:reflector at tvbf.org>
Sent: Monday, February 25, 2008 8:05 AM
Subject: REFLECTOR: Transponder Antennas

Hey all-

  John and I have a homemade ¼ wave transponder antenna mounted under the battery in the nose of the aircraft (completely) inside.  When we first started flying we got occasional complaints here in SoCal about losing our beacon.  Now it happens much more frequently and it appears to be when we are low and in the “ground clutter”.  The frustrating part is the GTX 330 shows a reply light even when ATC says they can’t see the code.  They are getting a primary but no code.

Where have other located their antennas and what type?

Has anyone tried the Archer or Advance Electronics Antennas I see in AS?

Craig

250hrs…XL-5FG Flying in Lancaster, CA


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