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Old 08-06-2018, 05:59 PM
Electrogunner Electrogunner is offline
Join Date: Aug 2016
Location: Quarryville ,pa
Posts: 393
Default Transponder antenna

I am installing my transponder ant just aft of the firewall through the tunnel. Well, where it needs to be because of my fuel pump and plumbing it ends up under the fairing that extends back under the fuselage, on my show planes cowl. Iím useing a lollipop ant. Iím thinking about making a bracket that will mount to the bottom of the fuselage and face the antenna forward at a 45 degree angle thus hiding it under the fiberglass fairing. What is the consensus in regards to the signal, since it will not be facing straight down.
RV10 N620RV
IO540 C4B5
Whirlwind 375HRT
Garmin G900X
First Flight 2/14/2019
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Old 08-07-2018, 07:32 AM
Bill.Peyton's Avatar
Bill.Peyton Bill.Peyton is offline
Join Date: Sep 2009
Location: St. Louis, MO
Posts: 1,675

Doing so would compromise the performance of your transponder. The quarter wave stub vertical antenna you are using should be mounted perpendicular to the ground plane. A sufficient ground plane is required for this antenna to operate properly. The size of the ground plane required for optimal performance of a !/4 wave monopole antenna is determined by the frequency you are transmitting, in this case, around 2.5" radius is required. Mounting it at 45 degrees, or any angle other than perpendicular for that matter, will affect the impedance of the antenna, thus increasing the reflected power (SWR). This may result in damage to the output amplifier in the transponder. I suspect that some of the newer solid state transponders may have built in protection for this.
Could you possibly mount it under the pilot or passenger seat?
Bill Peyton
RV-10 - 950 hrs
First Flight Oct 2012
Aviation Partners, LLC
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Old 08-07-2018, 11:24 AM
togaflyer togaflyer is offline
Join Date: Apr 2013
Location: Cleveland Ga
Posts: 518

Mine is installed under the pilot seat. Works good and easy to pull the cable there.
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