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Dave's RV-7A Builder's Log
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Date: 2-20-2016
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Number of Hours: 7.50
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Manual Reference:
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Brief Description: ADS-B & transponder antennas
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A beautiful February day in Kansas and I took advantage of it! Out to the hangar by mid-morning with the goal of figuring out the two antennas that would be connected to the ADS-B and transponder. As with all of the antennas—especially those that radiate—maximum spacing from each other is the objective. There are a couple of other conditions that must be met—for the transponder, the maximum cable length for R-400 is 6’1”…but the antenna must be a minimum of 3’ from the box. This put the antenna squarely in the 3’ radius of both comm antennas. So…I decided to put the ADS-B antenna on the right forward part of the lower fuselage, just inboard of the fuel vent. And, the transponder antenna just aft of the pitch servo bracket, in the middle of the aft fuselage. With this placement, everything is at least 3’ apart, but the only challenge will be that the transponder cable will be closer to 8’ in length. Two options if that is an issue—first, swap the ADS-B and transponder to the other antenna…or second, get more expensive cable and buy myself a little better functionality for the transponder. We’ll see once I power everything up.
Fabricating doublers followed the same basic sequence used during the comm antennas—the biggest challenge was that I lacked enough heavier stock to really make a good doubler for the aft antenna. To solve this, I decided to use some surplus angle stock and attach to the center rib. My logic was that I wanted to rivet the double to the rib and floor, thus giving me maximum support while also allowing me to put a hole in the doubler to allow the coax to pass through and proceed forward without conflicting with the elevator push rod. The picture shows the final result…it’s certainly stout enough. All that’s left for the forward doubler is dimpling and riveting.
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Forward antenna doubler
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Aft antenna doubler
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Forward antenna test fit
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