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francis.mandrake
20th May 2016, 14:34
My CFI told me something the other day that just doesn't make sense to me...
Please clarify if you know the answer to this one.

Skyhawks have two radios and two VOR's. Why are there two communications antennas but only one VOR antenna?

Here is what I was told:
One communication antenna is for transmitting and the other is for receiving. Since we only receive VOR, only one antenna is needed to serve both VORs.

BUT...
The Tomahawk that I used to fly had only one radio and only one communication antenna. Wouldn't that mean that that communication antenna is used for both transmitting and receiving?
Fly4Business
20th May 2016, 17:05
The two COM antennas on the Skyhawk are one for each COM and the reason is, both can RX and TX on their own antenna without the danger of one COM sending the TX directly into the RX part of the other. One antenna for RX and the other for TX is simply, a joke ... your CFI got you on that one :-).

One VOR antenna may serve for two NAV by introducing a duplexer to the antenna cable, because it is RX only, so far correct. But, the reason not to fit two VOR antenna is pure laziness. There are quite some Skyhawks with two VOR antenna, mainly the ones first fitted for IFR use on ancient avionics when there were no sufficient duplexer available.
francis.mandrake
20th May 2016, 17:12
That makes much more sense. Thank you.
nkt2000
21st May 2016, 10:17
That makes much more sense. Thank you.

The C172 Skyhawk I fly has two comm and two nav antennas. Comm on the cockpit roof and nav on the vertical stabiliser
Whopity
24th May 2016, 11:59
If two VOR receivers use the same antenna, then you do not have two independent aids.
A and C
29th May 2016, 17:55
That's how almost all light aircraft are rigged, one VOR/ILS antenna that is split at some place under the instrument panel to feed two VOR/LOC and GS boxes.

The latest Garmin split the VOR/ILS signal inside the box.

Mathematicly an antenna and length of coax cable is so reliable that failure is almost unheard of and far less likely than two independent failures inside the boxes.
piperboy84
29th May 2016, 19:04
Here ya go ! some further reading from an old thread

http://www.pprune.org/private-flying/542359-com-antenna-question.html