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Author Topic: Use existing aircraft Comm antenna for 2 meters?  (Read 5390 times)
NZ5E
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Posts: 75




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« on: March 07, 2012, 05:29:35 AM »

I purchased a Comant CI-292-3 bent whip antenna, to be installed during the annual inspection on the belly of my Mooney Bravo. Ended up not installing it, mainly because of ground clearance issues when in a landing pitch-up attitude.

Has anyone tried matching into an existing aircraft Comm antenna? I have seen mention of antenna switches for the purpose of connecting a handheld radio to an outside antenna that should work for switching the aircraft Comm antenna from the GPS/Nav/Comm to a 2 meter transceiver.

My Mooney has the blade type Comm antennas for 118-137 MHz.
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K1CJS
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« Reply #1 on: March 07, 2012, 06:10:24 AM »

It can be done, but what of the aircraft band radio when you have the antenna switched to the ham rig?  That is, what if someone were trying to call you on the aircraft frequencies--especially for an emergency?  I would think that it wouldn't be in your best interest to do that. 

Also, although the antenna could be used for the ham bands, you're going to lost quite a bit of your transmit power because of the mismatch--or in the matching box you use.  All in all, it's best that you put a separate antenna for the ham bands on your aircraft.  That way, you have the aircraft band available all the time--as it probably should be.
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NZ5E
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« Reply #2 on: March 07, 2012, 07:14:29 AM »

It can be done, but what of the aircraft band radio when you have the antenna switched to the ham rig?  That is, what if someone were trying to call you on the aircraft frequencies--especially for an emergency?

The plane has dual GPS/Nav/Comms, both with standby frequencies, so I would only be using the antenna for one of them and would therefore always have the other aircraft radio available.  I usually fly by instrument flight rules (IFR) on cross country trips and would have someone (usually my son) monitoring the aircraft radio for frequency changes, etc.

...All in all, it's best that you put a separate antenna for the ham bands on your aircraft....

That is the reason I purchased the Comant CI-292-3.  I have operated with a 2 meter antenna installed on the top of the cockpit in an Archer and a TSO'd business band VHF antenna installed on the top of the wing on a Maule.  I was really wanting to try a belly mount antenna on the Mooney.

Another detail is that I have the Mooney for sale to trade for a helicopter.  I have several trips planned that I will continue to make in the Mooney until it sells.  I think it is worth going to the trouble of connecting into one of the aircraft antennas, even with some loss, without making any permanent modifications to the plane.  The blade type aircraft antennas that I have now are known to be broadbanded.  I am thinking that the match will not actually be that bad.  I am hoping that someone has tried this.
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KB4QAA
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Posts: 2450




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« Reply #3 on: March 07, 2012, 03:24:40 PM »

http://www.aviator.cc/bonanza/ci292-3.pdf
Data Sheet.

3:1 SWR from 136-174Mhz

Yes it will work.  The impedance mismatch will mean a lower efficiency, but you will still get out.

IMO it is not good idea to add switches and alternate transceivers in an IFR setup.  I'm not sure the FAA will approve this.   Better to add a separate antenna.

Don't forget to have your A&P send the 137 to the FSDO for installation approval of the switch, coax lead and radio.
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W0FM
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Posts: 2057




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« Reply #4 on: March 08, 2012, 10:05:44 AM »

I've used a compact home brew VHF antenna tuner similar to this:

http://www.youtube.com/watch?v=z3joEsPLOpg

with a coaxial A-B switch on the VHF Marine antenna on my boat to use that antenna on 2 meters.  It's a "set-it-and-forget-it" arrangement and works quite well.  When the switch breaks the connection from the marine transceiver and makes the connection to the 2 meter ham rig, the tuner is permanently in line for the ham rig and is already tuned.

I don't know why a similar arrangement would not work with your VHF aircraft antenna.  Good luck.

73,

Terry, WØFM
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NA0AA
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« Reply #5 on: March 10, 2012, 07:24:27 AM »

I'm no longer current on required equipment for flight in IFR, but IIRC, the FAA prohibits 'shared' equipment for amateur use.

I believe the only exception to this is connecting thru your audio panel.

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NZ5E
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Posts: 75




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« Reply #6 on: March 10, 2012, 05:44:49 PM »

I'm no longer current on required equipment for flight in IFR, but IIRC, the FAA prohibits 'shared' equipment for amateur use.

I believe the only exception to this is connecting thru your audio panel.



I had an avionics shop connect an IC-706 in my Archer through the audio panel as Comm 3.  I did the same thing with a Motorola M1225 business band radio in my Maule.  Both installs required an audio transformer to match into the 600 ohm input impedance of the audio panels.  It was very nice being able to key the mic on the yoke and listen/talk through the aviation headsets.  The aviation headsets have really good noise cancelling microphones.  People would sometimes not believe I was in an aircraft because they could not hear any background noise.  I have the Lightspeed Zulu headsets with active noise cancellation in the Mooney which really makes for a quiet ride.

My wife and I flew the Mooney to Ashland, Kentucky and back yesterday.  I tried a quarter wavelength of wire in one of the side windows with the shield of the coax grounded to one of the screws that hold the interior in place.  It worked some better than the rubber ducky on my handheld, but not by much.  Nothing beats an outside antenna with a clear view in a metal aircraft.
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