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Emergency "T" in antenna lead?

R

Rotary10-RV

Well Known Member
Guys I need radio antenna advice. I am capable of wiring and setting up my radio antenna to spec but I wanted to ask somebody about a possible emergency antenna set up.
I have a icom hand held with the BNC antenna connector. I was wondering if the antenna coax can have a "t" in it going to a panel mounted BNC bulkhead connection. All shielded of course, with a cap when not used. Keeping a short patch cord to connect to the hand held if the main nav/com bites the dust in-flight. I am not sure if this wouldn't mess up the SWR and be inadvisable. Doubt if it would ever be used but you never know. If you need to connect to the main antenna it is almost impossible to reach behind the panel and do it while flying! I am building an RV-10 and I know I'd be banging on the stick if I tried such a thing! Suggestions?
Bill Jepson
 
DO NOT USE A SPLITTER OR TEE!!!!!

You have to isolate the unused radio lead, otherwise your pushing full RF transmit power into the unused radio!!!!!

Another option to the unit described below is an 2 position antenna switch as used by Ham operators. Mount in a convenient location, plug your handheld in when you get in the plane, and flip the switch when you need to. Not much different than an audio panel switch.

They have a very low insertion loss (<0.2 db), cost from $20 and up, but you will need to get some PL to BNC jacks, and make sure you get a unit capable of at least 150 MHz. If the switch is mounted conveniently enough, no reason not to keep both radios going at the same time (however, unlike the audio panel switch, you need to KEY the right radio!)
 
Thanks to all who responded to this post! I have some radio knowledge. (addmitedly a dangerous thing) I thought of using a switch box as well but was unsure if they hurt the radio output as well. The King box is a nice looking idea. The aeroelectric a functional one! Does the mini jack cause losses, or is it negligible? I like the solutions of some of the -6-7-8 guys where they mount the external antenna just forward of the spar and have direct access to the BNC if they have a radio failure. I don't think this option will work on the 10 since the front seats are so much further in front of the spar. Frankly forgot about pushing a signal into the dead radio Dave, thanks for the heads up. I'll try to start thinking RF again. I rather prefer the BNC connectors, (I can't put it on wrong as easily), so I will probably look for a switch box at a ham store. I wonder if they have one with a protected knob. I'd have to switch it by accident key up and blow my Garmin! Thanks again for the suggestions.

Bill Jepson
rotary powered RV-10
Working on Empecone, wings in box on floor!

ddurakovich said:
DO NOT USE A SPLITTER OR TEE!!!!!

You have to isolate the unused radio lead, otherwise your pushing full RF transmit power into the unused radio!!!!!

Another option to the unit described below is an 2 position antenna switch as used by Ham operators. Mount in a convenient location, plug your handheld in when you get in the plane, and flip the switch when you need to. Not much different than an audio panel switch.

They have a very low insertion loss (<0.2 db), cost from $20 and up, but you will need to get some PL to BNC jacks, and make sure you get a unit capable of at least 150 MHz. If the switch is mounted conveniently enough, no reason not to keep both radios going at the same time (however, unlike the audio panel switch, you need to KEY the right radio!)
Click to expand...
 
Being an electrical engineer and ham radio operator, I'd have to recommend against that contraption put together by AeroElectric Bob... It will introduce losses caused mainly by impedance mismatches. The big problem is that you can't know how good or bad that little switch box will be until you make it. with loose wires inside a randomly sized and shaped box the impedance of that little connection could vary wildly, and could even change with time as the wires move around in there. And where there are impedance mismatches, there are signal reflections, causing some of that transmitter power to be sent back into your expensive garmin (causing additional heat, etc) instead of going out the antenna (a.k.a. loss). And if this is significant depends on your definition. A 1-3 dB loss wouldn't be unreasonable for a box like that, which doesn't sound like much, but -3dB=1/2 power. (This is a complete guess, though, it could be much less or even more).

Your cheapest solution to this would be to buy 1 male and 1 female BNC coax connector (the females are a little more of a rare beast, but can be found). Cut your comm coax in an easily accesible location and put the female connector on the antenna side of the cut, and the male on the radio side. If you need to switch to your handheld, simply undo that connection and connect the coax from your handheld to the female BNC going to the antenna...
 
Suggestion #2

Consider easy access to the BNC connector on the antenna to attach a coax jumper to the handheld in case of emergency.

Someone pointed out a great place is to put you comm. antenna is under the belly off set to one side just forward of the front spar. Location of antenna in floor of the cockpit, between your legs, makes it easy to access. The BNC on the antenna is removed, which is goes to your panel comm. If your panel comm. is dead than it will not need the antenna. A short coax jumper can connect the handheld transceiver. If it's for emergency use only than that would be a good compromise. If it close to the spar it will be out of the way or you could add a little cover or guard.


For VFR and even light IFR a single Comm. TX/RECV is good enough. Comm. radios are real reliable. So the above method is good enough for the rare emergency backup use. If you want the get ATIS with the handheld an external antenna is not really needed. I agree all the little junction/switch boxes and chance of pumping RF right into the radio by accident and losses is not worth.


Occasionally it is nice to monitor a second freq. I find the rubber ducky on the handheld is good enough for this. However if you want more full-time roll for your handheld, you can have a dedicated second antenna. Consider a foil tape antenna primarily for RECV only and occasional TX. The foil tape should work better than the rubber ducky but will be less efficient compared to the belly bent whip.

Cheers George
 
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