External Antenna Access

[Romeo Tango]

I'm upgrading the avionics in my C170 (not too much, just to add a bit more safety features). One thing that is very important to me is an antenna jack to get to the external antenna with my transceiver. Having had complete electrical failures in other aircraft, and then having my handheld only get out about 3 miles, I really want to be able to connect my handheld to the airframe antenna.

The installer is convinced that you need to put in an entirely separate second antenna system, but I am equally convinced that if you put in a t-junction on the antenna jack to a BNC connector on the panel, and then make sure the panel-mounted radio is turned off, the handheld will be just fine driving the standard antenna.

Any guidance or feedback here?

[Bruce Fenstermacher]

I'm not an electronics/radio guy but what I understand from a friend who knows is that those splitters have electronic components tied between the input legs, which make them work. It is not simply a y connection like you might make with a light bulb circuit.

These splitters when installed actually slightly degrade the performance of both radios connected. So you would have less performance from you main radio all the time. This may or may not be significant. Does it matter that you can transmit and receive 40 miles when you may get 60 miles with out the splitter.

I'm not sure but I could guess that having and open connection like a BNC connector may open up your system to all sorts of radio interference or degraded performance. Also I believe those splitters are only made for nav systems not for com and that is for a reason. Believe me I asked cause I didn't want to buy and install a second antenna for my second radio either.

BTW you can probably find an antenna for what those splitters cost unless you have an electronics friend who can show you how to modify a $6 radio shack splitter.

Here is a thought. Have your avionics guy run a little extra length in your antenna coax. Loop this down just under the instrument panel near the front so that you could easily reach up under will flying and pull out the excess. With a BNC coupling in this area of the coax, you could easily unplug your main radio and plug in your hand held radio. Your avionics guy won’t like this either cause it's another connection which leads to resistance and degraded performance but it's better and cheaper than a splitter.

[Dave Clark]

Bruce has the right idea with the extra loop access. I've done this a lot in my planes and it's a simple cheap way to do it. Plus no paperwork required! I would not use a splitter nor would I want to drag around or look at another antenna. The ELT antenna will also work though probably not as much range as one of your com antennas. I used an elt antenna and a King handheld as my only com in one of my Champs very successfully.

[GAHorn]

RT, your avionics shop is telling you right. A handheld is already weak (as you know and is the reason you want access to an external antenna.) If you feed it's output into a dual-ended piece of coax with one end at an antenna and the other end plugged into an existing radio, you will lose more RF energy than if you'd simply used the handheld's rubber-duck antenna. Part of your RF output would feed into the circuitry of the panel mount radio, and even if turned off may damage that panel mounts output section. I'd encourage you to do this right and install a dedicated external antenna.
One solution I thought was beautifully thought up by an online friend who works at an avionics shop..... He replaced his ELT with one of those units that had it's own antenna which remained internal to the cabin. (An EBC unit perhaps?) Anyway, it left an unused ELT antenna on the external surface of the fuselage. He simply turned that unused ELT antenna into his handheld antenna and it works great.
If that solution doesn't fit your fancy, then I'd suggest a belly mounted bent whip, which would give you a good airborne ground plane and would be easy to access and install thru the floor inspection plates. The simplest, metal, (porcelain mount) antennas have no RF gain, so may not be much improvement, so consider one of the more expensive fiberglass antennas for maximum performance. You can view antenna choices at:
http://www.comant.com/ and http://www.avionix.com/ and http://www.aircraftspruce.com/

[N1478D]

Avionics West in Santa Maria, CA has a solution - the ICOM IC-ANTSB switching box. It allows the operation of one external antenna for both handheld and panel mounted radio. The switching box prevents reverse transmissions from the handheld into the panel radio. It's priced at $79.00. Comes with an adapter cable that replaces the rubber duck stub antenna on the radio and plugs into the switchbox. The assembly takes about 4" of clearance and the adapter protrudes thru a 5/16" hole. The test review claimed an easy 35 mile range with an ICOM, JRC, or YAESU handheld. Avionics West 805-934-9777.

Sidebar Note on "T" Fittings: "There may be some folks out there saying this is overkill. "Why I can just go out and buy a $10 "T" fitting and accomplish the same thing?" No can do.
While you can make the mechanical connections, transmitting it works very poorly. When there is a simple, constant mechanical connection and you try to transmit, the small signal from the handheld will try to go up both coax cables-the antenna and the cable to the radio in the panel. This tends to attenuate the already weak handheld signal."