Some lighter ideas and may be cheaper
647jc said:
I bought a 20 ft extension cable with fittings already on it but would really like to make my own of the proper length. The auto specialty place where I bought the 20 ft extension cable also told me if I ohmed the 20 ft cable conductor from end to end it would measure 'open'.
(What? 
) He said
this is correct for a long cable otherwise the length would load the receiver down and it would not work properly.
(What again?) Any advice on FM radio antennas, coax and fittings would be appreciated.
Thx,
J.C.
Everyone already addressed the advice audio guy gave you, thumbs down. FM antennas can be a 300 ohm (those flat two conductor wires) or 75 or 50 ohm antennas. The coax must impedance should match the antenna, but the length of the coax is not critical except the longer the more loss you get. I think that's where some confusion is from.
Like Warren, I used a portable radio and plugged it into the intercom. The wire coming from the head set jack (the headset or patch cord) was actually doing double duty as the antenna. It worked pretty good. The point is flying at 8,500 feet, a little foot and half wire in the cockpit got plenty radio stations. A coat hanger will work. However doing 190 mph TAS you go thru radio stations. Assuming typical fringe reception range of 100 miles, you are looking at 60 minutes max per station (probably less) as you scoot by X-C. I used the radio pre iPod. With the iPod I don't bother with radio, but sometimes its nice to get a game or radio show, so I'll pull the portable battery powered radio out.
You can do any damage to the radio with the antenna; it just won't work well, so have fun with it. It just an FM receiver.
I'd suggest may be consider something other than 20 feet of coax and a wing tip antenna. Nothing technically wrong that, should work great. I'd just consider the weight, 3.6 lbs for the coax alone?
I've got some ideas you might like, and it will not require the nightmare of running coax all over the plane (I'd avoid it like the plague):
Windshield antenna that could sit on glare-shield, off of roll bar or under cowl.
http://www.crutchfield.com/S-lBbFR7...=232750&I=12044UA200&search=am+fm+car+antenna
-or-
A "Hidden antenna, not sure how it works, its a new thing, but came up in a search.
http://www.crutchfield.com/S-lBbFR7...g=232750&I=12044UA20&search=am+fm+car+antenna
How about a real coil loaded antennas that are shorter than a typical full length FM antenna 26 inch to 32 inch:
http://redlemon.manufacturer.globalsources.com/si/6008816074481/pdtl/DVB-T-antenna/1002509715/FM.htm
or
http://www.installer.com/antenna/ant.html (scroll down)
or
http://www.installer.com/photos/44-uag10b.jpg
[You could put any short antenna, in the bag area (RV6/7) with the element next to the plexi. You could just put it under the cowl off the firewall or engine mount, RV-4 next to roll bar.]
On the "roll your own" front, you can take copper tape with adhesive back and place it on the inside of the canopy. People make VHF com antennas this way. Length for 87.5 to 108 MHz, about 25 to 29 inches for a 1/4 wave antenna. You can go shorter it just reduces the gain at the higher frequencies but it will not reject those frequencies. Solider center coax conductor to copper tape, The shield? Usually you ground it to the airframe, but it might work OK with out grounding it. You can by two yards x 1/4" copper tape for $2.00 on ebay.
http://www.semsupplies.com/Copper Tape-CCT-3M.html
Cowl? fiberglass the wire (antenna) in bottom of the cowl of proper length. When you remove cowl, you'll have a plug to disconnect it (no big deal, takes a second). A friend has a VOR antenna in the cowl bottom, works OK, about a 5 or 6 out of 10 reception. I would make what's called a Dipole antenna (see pic below) for the lower cowl. Its just wires of proper length. A dipole is two 1/4 wave or 1/2 wave antenna, so D+D is about 57" ideally. This is an elegant dipole and very efficient. If you can lay that down across the forward part of your cowl you might get some real range. Simple is good.
Gear Leg Fairing? Like the cowl you could make a dipole with one wire running down the front of the gear leg fairing and looping back-up the back of the gear leg faring. Note the feed wire of a dipole is in the middle not the end. This would be 70/300 ohm so you'd might need a matching transformer to 50 ohms, but it may not matter, its just a FM radio receiver. The worst you can do is loose gain or sensitivity. The gear leg bing shorter the antenna will be under the ideal 57", but the wire can run into the cowl area to get close. The possible down side? It could be directional, may be, may be not. The metal gear leg is not helping for sure. Others have tried this for their VHF comm radio. As a transmitter antenna it worked but poorly as I recall. Still it might be acceptable for FM radio reception. It will be hidden.
A homemade antennas could be fun, cool and educational. Its part physics and part black magic. I like the self contained store bought units, but reception with a good tuned homemade can be very good. As I said I doubt you can do any damage to the radio with the antenna; it just won't work well, so have fun with it. Your plan will work but the 20 feet of coax (3.6 lbs) and hassle installing it or removing is the down side I see. It's just a FM radio.
