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Portable nav antenna

rvdave

Well Known Member
I have a remote nav receiver I?m having intermittent problem with and desiring to hook up a temporary antenna to maybe put on the glare shield to determine if it is antenna or receiver problem. Do I need to obtain a cat whisker type antenna or build a homemade test antenna? Was remembering the portable comm radios with a vor/nav function operating with a rubber duck type antenna, would something like that work as a test antenna?
 
NAV signals are horizontally polarized, COMM signals are vertically polarized. That's why our COMM antennas run vertically, and the cat whiskers are oriented horizontally.

If you wish to build an antenna for test purposes you can do so with a pair of wires of the appropriate length run horizontally from a centre feed point. That feed point will need a device called a BALUN to convert the naturally balanced 300 ohm characteristic impedance of the dipole antenna to the unbalanced 50 ohm coax that you'll use to feed your radio. On-line suppliers of HAM radio gear stock these things and they're generally pretty cheap, under $10.

HINT: if this is just a quick test you can feed your test dipole antenna directly to 50 ohm coax. The impedance mis-match will result in the antenna being much less effective but it will still work for limited range.
 
The above is correct for a variant called a ?folded dipole?. For an ideal simple dipole, the feed point impedance is 73 ohms, close enough to 50 ohms for your purposes. It will be directional, with weak reception off the ends.
 
A tv antenna balun of 300 to 75 ohm should work, right? If I take two rods of appropriate length, attach balun at center fed to coax I?m thinking then that this should work for a test. Next question, what is an appropriate length?
 
You all are making this too hard.

Two pieces of wire, each ~25” long (standard dipole equation, total length (in ft)=468/freq in MHZ). In a perfect world feedpoint impedance is 75 ohms, but for us this is close enough to 50 ohm coax that the difference is not measurable.

Piece of coax connected in the center, one wire to the coax shield, one to the coax center conductor. Don’t bother with a balun.

Stretch the wire out from the center point - and it is fine to have the end droop down. So running this along the top of the fuselage with some tape is a simple approach. This now “inverted V” antenna also receives better on the sides than a horizontal dipole.

Don’t get wrapped around the axle on vertical vs horizon polarization. While this does make a difference in a perfect world (as in 100 mile range to hear a VOR instead of 60), for testing any resonant antenna is fine. Rubber duck type antennas however have an amazing amount of loss (there is no free lunch) but should pick up close VORs.

Carl
 
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What Carl said. If the goal is checking for live/dead and you have a good strong VOR signal available, just hook it up to a known-good comm antenna. Same coax, same impedance, and not that far away from tuned. It's receive only; you won't kill it.
 
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