[MRCA] Portable Antenna Woes

[Christopher Bowne]

Gene- don't forget about the standard formula for the length of a half wave dipole , 468 divided by the frequency in megahertz to determine the overall length in feet.  If you cant read those markers on your antenna, use a tape measure to lay out the antenna length for whatever frequency you are going to operate on.   The standard frequency formula will get you in the ball park, but the actual  frequency for lowest SWR will vary depending on factors such as height above ground of the antenna.   If you want to tweak for best SWR, use an in line SWR meter at the transmittter and slightly lengthen or shorten the antenna.  With your adjustable antennas that is easy, when working with a length of wire its always better to go at first try go long and nibble away at it and watch the SWR come down at each try.

I use standard dipoles that I have cut from 16 gauge stranded insulated wire in my field antenna kit.  To cover both the CW and phone ends of the 80/75 meter band, I have nominal 5 foot extensions for each end that I connect on to the antenna that I tweaked to resonate for best SWR on 3885 to resonate it down in the CW end of the band near 3570.  If you want to be fancy about it you can have insulators at the end of the 3885  inner section and put the 5 foot extensions out past the  inner section with a second set of insulators, and use a jumper to take them in or out of the circuit.  If you are  using braided nylon or other similar non-conductive guy rope you can just tape or wrap the extensions right on it outboard of the inner section when you connect them for up for CW operation.

To make it easier to measure out the dipole initial length, just measure one half of the overall dipole length to make up one half of the antenna and then use it as a template to cut the other half to the same length.

Of course if you want to make a 1/4 wave wire for a vertical/ horizontial NVIS wire, use the formula 234 divided  by the frequency.  I have used a 1/4 wave wire connected on my Tacoma's mobile antenna mount in lieu the normal whip and it will give you about 10 dB improvement over the whip, working against the vehicle body as a counterpoise, but even a very low full dipole or inverted vee is best in the field if you have the room to put one up.  In practice, the 1/4 wave wire usually ends up as a nearly horizontial wire from the feed point to however high I can get the end up over a tree branch, typically about 15 to 20 feet.  This  arrangement is better for NVIS work than a mostly vertical antenna. ________________________________________________________

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-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/

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