Monday, June 18, 2012

Simple Antennas for the Amateur Radio Operator--a continuing series

More emergency antennas for hams:

There is a wealth of wonderful and somewhat curious antenna ideas in the amateur radio library.  I have several books published by the ARRL which I 've found useful in my restricted space environment.

As I was paging through "More Wire Antenna Classics, Volume 2" (Copyright 1999-2006 by the ARRL), I ran across two articles on using ordinary lamp cord for both antenna feedlines and antenna elements.

"A Zip-Cord Special Antenna" on page 1-6, Chapter 1 (taken from the "QST", May 1972) describes how one ham pressed about 80-feet of ordinary lamp cord into service as both feedline and radiator for  a 75-meter  schedule he was running with some of his friends.  Apparently, this operator was on vacation and lacked some materials to maintain his contacts with  his hometown.  He found that the emergency dipole performed well in his temporary location.  He also fashioned a version for 40-meters, which he found useful on 15 meters.  While this antenna was truly a compromise affair, it did allow this amateur to maintain a schedule with his friends at very little cost.

On the next page, 1-7, Chapter 1, Jerry Hall, K1TD, did a careful analysis of a zip-cord antenna at the ARRL laboratory.  His findings were first reported in the March 1979 issue of "QST".  Hall found that the zip-cord used as a balanced feedline had an impedance of 105-ohms with a velocity factor of 69.5 percent.  Hall believes you could use this type of feedline on 80 and 40 meters if you kept power low and the feedline length under 100 feet.  A graph constructed by Hall shows significant line loss above 7 Mhz. 

Hall says zip-line used as a radiator presents little problem for the operator.  In fact, the insulation on the wire could add to the durability of the antenna elements.  As Hall states, "How efficient is a zip-cord antenna system?  Well, that does depend on the length of the wire used for the feed-line section and on the frequency.  In a pinch on 160, 80, 40, and perhaps 20 meters, communications can certainly be established with this kind of antenna.  For higher frequencies, especially with long line lengths for the feeder, your're on your own."

And, there you have it.  Another idea worth trying if you're short on coax or someother kind of balanced line.  Although I haven't used an antenna fed with zip-cord, I have made my own balanced line feeders out of #14 gauge house wire.   My homebrew feed line wasn't pretty, but it did the job.

I trust your Father's Day went well.  Take care.  'See you down the log.

Aloha es 73 de KH6JRM--BK29jx15