[Antennas] Insulated Antenna Wire

Mon Mar 27 11:23:32 EDT 2017

When using insulated wire for antennas one should be aware of the 
shortening effect of the insulation. 
Years ago a friend took his Gem Quad down when he moved. Before putting 
it back up he sleeved all the antenna wires with thin Teflon tubing. He 
felt the Teflon might prevent ice from forming during ice storms. When 
we put the Quad back up the F/B on all four bands, 20, 17, 15 and 10 
meters were terrible. In fact the reflector tuning stubs were not able 
to improve the F/B. When we removed the Teflon tubing, maximum F/B was 
easily attained by adjusting the reflector stubs. The Teflon tubing made 
the Reflectors electrically too long.
Depending on the thickness of the insulation and the material, the 
shortening effect of the electrical length of the wire can be from 1-4% 
.
For example, if you are making a wire dipole for 20 meters centered on 
14.175 and use the formula 468/f the length would be 33.01' for bare 
wire. If you use insulated wire for that same dipole, it may have to be 
shortened to 31.69' based upon 4%.
When making wire reflectors or directors with insulated wire, the 
shortening effect of the insulation should also be considered for proper 
operation.
EZNEC does have a feature for entering wire insulation and calculates 
it's results based on your entries.
Here is a link with some more technical aspects of insulated wire and 
modeling.
http://www.antennex.com/w4rnl/col0105/amod83.html
Of course the best way to 'tune' an antenna is by physically adjusting 
the wire lengths at the antenna's final installed height for optimum 
performance either for lowest SWR, maximum forward gain or maximum F/B.
Jay   NE2Q