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This is our new articles section. Over the past year I have written a series of articles for our club newsletter and at this time I will convert them to html and present them for here as time permits.BUILDING WIRE: Antennas out of common household electrical wire is very has become very popular because of the availability, low cost and in some cases the need for an insulated antenna. Wire such as No. 12 or 14, AWG THNN solid or stranded, poses the basic issue of what is the velocity factor for this wire and how long should I cut it for a given frequency?.: DETERMINING THE VELOCITY FACTOR OF COMMON WIRE :.
Normally when we use the formula of 492/Freq in MHz (a half wave in free space) we would get the length of the antenna wire in free space. However in all practical applications the electrical wave traveling down the wire is not capable of traveling at the same speed as in free space. Factors such as, the wires insulation, the capacity of the wire, end effect, height above ground, trees and other surrounding objects all effect the speed at which the wave travels down the wire. This reduction in speed is referred to as the wires velocity factor and is usually expressed in a percentage and referred to as “K”. To offset some of the factors affecting the K of a wire a compromise formula of 468/Freq in MHz or .95 velocity factor is normally used to determine the given length of a un-insulated wire. This will usually get you in the ball park and a bit of pruning will have to be performed to bring the antenna into resonance at the desired frequency. Depending on the actual K vs. the cut length one guesstimates this could be a time consuming effort.
What follows is not quite as accurate as knowing the true K of a given wire because you don’t know what the reactance or the phase angle of the wire is but still yields a suitable approach and does work very well for amateur purposes.
As stated the number 468 that we all use assumes a .95 velocity factor which accounts for end effect and other proximity effects. The base number is actually 492 which is a half wavelength in free space. The corrected figure of 468 is 492 X .9512 (K). Understand that the velocity factor for any conductor varies slightly depending on its surroundings.An antenna in free space has one velocity factor while the same antenna close to the ground has another. Put up an antenna using the formula 468/F. If the antenna doesn't work where you want it to, set the rig’s power to 5 or 10 watts and tune across the band while watching the SWR. When you get to minimum SWR you have found the operating point for the antenna. Notice I didn't say resonance, because minimum SWR does not always occur at resonance.
OK, now multiply the frequency of the lowest SWR times the length of the antenna. This will yield a number either higher or lower than 468. Now divide this new number by the frequency you wish to operate on and it will give you the correct length to cut your antenna too. This assumes you will put the antenna in the same place at the same height. The good thing about this is it will save you hours of lowering the antenna to prune it and putting it back up again. The velocity factor of your wire antenna is simply the new number divided by 492. So if you got a number of say 453, divide 453/492 = .92(K). Now if you use the same type of wire for additional antennas you will achieve your goal of minimum SWR at the operating frequency of choice with much less effort.
EXAMPLE:
Operating frequency: 03.990 MHz
468 / Freq. in MHz
468/03.990 = 117.293 Ft.
Frequency of lowest SWR as observed on an SWR meter while transmitting = 3.870MHz
Length of the antenna =117.293 (original antenna length) X 3.870 MHz (lowest observed SWR) = 453.9, divide this new number by the desired frequency. 453.9 / 3.990MHz = 113.765 Ft.Cut the antenna to this new length and it should be very close to minimum SWR at your desired operating frequency.
Now divide 453.9 (new number)by 492 (base number) = 0.912 this is the velocity factor for the wire you used. Besides the time consuming task of multiple trimmings the big payoff is that as long as you stay with the same wire you have an accurate velocity factor for future antenna construction projects. While I have yet to try this approach on a OCF (off center fed dipole) i.e. A Windom antenna, the process should work. Just compute the overall length of the half wave wire and then offset the feed point 30%. WA2NTK
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last update03/23/10