3
$\begingroup$

I recently purchased a 500-foot roll of military surplus 14 AWG stranded copper wire at a hamfest for next to nothing . It is Prestolite M13486/1-6 (spec sheet).

It has very thick chlorinated polyethylene (CPE) insulation. The outside diameter is nearly 1/4 inch (.235 to be exact). My plan is to use it to lay out radials for a ground mounted vertical, and my assumption is that it should work just fine.

However, my knowledge does not extend very far into dielectric constants (actually, my knowledge pretty much ends right after looking-up what the dielectric constant of chlorinated polyethylene is: 5.5 at 1kHz).

In the past, as many do, I have used 14 AWG stranded copper THHN for radials in ground mounted verticals with good results. As far as the copper conductors are concerned, THHN is identical to the Prestolite. Both are comprised of 19 strands of 29 AWG wire.

The difference between the two products comes down to the insulation/jacketing:

  • Prestolite uses CPE that is 57 mils thick. CPE has a dielectric constant of 5.5
  • THHN uses PVC that is 4 mils thick. PVC has a dielectric constant of 4.0

So, there are obviously significant differences between the two in terms of insulation/jacketing and by definition, they will perform differently. My question is: Will these differences result in any practical, discernable difference in performance?

| improve this question | |
$\endgroup$
3
$\begingroup$

Developing this comment of W2ASC (above): But, given the fact that radials are effective even when buried in a few inches of soil, my assumption was (and still is) that whatever difference exists between the two insulation types is - for all practical purposes - negligible. So, my question is probably a bit academic. I still believe my initial assumption was probably correct, I just don’t know how to do the math to confirm it. Can anyone confirm or refute?

Below is a comparison that might be useful, showing the effects of bare vs. insulated buried radials (other things equal).

enter image description here

| improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ I don't see the blue plot. I presume that's because the red plot is right on top of it, meaning that there is a negligible difference in performance between the two radial systems? $\endgroup$ – rclocher3 Feb 3 at 14:26
  • 1
    $\begingroup$ @rclocher3 Yes. You can see some blue if you zoom in. $\endgroup$ – Mike Waters♦ Feb 3 at 15:34
1
$\begingroup$

Thanks for joining us on Ham.SE! Insulation on wires will affect performance if they are part of the antenna's system of resonance.

Just as with coaxial cable, adding insulation to a wire decreases the speed at which electromagnetic effects propagate:

$$\lambda=\frac{vc}{f}$$

where $v$ is the velocity factor of the insulation and is less than or equal to 1.

To get an idea of the effect of the two insulating media you cited, I simulated dipole antennas in NEC 2. With no insulation, the resonant frequency of a dipole in free space was 14.16 MHz. With the thin THHN insulation, the resonant frequency dropped to 14.08 MHz, a relatively modest but measurable decrease. With the thicker Prestolite insulation, the resonant frequency dropped to 13.36 MHz.

The ratios of the resonant frequencies indicate the amounts by which you should shorten your radials if they are part of a resonant antenna system.

| improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ Thank you for your answer, and thereby causing me to refine in my own mind what I was wondering about. Since I am planning to use the wire as radials laying on the ground, electrical length/resonant frequency as a function of the insulation’s velocity factor is not terribly relevant, since a ground radial cannot be tuned with any level of precision in the first place. Putting any wire on the ground detunes it. What got me questioning the Prestolite’s insulation was how much thicker it was in comparison to THHN that I instinctively felt it couldn’t possibly work as well (cont'd) $\endgroup$ – W2ASC Jun 16 '19 at 15:33
  • 1
    $\begingroup$ (cont'd from above) But, given the fact that radials are effective even when buried in a few inches of soil, my assumption was (and still is) that whatever difference exists between the two insulation types is - for all practical purposes - negligible. So, my question is probably a bit academic. I still believe my initial assumption was probably correct, I just don’t know how to do the math to confirm it. Can anyone confirm or refute? $\endgroup$ – W2ASC Jun 16 '19 at 15:34
  • 1
    $\begingroup$ N6LF has dealt with this question in great detail. This and other sections of Rudy's site are recommended reading for all with a serious interest in the subject. $\endgroup$ – Brian K1LI Jun 16 '19 at 15:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.