Hi Steve,
PVC or similar jacketing adds about 2 - 3% of electrical length to the
wire. I don't know why, but it does consistently. In my experience
with making wire 4-squares, and putting aside coupling between the
elements, if I want the antenna to resonate ~ 3.55 mhz and I am using
#12 THHN with the PVC jacket, I would ideally cut the elements 2%
shorter to resonate ~ 3.62 mhz.
It's not exact, but it produces very close results. The best strategy
for me is to cut the elements for the normal length, measure the
resonant frequency (minimal dumped power) on the array and then I am
prepare to trim ~ 2% of the length off of off each leg and then
remeasure.
Example... I make 4 wire elements out of #12 THHN and the minimum
dumped power is ~ 3.500 mhz - 2% too low.
I then multiple the physical length of the elemnent x 3.500/3.550 and
then subtract that from the length of the cut element. If that is 1.5',
then I cut 1.5' off each element and remeasure the dumped power and see
at freq. it is minimized. This is actually a better method that the
first one, because you are making a length adjustment based on a known
length of wire and actual resonant freq. and then just setting up the
ratio of correct element length based on a known measured resonant
length. But either method works quite well and is easy to do.
73
Bob, KQ2M
On 2022-09-19 16:56, Steve Maki wrote:
Wondering whether there is any substantial difference between #12
solid THHN and #12 PVC jacketed copperweld (steel) in terms of cutting
a dipole to length.
I'm in the process of installing a K8UR style 80M four square. I
trimmed the first element to length using THHN. Then I decided that I
should use steel wire for at least the top half of the elements
because of the high wire tension I ended up with after pulling the
feed-point out far enough from the array center. It's an issue with
not enough acreage around the tower.
Even so I ended up with a 50 ohm antenna rather than the desired 30-40
ohm antenna per the DXE instruction manual. But that's an entirely
difference issue (though I'd appreciate any comments on that as well).
Should I expect the steel wire lengths to be pretty close to the THHN
lengths, or should I re-do the initial step of carefully cutting the
first element in isolation?
TIA!
-Steve K8LX
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|