Unfortunately, there is some bad info there. IIRC Brian has updated his coil
program several times and the answers are much different with the newer
version. The Cushcraft coil is nowhere near as bad as reported. Steve's
measurement "system" is highly suspect. Measuring physically large inductors is
far from trivial. For example see: www.hamcom.dk/VNWA/How to measure the
resonance frequency and Q of an air coil-rev1.pdf
Another question to be asked is whether higher Q is actually necessary. My
modeling, using AC6LA's wonderful AutoEZ, which allows all kinds of "what ifs",
allows the exploration of this. For example I modeled a 40-meter dipole, 60'
above average ground. I don't know Cushcraft's element design or coil placement
but as a guess I placed loading inductors at 50% of the length of each half
element.
Using the supplied dimensions, which I have not verified, K6STI's coil program,
and my favored ON4AA air-coil inductance calculator, I come up with ~ +j370
with Q = 285 with Brian's calculator and ~ +j400 with Q =320 with ON4AA's
calculator. Also note that going from an air to fiberglass former makes very
little difference in Q, contrary to popular belief.
Placing loading inductors of +j370 at 50% out on each half element and using the
resonating function in AutoEZ to adjust the tubing length for resonance at 7.1
MHz I get 22.5 feet per side. By serendipity, for this length and height above
this ground, the match is nearly perfect. Now I setup a variable for Q and
incremented it from Q=250 to Q=1000 in steps of 50 and looked at average gain at
each step. (Average gain reports the less than perfect results due to losses in
the element)
For Q = 250 the average gain was -1.28 dB. For Q = 1000 the average gain was
-1.12 dB. So, for this case, the "gain" from going from a relatively easy to
construct coil to a heroic coil is less than 0.2 dB. Is it worth it?
As always, I standby to be corrected.
Wes N7WS
On 5/21/2021 5:28 PM, john@kk9a.com wrote:
VE6WZ has an interesting webpage on Cushcraft's loading coil construction
(68 turns of 12AWG wire) and its loss:
https://www.qsl.net/ve6wz/CC_coil.html
John KK9A
Dave Thompson K4JRB wrote
That optibeam sounds great but I reburbed my old 2 el Cushcraft 40 and its
still a killer. I see no reason to change.
The cushcraft usually beat out a 3 el Telerex at n4RJ and several 4 el KLM
40's nearby. It worked so good that Tom N4KG (SK now) in Huntsville called
it a killer antenna and ordered one for himself. And no I left it alone and
did not modify it as a moxon.
73 Dave K4JRB
_______________________________________________
_______________________________________________
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
|