[TowerTalk] OptiBeam OB2-40M

[Wes]

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 at 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
>
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