Wes,
You are correct.
The original numbers for the CC coils from Brian's program were far too
pessimistic, especially regarding his form loss algorithm.
Brians new coil program is more realistic.
In my opinion, the biggest "gain loss” on the XM-240 design comes from tuning
the reflector too low in frequency in order to maximize the SWR bandwidth, and
achieve a direct 50 ohm feedpoint match.
CC did this because it yields a nice plug-and-play Yagi with a huge SWR
bandwidth that will cover the full band from CW to SSB.
Of course, the “problem" with retuning for maximum gain and F/B is a narrow
SWR bandwidth that will not cover the full band and therefore the need for
relay switched inductor element tuning to move across the band.
This is really not that difficult to implement, but likely more complex than
most operators want.
Also, a feedpoint matching network will be required (hairpin or L) to match the
lower feedpoint impedance.
Steve, ve6wz
> On May 22, 2021, at 8:40 AM, Wes <wes_n7ws@triconet.org> wrote:
>
> 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
>>
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