Steve et all
So what is the "penalty" in DB (gain) for the using the increased
bandwidth settings on this beam vs an optimized for gain approach? For
the sake of discussion ignore the F/B degradation ...just raw dBd or dBi
gain comparison
Dave
NR1DX
On 5/22/2021 11:11 AM, VE6WZ_Steve wrote:
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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