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[TowerTalk] 40-2CD performance and modeling topics

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Subject: [TowerTalk] 40-2CD performance and modeling topics
From: (Healy, Rus)
Date: Fri, 11 Jul 1997 12:43:32 -0400
I'm about to finally erect the reflector and boom of my 40-2CD, so I
thought I'd revisit some modeling topics I looked at several months

I found something interesting just the other morning (like it or not,
you get lots of *early* morning "awake" time when you have a new baby!).
I had optimized the original model some time back using K6STI's AO 6.0
software to find an optimum reflector length for bottom-of-the-band
performance (that's what I care most about; if the antenna works on SSB,
that's fine--but it's not important to me). In optimizing the antenna
and comparing its element tip lengths to what the Cushcraft manual says,
I found that my "best CW performance" model had considerably shorter
element tips than Cushcraft's recommended *SSB* settings! Knowing many
40-2CD owners, and Cushcraft, I couldn't believe that the antenna could
be that far off from the Cushcraft recommendations. If it were, the SWR
curve (as computed by W7EL's VFB EZNEC 2.0 software) would be just
awful. So I looked deeper.

Logically, the loading-coil values (S parameters) used in the models had
to be wrong--or both software programs were way off the mark, and I
simply don't believe that. I originally used 12.93 microhenries of
inductance with a Q of 161 (series resistance of 3.26 ohms) in the
models. When I stuck Cushcraft's CW element tip lengths into the models
with these inductor constants, I found that the antenna wasn't resonant
anywhere near the band. So, not knowing the origin of the 12.93
microhenry inductance number (speculatively, it may be something someone
calculated based on the number of coil turns, wire size and pitch), I
decided to twiddle the inductance until the antenna's SWR curve
correlated better with Cushcraft's claims (and the QST and CQ reviews of
the early '80s). I didn't fiddle with the series resistance (Q) number
from the unknown source of this data. I figured that, if I could get the
new SWR curves to make sense based on what I know of the antenna, I'd
try optimizing the design again with the new inductance values.

The magic inductance number turned out to be 12.0 microhenries. When
tested in the models with the Cushcraft CW, Mid and Phone dimensions,
this gives a good match to their claimed resonant frequencies, and the
bandwidths, gain and F/B are pretty much as expected (more on this

So I revisited the "optimized" element tip lengths, starting with the
Cushcraft CW dimensions. I left the driven element alone, since it has
very little effect on the pattern and, in my case, the driven element is
already on the tower; I don't care to reset the tip lengths tomorrow
morning when I'm up there solo putting the antenna together. It turns
out that if you reduce the reflector tip lengths from 80 to 77.43
inches, the F/B ratio at the bottom of the band goes above 10 dB. A good

Here's a table that gives the important details at 95 feet over average
ground, from EZNEC 2.0:

Frequency        Gain         F/B          SWR
7.000                10.2 dBi      9.3 dB    2.2
7.025                10.4          11.2          1.7
7.050                10.2          10.8          1.5
7.075                10.0            9.6          1.4
7.100                  9.7            8.3          1.5

7.150                  9.3            6.5          2.0

7.200                  9.0            5.3          2.6

7.300                  8.6            4.0          4.5

I may decide to tweak the response a bit lower in the band, but this
looks like a pretty good setup. My radio and amplifier don't really care
about a 2.2:1 SWR at the bottom band edge, if this turns out to be real.
I have a suspicion that it may be a bit high, since the Q in the models
gives results that show a slightly narrower SWR bandwidth than the QST
and CQ product reviews indicate.

I hope this is of some use to those of you with 40-2CDs and those
considering them, and to those who just like to play with antenna
modeling. (Thanks to K1KP for the original AO model.)

--73, Rus, NJ2L

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