Yesterday I made the comment to K4GMH and NR4M that I rather liked working
with the older shorty 40 designs such as the CC, Mosley and KLM because they
are essentially turnkey. That is one puts them on the tower, whether at 50
feet or a 150 and they seem to perform quite well.
The comment comes after spending countless days putting ef240Xs on K4GMH's
130 ft tower and tuning (?) them. Fortunately we eventually developed an
easy means to tilt the driven element and reflector into a position making
them easy to reach.
I think we're done with Mike's antenna, at least until Spring. The top
antenna is at 140 feet and the bottom at 80.
The top antenna was essentially done trial and errory as was the bottom one
initially. Yesterday we reverted to removing the shorting bar on the
reflector and tuning it to 6925khz or so as recommended by F12. Then after
replacing the shorting bar, we worked with the driven element hairpin and
linear loading to get a good match around 7050. Mike's primarily an RTTY
(darn good one at that) kind of guy, but he does a lot of cw, so we won't
hold it against him!
Now, in comparing the two antennas we find that the 2 to 1 SWR points on the
top antenna are significantly broader than the bottom. Initial checks that
are far from scientific seem to indicate that the top antenna does have
pattern and F/B. The same checks on the bottom antenna seem to indicate a
sharper pattern and better F/B.
I think with time that Mike may get to the point of determining whether he
likes performance of one antenna more than the other. I also expect that in
dealing with the heights that he is, the difference in capacitive affect to
ground will be insignificant, so we could reset the linear loading and the
hairpin on one to closely approximate the performance of the other.
In operation, Mike has been very happy with the top antenna. I expect he'll
be equally pleased with the bottom one.
At my location I put a linear loaded KLM 2 el 40 up about ten years ago.
Again it's not scientific but it's got F/B on the order of 12-15 db and 2:1
swr points easily cover the lower 150khz of the band. While the vswr is
around 2.5:1 around 7200, the tuner in the transceiver doesn't mind and it's
been quite competitive in the few fone contests that I really participate
in. It's at about 75 feet. The point is I put it up and haven't touched it
Likewise we put a 2 el Mosley coil loaded 40 up at NR4M about 10 years ago
and it played very well, except when hit by lightning or when a balun
developed problems. Again we put it up and had to do no tuning.
Based on the experiences of K3NC, the XM240 requires no tuning as well. See
recent discussion on the tower reflector regarding ground the reflector on
the XM240 to better approximate ithe small interaction with other antennas
experienced with the earlier CC antenna.
My gut feeling is there's probably not a lot of difference in performance
between the antennas discussed. The drawback of the ef240X is obviously the
need to tune it. Having said that, it is quite light and if you don't have
obstructions on the tower, it's easy to tilt it to reach the elements for
adjustment. F12 provides an information sheet that gives good guidance on
tuning the antenna based on resonating the reflector at a specific frequency
and then tuning the driven element to resonance/match.
73 de Larry K7SV
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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