OK you two (Ward and Steve), time to do this test over again with more recent
antenna offerings. I'd like to see the stepping kind of antenna included, and
a log, plus some others. Pick the top three or so from the old test and line
them up against the new ones. I'd buy that book!
Chris
KF7P
On Oct 16, 2012, at 16:12 , Ward Silver wrote:
> Mosley's PRO-57B claim of 8.5dBd gain on 20m with only a 24' boom is
> incredible
That gain spec originated many years ago when it was common practice to include
ground gain (up to 6 dB) in the figure. The practice of only using free-space
gain in dBd is a relatively recent innovation that came about after antenna
modeling became widely used. Without the ground gain included, a free-space
gain of 2.5 dBd (4.65 dBi) on a 3/16-wavelength boom is quite a bit more
reasonable. (It's probably a little higher than that with no effects from the
feed line causing possible pattern distortion.) On 20 meters, the PRO-57B was
within reach of the other designs and you wouldn't have noticed great
differences on the air.
As a useful benchmark for sanity checks, I recommend the optimized set of Yagi
designs developed by Dean Straw, N6BV for the ARRL Antenna Book. The program
YW (Yagi for Windows) gives reasonable, reproduceable gain figures for Yagi
antennas. The venerable NBS Tech Note 688
(http://tf.nist.gov/general/pdf/451.pdf) also provides some guidance about what
is practically achievable.
Remember that fixed-spacing tribanders are going to be a compromise on two of
the bands so you're going to have tradeoffs. And even an antenna with modest
gain can be wicked loud if it is at the right height at the right time on the
right band.
73, Ward N0AX
> That is correct. We used a Force-12 1:1 bead balun (i.e. common-mode choke)
> on all of the other antennas, including the reference dipole, except for the
> KT-34XA which came with a 4:1 balun/transformer.
>
> There are a number of stations with good signals, using Mosley beams. Most
> notably, K4RO has a pair of them and does quite well. He also has an
> extraordinary ridge-top location and has taken great pains to locate the
> antennas and towers to take advantage of that location. As in real estate -
> location, location, location. I used a Mosley Classic 36 at W0EEE for years,
> myself, with pretty good results.
>
> The Mosley manual we had with the loaned PRO-57B (I think it was a B model)
> gave specific instructions how to construct the coax pigtail and said
> specifically not to use any additional devices at the feed point. It has
> been a long time since we did the tests (11 years, I think) and it is quite
> likely that the manual has been changed to recommend a common-mode choke. I
> would applaud that change as good practice. Feed line interaction could have
> certainly affected the results on one or more bands. We followed the rule of
> doing what the manufacturer said to do.
>
> 73, Ward N0AX
>
> Date: Tue, 16 Oct 2012 10:14:21 -0400
> From: john@kk9a.com
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] If you had a choice
> Message-ID: <4ef571bb19327019e3bfc21e35f9638a.squirrel@www11.qth.com>
> Content-Type: text/plain;charset=iso-8859-1
>
> Was this test run with no choke at the feed point of the Mosley and the
> other antennas in your test had one?
>
>
> To: "Towertalk Reflector" <towertalk@contesting.com>
> Subject: Re: [TowerTalk] If you had a choice
> From: "Ward Silver" <hwardsil@gmail.com>
> Date: Tue, 16 Oct 2012 08:09:51 -0500
>
> A cautious word of defense on Mosley...the PRO series was definitely not
> the top performing tribander in the tests (www.championradio.com) but they
> are built like tanks (that also means they are very heavy antennas) and do
> stand up to the elements, particularly ice loading. The Mosleys would
> probably also benefit from decoupling the feedline shield with common-mode
> chokes (ferrite or
> coiled-coax) although the manual I saw during the test said not to use a
> balun for some reason - that may have changed.
>
> I agree with whoever said to avoid quads in Alaska - unless you like
> rebuilding antennas :-)
>
> Check with other KL7s in your area as to what they have put up that makes
> it through the storms and gives adequate performance on the air. In
> extreme weather environments, there are more requirements than just gain
> to worry about.
>
> 73, Ward N0AX
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|