[TowerTalk] If you had a choice

[Ward Silver]

>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 at kk9a.com
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] If you had a choice
> Message-ID: <4ef571bb19327019e3bfc21e35f9638a.squirrel at 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 at contesting.com>
> Subject: Re: [TowerTalk] If you had a choice
> From: "Ward Silver" <hwardsil at 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
>
>