Hi Jim -
Would you like to add any comments regarding LP's. Granted, only as much
"gain" as a good two or three element yagi and no killer as far as F/B is
concerned, but no traps, no "critical" tuning, only one feed line, one boom
and only one rotator. For that matter the TH-11 uses a small log cell, with
no traps for the "driven" element(s), and combined with parasitic directors
and reflectors, with relatively few traps, should be a fairly effective
antenna, as should be the Tennadyn LP's
Gene / W2LU
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <towertalk@contesting.com>
Sent: Thursday, October 24, 2013 11:26 AM
Subject: Re: [TowerTalk] Tower and antenna decisions
On 10/23/2013 9:10 PM, Avery Davis wrote:
TH-11DX at 34 feet?
HexBeam at 50 feet?
As to gain figures for these antennas -- around 1999-2000, Ward, N0AX, and
Steve, K7LXC, set up and measured a dozen or so tribanders on a tower
under carefully controlled conditions. A few years later, Tom, N6BT, did
similar work. Both found that the CLAIMED gain numbers for all of them
(except Tom's Force 12 designs) were wildly inflated, often by as much as
6 dB. Two antennas that Ward and Steve measured actually had NEGATIVE gain
as compared to a resonant dipole at the same point. You can buy a report
on their work from K7LXC's website. It's $20 well spent. Steve also sells
N6BT's "Array of Light," which includes his measurements and lots of
excellent discussions of various antenna designs. I think it was $35.
One of the points that Tom made in his book (really a somewhat
disorganized collection of essays he's published over the years) is that
the most efficient antennas are simple monoband Yagis, and that when traps
are added to cover multiple bands, the traps burn some of the transmitter
power, and the shortened elements reduce radiation resistance (the part of
the antenna impedance that accounts for radiated power), which also
reduces gain. Thus, the gain of an ideal 2-element or 3-element Yagi is
the BEST that can be achieved by an antenna with the same number of
elements for that band. The advertised gain figures for the antennas that
Ward, Steve, and Tom measured were 3-6 dB greater than for the ideal
Yagis!
I would look at one of lighter-weight antennas that does not use traps. My
3-element SteppIR (the original with no trombones) works very well, have
been essentially trouble-free for the four years it's been up, and is
lighter in weight than most other antennas you are probably considering,
which makes it easier to install safely. It has the efficiency of an
optimized single-band Yagi on all bands, including the WARC bands. With
the added fixed 6M element, it's also a very good performer on that band.
No, it's not cheap, but given your constraints, it's a very good choice.
Even the simple 2-element SteppIR would be a good choice, and can easily
be handled by one person. I learned that when I helped N6KJ assemble his
for Field Day several years ago.
I'd also look at the Force 12 antennas, none of which use traps, and which
provide very good bang for the buck. The two that Ward and Steve measured
were the best performers in both groups they measured -- small tribanders
and larger tribanders.
73, Jim K9YC
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