[TowerTalk] Tower and antenna decisions
jim at audiosystemsgroup.com
Thu Oct 24 11:26:01 EDT 2013
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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