Date: Mon, 06 Apr 2015 22:47:01 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] antenna choices
Now, look at the Optibeam numbers and description. This is a 2-el Yagi
on all bands except 10M and maybe 12M. The gain numbers are 4.3-4.5 dBd
which translate to 6.45 - 6.65 dBi (free space). That's slightly better
than a near perfect 2-el Yagi, so there's either some design magic or
some specsmanship happening here too. :)
### Believe it. The extra eles on the boom will act like quasi directors
and reflectors. You can see the same effect on the W3JK dual band
yagis as well. The monoband versions of the same ant have slightly less
gain. It was modeled on NEC-4. The F-12 5 BA is a 5 band yagi,
but only has ONE ele on 12m. Yet it has loads of gain on 12m.
The F-12 N1715XL is a dual band 17+15m yagi..on a 36 ft boom.
Only 2 eles on 17m, yet has 6.0 dbd gain on 17m. The 4 x 15M directors
look like quasi directors on 17m.
So -- repeating the question, what do you hope to achieve by going to
another antenna? There seems to be no question that the Optibeam is
ruggedly built, but it's unlikely that it will perform measurably better
that what you have. Folks are always saying that whatever they have is
wonderful, usually based on all the DX they worked. But the basic facts
are that if you want more gain, you need more elements PER BAND and a
longer boom, also between elements. For example -- with the Optibeam,
there are two 20M elements and they appear from the photo to be only
about 8 ft apart. That's awfully closed spaced for 20M -- the SteppIR
has 3 el 8 ft apart, and that spacing is short for 20M.
The OB 11-5 (3-el/band, 19 ft boom) looks to be the performance
equivalent of the 3-el SteppIR, the 16-5 (4-el/band, 33 ft boom) the
performance equivalent of the 4-el straight SteppIR. At the current
EU-dollar value, the 11-5 is about $1,650, the 16-5 about $2,640. The
SteppIRs are about $2,000 and $2,800 respectively.
73, Jim K9YC
### The shipping costs on the optibeam ants is extremely expensive
to Canada. A buddy enquired about an optibeam 40m rotary dipole..and
shipping alone was $300.00 cdn funds..which is nuts. Optibeam makes
good stuff... but Im not impressed with the use of metric tubing. Another buddy
finally found a US source of metric tubing a few yrs back. Dunno if they are
still in
business or not.
## I can buy one helluva lot of 6061-T6 or 6063-T832 tubing for $2800.00
Price out 6063-T832 from DX engineering. A single 3 foot piece of .375 inch
OD tubing..with a .058 inch wall thickness is a measly $1.48 !! You can
get it
in either 3 or 6 ft lengths. You can also buy it with the slots one end
only... or no slots
at all. No slots on either end..and price drops a whole bunch. Prices go up
as OD increases.
## F12s boom to mast plates are second to none..and easily replicated. I used
.375 inch
thick plates, one on the mast..and the 2nd one on the boom. They just mate
with 4 x bolts.
A 5th bolt, pinned by the mast..is what the boom plate is mated to initially.
Takes all of 30 secs
to install the yagi on the tower. DX eng jumbo mast clamps used on the mast.
And SS u bolts
with solid AL saddles used on the boom. Every other boom to mast mounting
scheme, using a single
plate is fubar imo.
## This is not rocket science here folks. This is one place where you can
potentially save a lot of
$$. If you do hb a yagi, Id stick with .058 wall stuff, then no swaging
required anywhere. You
can double wall sections of the ele very easily.... to beef it up for greater
strength. A few mins
spent with yagi stress or the eq program from dx eng will show exactly where
any weak spots are
in any mech design.
## Now this all assumes no traps involved..and no steppir motors used.
Jim VE7RF
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