Date: Sat, 5 May 2018 17:34:33 +0000 (UTC)
From: Timothy Coker <n6win@yahoo.com>
To: <kstover@ac0h.net>, <rlvz@aol.com>, <towertalk@contesting.com>
Subject: Re: [TowerTalk] Need recommendations for a 40-10m. Yagi
<I could be wrong... but I believe your not looking at an apples to apples
windload between the two.
<I?m seeing 16.5 sq ft for the JK Mid Tri w/40m and 17.8 sq ft for the
Optibeam. Notice how Optibeam only lists one rating where as JK lists three
which can confuse things in that Ken is giving more information.
<As for the aluminum being the same... way smarter guys here than me. I?ve been
curious of late if that?s really the case... I?ve been reading about some boom
failures at other stations of late and there are some very adamant hams talking
about the quality <differences in aluminum used. Could just be angry folks, I?m
no mechanical expert.
<If the original op is saying he?s mainly into the contests he?s probably going
to want to keep his contest yagi?s as clear of his WARCs as possible in order
to avoid possible interaction so that he obtains better performance during the
contests.
<I first started out trying to do everything I could on 10 feet of mast
sticking out of Rohn 25G. Who knows what was happening with a TH7DX at the
thrust bearing, an A3WS/30m 5? above and a XM240 driven element at 10? above.
Oh and toss in an 80m <inverted V a foot below it all! :)
<Going forward I?ll put my WARCs on separate structures or as temporary wires
as needed. Just less for me to worry about when people aren?t returning to my
CQ?s.
<That?s me though. Try to make it so the scores aren?t so much limited by
hardware and more by me.
<Tim / N6WIN.
## The JK Mid tri with 40m IS 24.75 sq ft of projected area. Projected
area means length X width for each ele. Then total up all the areas of
all the elements. The 16.5 sq ft for the mid tri with 40m is the old C
spec.... no longer used.
The old C spec took the projected area.. then multiplied it by .666 to get...
effective area. This is what F12, hy gain, etc used...and plenty of ant
makers still use the old C spec. To get from old C spec ratings.. back to
projected area, just multiply by 1.5. EG: 10 sq ft projected = 6.666
sq ft using c spec. 6.666 X 1.5 = 10 sq ft. For all you folks using
yagis with the old C spec ratings.... multiply by 1.5 to get reality.
## 16.5 X 1.5 = 24.75 sq ft projected... which is what the mid tri + 40m
actually is.
## I would not even mess with the F or G spec, too convoluted and requires
the actual installed height to be factored in, plus a ton of other variables,
including wind exposure etc. IE, buried in a forest of trees, in the middle
of a wheat field, on a ridge, or next to salt water, etc.
## When the projected area is run through the mech software, to calculate max
wind rating, like when using DX engineerings ...Yagi mechanical, it will
spit out max wind survival for any spec you want, including F, G, C.... and
also UBC-97..with its associated A, B, C, D.. wind exposure. It will also
spit out.... No spec. No spec is what you get when using a wind tunnel...
which is good enough for myself... and is a bit more stringent vs the old C
spec. If you installed a wind speed indicator at the top of your mast, you
will get the same results as using a wind tunnel. Yagi mechanical will also
spit out the ele deflection at any wind speed you want, from 0 up to max wind
survival. Ken uses NO spec to calculate the max wind survival ratings of his
yagis. Its very conservative imo. Also when the eles deflect, the tips shed
wind, aka reducing the loading. That alone will add an additional safety
buffer on top of what the NO spec spits out. 40M el
es will deflect more than a 20M ele, and unload even more. The JK 40M els
dont sag, and dont require a truss.
## Several folks have had severe issues with the way optibeam splices their
square boom sections together. I dont like their boom to mast assys, it
uses 2 x muffler clamps for the mast. Nobody uses muffler clamps for
anything these days, esp the types with sheet metal saddles, as depicted.
https://www.dxengineering.com/parts/dbo-ob12-4w-p JKs version wont budge
or slip on any mast. Optibeam uses METRIC tubing sizes for the
elements..and also the square and also their round booms.
Unlike 6063-T832, 39 ksi yield strength...... or 6061-T6, which uses 40
ksi yield strength..and both using standard tubing sizes and wall thickness,
METRIC tubing does not telescope with a snug fit. They make a sloppy fit and
have to be either shimmed, or swaged.... pita. The deal breaker for myself is
that METRIC tubing is a bitch to find in Both Canada..and the USA. A buddy
finally found one source for metric tubing in the usa....and it was special
order, and expensive. That may have changed. So even if you can find
replacement METRIC tubing in the usa, next up you will have to get the ends
carefully swaged. Any swaging process has to be done dead one. Too little or
too much, and you are in trouble..esp if too much.
## If you are hell bent for the optibeam, even if you ordered it asap, it
wont ship from Germany until August 13, 2018. $3378.00 vs $2695.00
for the JK. The $683.00 difference is not exactly chump change either..and
better put towards the JK-801. The optibeam uses an included 2:1 balun.
That would kill it for me right then and there. The JK uses a simple 1:1 CM
choke for the 20-15-10m portion.... and a 2nd 1:1 simple CM choke for the
40m portion.
## JK WELDS the ends of their 40 loading coils, made from solid .25 Al rod,
comes in large rolls. Zero resistance connections. I would install the jk
mid tri with 40m just a few inches above the top of the tower, and the 801
towards top of mast..then call it a day.
Jim VE7RF
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