>From 1992-1997 I had a 65' long 20M yagi at my W0ID QTH on the Continental
Divide at Breckenridge, CO. The 80' tower was tightly surrounded by ~55'
lodgepole and spruce trees, making "up-and-down" tuning totally
impracticable.
Designed the yagi with YO, using a gamma match with a motor-driven variable
capacitor... an air variable with a little 1 RPM, gearhead reversible d-c
motor from Grainger. Probably I was lucky, but after setting gamma
dimensions as best I could (via YO I think, but also used EZNEC for some
things ... not sure), we got the big beam at height on the tower. From the
shack, using a simple DPDT center-off reversing toggle, it was easy to dip
the in-shack VSWR right down to as close to 1:1 as I could read. Move from
one end of 20M to the other, adjusted again to near-perfect match in -
literally - 15 sec.
Used the same technique and structure on 80/75. First built an ~1/40 scale
model of the thing out of small brass tubing & solid cu wire for a
feasibility check. Extended the 3" OD 20M boom to 80+' with 1-1/4" tubing;
inserted calculated loading coils wound on 3" oak dowels, then 6063-T6 ends
to approx.100' overall to resonate @ ~3.75 MHz. Gamma feed of (~20' ?) #12
wire spaced ~12" from boom, its cap also tuned w/reversible motor. Easy to
get near-perfect match from the shack - all the way across 3.5-4.0 - by
tweaking the cap & watching the SWR meter. Seemed almost too easy.
On one occasion a VERY strange thing happened. Lightning struck a pine tree
~100-130' from the tower -- FLASHED and BANGED literally simultaneously in
the shack on opposite side of house. I located the pine; nearly all its bark
was in little pieces forming a circle at its base. Fired up the rig and
noted SWRs >2:1 on both 20M & 75/80M. Very surprised to discover no antenna
damage - small re-adjustment of 20M & 75M gamma caps restored near-perfect
VSWR. Concluded that d-c current induced in unshielded control cables
running about 175' underground and ~100' up inside the tower had actually
turned both motors a fraction of a revolution ... maybe 10 degrees or so.
A 100-year, 100 mph windstorm wiped out that 30-year-old boom - same storm
that literally logged 20,000 acres in Routt County (Steamboat Springs ski
area vicinity) ~100 miles north.
Planning to resuscitate the same matching system on a beam or two going up
soon here in the Blue Ridge foothills of Virginia. (Obviously it has nothing
to do with the antenna PATTERN - only the match.
73,
Dick W4ETO (new call of the Blue Ridge Alpha Club) / W0ID
----- Original Message -----
From: "john@kk9a.com" <kk9a@bellsouth.net>
To: <towertalk@contesting.com>
Cc: <k7sv@comcast.net>
Sent: Monday, August 24, 2009 7:24 AM
Subject: Re: [TowerTalk] ANTENNA RESONANCE VS HEIGHT ABOVE GROUND - DATA
POINTS?
>A model will give you an accurate SWR curve for any height and would
>provide the best answer to your question. I think 15' is too low for
>adjusting, I would pull it up to 35 - 40' high which should get you close
>to your final settings. What are you adjusting? Some people have had
>success adjusting a hairpin match by pointing the antenna straight up and
>leaving the reflector on the ground. OWA antennas require little if any
>adjusting.
>
> John KK9A
>
>
>
> To: <towertalk@contesting.com>
> Subject: [TowerTalk] ANTENNA RESONANCE VS HEIGHT ABOVE GROUND - DATA
> POINTS?
> From: "Larry
> Date: Sun, 23 Aug 2009 21:54:06 -0400
>
>
> Hi all,
>
> We're in the midst of building a fairly long boom yagi for 20M. When
> completed it's going to be at 190ft. We expect to initially tune it (T
> Match) at 15 ft and then tram it to 80 ft or so for further tuning before
> bringing it up to full height.
>
> Initial thought is to tune it 200khz low at 15 ft, but that's just a
> guess.
> Are any of you aware of data available that might make this a little
> easier
> than simple trial and error. This will no doubt display a curve that
> sharply
> rises as one moves an antenna from ground level to some height above
> ground
> and from there on up to our working height the percentage of change
> becomes
> fairly small. I'd guess from 30 feet on up the rate of change would be
> greatly reduced from what one would see between ground level and that
> height.
>
> As I think about the question, I find I'm asking myself if there is
> anything
> in modeling that might provide some answers, but if that were possible I'd
> expect that Steve NR4M who developed the design based on YO would not be
> looking for your help!
>
> 73 de Lar K7SV
>
>
>
> _______________________________________________
>
>
>
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