[TowerTalk] Fwd: More about C-3 10M reflectors

[Steve Zettel]

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Date: Mon, 26 Oct 1998 14:42:43 -0700
To: gkurzenknabe@piezo-crystal.com
From: Steve Zettel <zettel@homer.libby.org>
Subject: More about C-3 10M reflectors
Cc:
Bcc:
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>Hi Steve,
>I saw your post on the "Towertalk Reflector" re an add-on 10 meter
>element to the Force 12 C3 .  I just got a C3 and did not get it up yet,
>so when I saw your post I ordered the kit.....
>Question......  There were not any instructions with the kit.  In your
>post it says it mounts 65" from the 20 meter reflector.  Is that actual
>spacing or center to center?    How did you drill the boom to accept the
>new boom to element assembly.  I thought about just laying the boom on
>the garage floor which will give me a flat surface to align the new mount
>before I drill?  How did the 10 M SWR look after you got it up?  What
>holes did you use in the 10 M elements?  That concerns me based on some
>of the chatter on the reflector about 10 M SWR .   I want to put it up
>once!  Your thoughts and general comments will be a big help.
>Thanks and 73,
>Glenn, K3SWZ
>gkurzenknabe@piezo-crystal.com

Hi, Glenn,

I am going to copy this to the reflector, as there is still some interest
coming my way about adding the 10M reflectors to the C-3

I'll attempt to answer your questions in order:

First, with my 10M reflector order, I received a reprint of the page in the
C-3 assembly manual showing the elements, locations, and dimensions.
Penciled in was the was an arrow pointing to the boom, with the notation
"65 inches". Looking at the way the  original drawing is labelled, and
verifying it with some measurements of my own when I took my antennas down,
this appears to be center-to-center measurements. And, as I found out (read
below), it is relatively non-critical anyhow.

The garage floor method will work fine. I used a flat workbench top on
sawhorses to accomplish the same thing --either way will ensure the element
mounting brackets are in the same plane as the others. F12 supplied all the
necessary hardware with mine, including my choice of pop-rivets or
self-tapping screws to fasten the new element mount. I opted for the
rivets, as the other elements mounts are attached. Be prepared for a hard
pull to pop the 3/16" SS blind rivets!

10M SWR looked good to me, slightly biased to the low end of the band
(which is my preference, anyway). I did not achieve "edge to edge" 10M
coverage, even with 140' of coax attached. My 2:1 VSWR bandwidth was from
28 mHz to about 29.2 mHz. I used an Autek RF-1 Analyst, 100' of RG-8X
attached by a barrel connector to the 40' of RG-213 I use to phase the C-3E
with the other two C-3E's in the stack, a Force 12 "current" balun at the
antenna feedpoint, and a Wireman "current" balun at the RF-1. The C-3's
were all hoisted about 50-60' in the air via a tramline for the
measurements, then lowered again to pop the rivets I had left loose for
testing/adjustment. I don't have any strong HF or BCB radiators nearby that
might false the readings of the RF-1.

I used the "mid" position for all of the adjustable elements -- the 10 and
15M driver elements -- as called out in the C-3 instructions. This is with
one unfilled rivet hole showing on the 3/8" (tip section) tubing ouboard of
the end of the 1/2" tubing, i.e. two rivets placed in the end of the 1/2"
section. BTW, the additional 10M reflector was supplied fixed-length and I
left it as such at 108" (boom centerline to tip).

I, too, have noted the comments on narrow 10M VSWR bandwidth. Some of that
concerns the C-3S, which I understand has one less 10M driver element than
the C-3. My stock C-3's didn't cover all of the 10M band either, but did
give me 1 mHz+ coverage from 28.0 mHz to over 29 mHz. I noted no
detrimental effects to 15 or 20M by adding the reflector; I don't operate
12 and 17M much, so I can't say what effect, if any, the 10M reflector had
on those bands. If I do operate 17M, I usually use my 40-2CD 40M beam at
98' instead. On 17M the 40-2CD still has the forward lobe in the same
direction, instead of reversed on the C-3. In any event, my Ten Tec radios
are very forgiving of VSWR and don't fold back at 2:1 as do some others.

One item to pay particular attention to is the alignment of the elements in
the open sleeve "driver cell" on the C-3, i.e. the 20M driver/fed element
and the 15M and two 10M driven elements. Steve Morris (K7LXC) and Ward
Silver's (N0AX) Tribander Test Report notes that alignment of the Bencher
Skyhawk and the Force 12 C-3 driver cells is critical for good VSWR
performance, and that they could actually "see" changes in VSWR and
received signal strength as the elements moved around. There is provision
for some adjustability when tightening the element to element mounting
plate u-bolts, so I snugged the bolts up by hand, checked the element
spacing at the boom and at the element tips, pushed the u-bolts forward or
back in their slotted holes as necessary to achieve alignment, and then
alternately tightened the nuts a half turn at a time until I had everything
as tight as recommended (one half turn past where the element can't be
rotated in the insulating sleeve by hand).

Where I live in Montana is very calm (average windspeed over the year is
only 2 mph; snow will sit for weeks in the winter in "caps" a foot high on
fence posts!), so I felt no need to fabricate spacers out of lexan or
similar, to maintain driver cell alignment. Others have felt they need to
do this in their locales. Mike Lamb, N7ML, lives in a lot windier part of
Montana than I; perhaps he would care to comment about his experiences with
his 6-stack (!) of C-3's.

I spent quite a bit of time modelling and attempting to optimize the
placement of a 10M reflector on the C-3, using Brian Beezley's (K6STI) AO
6.5 and NEC/Wires software. My conclusions are that the 10M reflector
position is relatively insensitive to changes in boom location over a
surprisingly wide range, and that changes to the tip dimensions of any of
the reflectors or the 10M director have little effect on the feedpoint VSWR
(or substantive gain or F/B) over a large range of adjustments. I made many
optimizing runs in AO where I literally left the computer on all night,
chugging away on iteration after iteration and ultimately decided that the
Force 12 recommendations were as good as any, and better than most. It was
an interesting learning experience. A sense of proportion is called for in
modelling,too; it is possible to chase 0.10 dB changes for days, and to
resolve element measurements down to .001" when the best I can hope to cut,
without setting up jigs, is to the nearest 1/8" over the typical element
length.

Hope this helps. Good luck with your C-3!

Steve  KJ7CH
near Libby, MT USA


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