Interesting we have the same first and last name.
It is sad that Force12 could not ship you all of the pieces for this
antenna, this is not an inexpensive product.
I have an older C3S that I purchased used and brought to St Croix in March.
I noticed that the predrilled element spacing did not match my manual. I
assembled it using the already attached element brackets and temporarily
mounted it about 30' high on the side of a mountain 300' above the sea. It
seemed to work well however I also noticed higher SWR (maybe 2:1) in the 10m
segment that I used. I do not recall where is was resonant. Any modeling
software should be able to give you a good estimate on how much to adjust
the element. I would love to hear your final results.
John KK9A aka WP2AA
To: towertalk@contesting.com
Subject: [TowerTalk] Help with Force 12 C3S 10-Meter Adjustments
From: John Bayne N4EEB <john.bayne@earthlink.net>
Reply-to: John Bayne N4EEB <john.bayne@earthlink.net>
Date: Sun, 12 Nov 2017 12:46:29 -0500 (EST)
Hello friends,
This is my first post to the list.
My questions are based on a Force 12 C3S constructed to the 80 MPH diagram
in
this manual: http://bit.ly/F12-C3S
I purchased a new C3S from he Colorado version of the Force-12 company
shortly
before they seemed to disappear. Unfortunately, my kit was missing several
element tubes, and a lot of the "pre-drilling" mentioned in the manual was
not
done. That being said, after I ordered the replacement aluminum from DXE
and
fabricated the missing pieces, the antenna went together fairly easy after
drilling the missing holes, etc.
Having done as much research on potential problems as I could do, I noticed
hams commenting that the 10m 2:1 curve was VERY high. So, in an attempt to
compensate/mitigate this in the original build, I extended each tip on the
10m
driver from the specified 36.5" to 37.25 inches.
The antenna was installed in the clear at 33 ft. - It is fed with 97 feet of
LMR-400-UF terminating to a 5KW JK 1:1 balun. 2" long, 1/2" wide copper
strap
connects the balun to the 20m driver.
A blanket statement: SWR curves on 20M and 15M (and performance) is
perfect.
I am very happy.
As feared, the 10m SWR curve is way off for the type of operating I do. I
mainly operate CW, and phone from 28.3 to about 28.7.
The measured SWR curve on 10m is as follows: (Measured on a RigExpert
AA-30
and also the SWR display on a Palstar HF Auto tuner in bypass - both in fair
agreement)
2:1 Lowside Freq: Approx 29.220 MHz.
2:1 Highside Freq: Approx 29.820 Mhz.
Frequency of lowest SWR: Approx: 29.417 MHz.
Rotating the antenna 360 degrees does not change any of the SWR readings, so
nothing around the antenna is skewing things.
I have bad O.C.D. with projects like this. I know I'm probably overthinking
the solution to this, and that's why I'm posting here. I realize that
simply
lengthening the tips on the 10m driver will adjust the SWR curve lower. I
also
know that experimenting prior to final riveting in place is the best option.
I
do not climb. I have a friend coming over on 11/21 to lower the antenna to
allow drilling out rivets and making changes. He does not have a lot of
time.
I would like to pursue the best plan available first. That being said, here
are
my specific thoughts and questions:
It was suggested by someone familiar with this design to try simply
lengthening
the 10m driver tips each by 1.5 inches and seeing if that works. (Leaving
10m
reflector alone.) What do you think about that?
My OCD brain has come up with a different first step:
Looking at the antenna diagram as printed, the length of the 10m reflector
is
7.035% longer than the driver. The distance between the elements is fixed
and
cannot be changed.
The manual states that a .750 inch adjustment in each 10m driver tip will
result in an approximate change of 250 KHz.
The distance of the SWR curve now vs where I need it to be, is approx. 1220
KHz.
Doing the math, the driver would seem to require a total lengthening of 4.88
1/4" increments to bring the lower 2:1 point down to approx 28.000 MHz.
If this way of looking at it is correct, the new total length of the 10m
driver
would increase to 207.26 inches, which would be accomplished by lengthening
each tip to approx 40 5/8" vs the current 37 1/4" tip lengths.
Am I thinking about this correctly?
Now, OCD on to the reflector! How can one make such a large change in the
driver and not the reflector?
If I change the total length of the 10m driver to 207.26 inches, the 7.035%
difference in the stock reflector measurements goes out the window.
However, if I increase the total length of the 10m reflector to 221.84" by
increasing each reflector tip by 4.42 inches (each tip now to be 47.75
inches
instead of 43.5"), the totality of the adjustments are somewhat linear and
symmetrical. I think.
So, there you have my options.
1. Lengthen the driver tips by 1.5 inches each, leave the reflector alone,
and
see what happens?
2. Adjust the driver tips to best SWR curve and leave the reflector alone?
or
3. Start with my math above and plan on starting with the full driver and
reflector adjustments?
Any help would be appreciated. I'm looking forward to learning a lot from
your
responses.
73, John N4EEB
n4eeb@arrl.net
(If you contact me off list, your email will bounce, but I will still get it
and add you to my white list. - Thank you.)
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