After twenty years (and 298 countries), I decided to upgrade my roof
mounted
TA33jr. The C3S met my requirements ? claimed broadband coverage for
10,
15, and 20 meter bands plus 12m and 17m via a tuner, gain and a clean
profile
on a 12 foot boom to fit my 15.5 foot wide row-house lot.
Dave, K9NX, Eric, WD3Q, and other TTers described their experiences
and frustrations in getting their C3S and C3SS to ?behave.? Being able
to use
their experiences, attempted solutions and SWR curves was very helpful ?
hopefully these notes will add to theirs and be of use to someone else.
www.contesting.com/_towertalk/9808/0044.html
www.contesting.com/_towertalk/9808/0100.html
Construction:
C3S is heavy duty by comparison to the TA33jr. The manual is relatively
easy to
follow, but has references to the C3 model that has two driven elements,
causing
some confusion. The manual also suggests that you check all
measurements to
be sure they didn?t mis-mark anything. RTFM ? mine had the 20m element
outboard of the centre sections labeled wrong, but the pre-drilled rivet
holes still
aligned. Also, one 10m element section needed a new hole drilled. For 10
and 15
meters, don?t rivet the tips in place until you?ve tested the antenna ?
tuning of the
C3S is high for the DX portion of the bands. I set the lengths for the
lowest
frequency on 10 and 15 (more later).
Because of limited ground space, I did final assembly on the roof. The
boom to
mast mount is something Force 12 is proud of ? it allows the boom to be
tilted at
about 30 degrees from horizontal to work on the elements. You?ll need a
rope on
the end of boom to hold the end you?re working on down after mounting
the first
elements on the other side of the pivot point. Also, if you choose to
mount the
boom first and then slip the elements into their U-bolts, use more than
a few
turns on each nut or the vibration of sliding the elements in place will
work them
loose ? looking for stray nuts and washers in gutters, flower beds, etc.
is time
consuming.
Installation
About one month of up and down on the roof was needed to get the C3S
ready
for use. The ?problems? encountered had a number of sources ?
mis-assembled
elements, bad feedlines, bandwidth specifications which are
questionable, and
installation limitations. What?s key to any debugging exercise is good
records.
After each change, I ran a complete set of SWR curves and recorded them
in a
spreadsheet with graphs. As my frustrations grew, reviewing each
previous step
was helpful to determine the next step and avoid trying the same thing
twice.
1. Initial test ? best SWR on 20m was 2:1, 15m was 2 or less, and the
low end
of 10m was 5:1! I decided to tackle 20m first ? it has no adjustments
and the
best SWR prospects given the specs which say 1.6:1 worst across the
band.
The Tower Talk postings about bad baluns and feedlines lead to changing
the
existing heliax feedline with little effect. After dancing on the roof
with a tape
measure, I discovered the mis-marked 20 meter elements. A few minutes
to
drill out rivets and exchange the outer portions of the elements yielded
1.8:1
on the band edges and 1.5:1 mid-band. SWR curves on 15m and 10m did not
change, suggesting that adjustments on one band shouldn?t have much
effect
on the others.
2. The 15m SWR curve was less than 2:1 across the band although high at
the
low end, so I left it alone.
3. Ten meters was next. The reflector and driven element were
lengthened to
their maximums (two rivets, not the normal three). The low end of 10m
was
now 3.7:1 with 1:1 at 28.7, not 28.35 as per the manual. The Tower Talk
postings suggested a bad balun and a FAX to Force 12 got a similar reply
and added that interaction with near objects could also be a factor.
Rotating
the antenna showed little variation in SWR to indicate an obvious
resonance
coupling, surprising given low mounting height (10? mast on roof ?
clears
chimney by about 3?).
4. The time consuming, frustrating part ? replacing the ferrite balun
with a coax
coil style, interchanging and replacing all connecting cables between
the rig
and the Bird wattmeter, and bypassing the patch panel used to bring coax
and rotor cables into the shack. And everything had tested OK prior to
installation. Now the shack was completely ripped apart and I had pages
of
graphs ? but no magic break through!
5. The old voltage balun was put back in place; it seemed to do a better
job on
feedline currents than the coax loop. I lengthened the 10 driven
element tips
to one rivet hole, then drilled another for security. Thanks to Lee,
W4RNL,
forpointing out that changing driven element length in a fixed beam
design
only changes impedance for a reasonable excursion (great web site!
funnelweb.utcc.utk.edu/~cebik/radio.html). Ten meter SWR still 3.2:1 at
the low end.
How does it work?
The C3S seems to work as well or better than the old TA33jr. I break
the piles as
easily as ever with my mighty 100 watts and have worked two new ones.
SWR
on all bands isn?t what I?d hoped, but the rig will tolerate it without
much power
turn-down. The C3S has front to back ratio and gain on 10, 15 and 20.
The two
WARC bands,12 and 17 do cover with a tuner, there is some directivity
apparent;
17m is backwards as advertised. The discovery that trapped antennas mask
feedline or other problems was a big surprise. Or is it that mono band
antennas
are fussier? Or both?
Things on my list to try next (suggestions are welcome!):
1. Replace the voltage balun with a bead current balun to see if some of
the
anomalies are feedline current related. The coax coil balun didn?t seem
to do
the job.
2. The antenna is currently on a one foot mast above the rotor, so
raising the
antenna would move the rotor out of the element plane more. The wind
load
would put more strain on the mast-mounted rotor which is the guying
point.
3. Lengthen the 10m driven element tips to get the SWR down for the
bottom
500 kHz ? unless someone can suggest a non-metallic solution.
Specifications
Finally, how can the specs go without a comment? Emphasizing no trap
losses
as a major selling point and then using coax loss to make the bandwidth
specs
?look good? is misleading. If you?re serious enough about minimizing
loss to buy
a premium priced antenna, you?re not going to use RG-8X and incur 2.3 dB
feedline loss for a 100? run.
Also surprising is the ten meter centre frequency, 28.800, especially
given that
we?re moving into a sunspot high. High performance antennas attract the
DX
crowd, which is 28.0?28.6 ? many may not know the band goes up there
(OK, I
do have a 10-10 number, but you get my point).
Any comments or suggestions would be appreciated.
73, George VE3YV/K8HI ve3yv@rac.ca
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