Tom, N6BT writes:
>I'll conclude with a short personal note (from me, not the company) to
someone who thinks that chosing a C-3 over an "XA" would be smoking
something, since I have done my fair share of contesting and DXpeditions:
any day take a C-3 - smaller, lighter, less tower strain, no 500
parts to assemble and maintain, no power limit, maybe not always
through before an XA on 10 mtrs, but 20 is where the horses run.
- other numbers might look good, but they are only numbers.
*********************************************************************
I'll reprint the original post question as well:
>The recent mention of Force antennas reminds me of a question that's
been rattling around for some time. I keep hearing the C3 referred to
as a set of monobanders sharing a single boom. There are six loaded elements
but no traps, so this appears to translate to three two element yagis
interlaced on a boom. How could these yagis possibly compete with the
other trapped yagi designs that have 4-5 active elements per band? What
am I missing?
Regards,
Pete
KS4XG
*********************************************************************
And now I'll comment:
The original poster made no mention that the antenna was to be used
specificly for a DXpedition. If he had, I also would have recommended
the C3, as I prefer it's ease of assembly over the rest, and in the
typical DXpedition setting, such as low height, near or over water,
the longer boom length of other antennas would not be effective.
Tom invites the readers to model his antennas. He also states
that ' All the elements
carry current regardless of the frequency of excitation (20-10)'.
Well if you do model the C3, on 20 meters, over real ground, using
NEC2, you'll find that Tom is correct - the front (10 meter) element
has a non-zero current, which is down 28.5 db from the current in the
driven element. Compare this ratio to the ratio of currents in the front
element of a PV4 to the current in the PV4 driven; -6.3dB. Do you
really believe that an element carrying less than a twentieth of the
current will be contributing anything significant to the pattern?
To further convince yourself of this, model the whole C3 on each
band, then start removing non-resonant elements from the simulations.
You have to leave those elements near the driven for the feed system
(an ingenious open-sleeve design) to work. Note the minimal degradation
in gain and F/B as you remove elements!
If you can get to the point where you accept that the elements which
are not resonant on the operating frequency are NOT really contributing,
you now have to accept that on any given band the antenna is really
a 2-element yagi, whose boom length is considerably shorter than
most 'brand X' antennas.
Check out the accepted antenna references - especially Dave Lawson's
(W2PV) book. You will find that 2-element beams cannot compete with
antennas having 3 or more elements in the F/B department!! If you're
wondering, check out Force 12's number's for F/B.
If efficiency is the key, and you're willing to settle for 2-element
F/B, then the antenna for you is the good old stand-by QUAD. It will
out-model and out-perform the C3 in forward gain, due to the gain
of a single loop over a dipole being carried over to the quad design.
And it should have the potential for excellent efficiency, as there
are no traps, and the radiation resistance is very high - 100 ohms
or so, depending on the band.
I'd like to summarize my conclusions on the C3 as follows (repost):
I have done a lot of modelling on mono-band models of the
KT-34XA (my existing antenna on 20-10), and I have modelled the C3.
My conclusion is that the C3 just cannot compete with the XA in the
forward gain department. Here are my observations:
1. Force12 C3 series antennas are well-built, using many desirable and
advanced construction techniques. Specificly rivets, and their mast-element
and mast-boom connections look really nice to me.
2. Force12 C3 series antennas are full-size, no loading 2-element beams
on 10-20. This means they can't be beat for efficiency.
3. As far as Force12 advertising claims, I'm skeptical. First, comparing
a C3 to a KT34XA, the XA is really 4 elements on 15 and 20, and 5-6 elements
on 20. If you check with the experts, or do some modelling, you'll
find that the XA comes out 4-8 dB better in the forward gain department.
4 dB is 60% of the power. To equate the two antennas' performance would
require that the XA dissipate almost 1KW in the form of heat. That would
be 100 watts per trap (there are 10 traps in an XA). I think that much power
would be enough to melt the plastic end caps out of the traps.
The traps of the XA are made out of three materials - air, aluminum,
and polyethylene (?) capacitor caps. Air is very low loss, the aluminum
tubing is of reasonable diameter so skin effect losses are minimal, and
the capacitor end caps constitute such a minute percentage of the capacitors
that they can't dissipate 100 watts each. The
upshot is that while there is merit in the efficiency arguement, there
isn't 4 dB of merit. I'd be willing to accept that antennas with lumped
traps like a TH7 or A4 etc would have higher losses. But the XA uses
linear loading and air-capacitors - you can't get much lower loss than that.
4. Consider this - a 2-element quad would have about one dB more gain
than a two-element yagi (such as a C3). A 2-element quad would also
be at least as efficient as a C3, if not more efficient, because of the
higher radiation resistance. I've had a 2-element quad up at the same
time as the XA, and while it comes close, the XA beats it 99% of the time
-usually by about 6 dB.
5. Force12 is very careful about making their most dramatic statement
always appear as a customer quote - they never directly state the following
as their own claim:
"The C3 outperforms every trapped tribander, regardless of boom length"
(quote may not be exact).
6. Force12 claims that all 7 elements are active on all bands. If this
means that the current flow in an element is non-zero, I guess they
are technically correct. If it means that the other elements contribute
significantly to gain or F/B, I disagree. I modelled the C3 on 20
meters, and got reasonably close numbers for gain and F/B to their
specs. Then I removed all the 10 and 15 meter elements. The pattern
change was really slight. Much less than .5 dB.
My conclusion was that the C3 antennas are really nice, well engineered,
well-built 2-element beams. They'd be a great replacement for a TH-32,
or even an A3 (if you buy the efficiency arguement). But they will not
provide the forward gain of an antenna with truly active elements on
a boom almost twice as long. And they won't beat a 2 element quad, either.
Before closing, I will repost my crude analysis of trap losses in the KT-34XA.
I'm sure many will be quick to suggest sources of measurement error; however
the magnitude of the numbers will make you think...
In the course of various discussions on this and other reflectors, there
has been considerable debate on the real effect of trap losses in trapped
triband antennas. In particular, Force 12 markets their C3 antenna with
the assertion that 2 elements per band, with no traps, results in
more true gain than more elements, with traps.
In an attempt to get a handle on this subject, I recently made some
measurements which would serve to give some idea as to the truth of
these arguements.
Part of this summer's antenna projects at K1KP involved removing a KT-34XA
from the top of my tower while the tower was being rebuilt. I've had
the XA down on the lawn for most of the summer.
This particular antenna was first purchased as a KT-34 in 1983. I purchased
it used from the original owner in about 1988, when I did the first rebuild
on it. I added the XA kit and did another rebuild, in 1993. So when the antenna
came down early this summer, it had 3 seasons on it. It was working properly
on all bands.
Initial visual inspection showed 7 out of 40 of the blue capacitor caps
were cracked. I also did DC resistance measurements of the traps and
found that most traps had a total series resistance of 30-50 mOhms
(milliohms). Two of the traps had resistances of 200-300 mOhms. I traced
the high resistance to the large end of the most outboard shorting
strap (15 meter section).
I called KLM and spoke to Bruce about the caps. He said they should have
lasted 10 years in normal UV exposure, and that there were only 3 explanations
for their premature failure. These were (1) installation with a hammer;
(2) installation using oils as lubricant; or (3) bad lot of plastic.
I told him I hadn't used (1) or (2), and he very kindly sent me a replacement
set for NO CHARGE, to replace the set I had bought for the prior rebuild.
I'd call that great service from an antenna maker!
I rebuilt the antenna using the new caps. Bruce told me that the best way
to install the caps was to warm them up in hot water. This is a great tip!
They went on really easily once they were warm.
In an effort to eliminate the high resistances in the 15 meter straps,
I drilled the bottom side of each end of the straps into the element tube
and pinned them with a 1/8 inch pop rivet. This ensures no rotation of the
straps around the elements. We'll see how this mod works out in a few
years.
Out of curiousity, while I had the antenna apart, I measured the resonant
frequencies of each element. They were pretty much as expected. This
post is pretty long so I won't include that data here - but I'll supply
it directly to anyone interested.
Next, I attempted to quantify the power loss of the traps as follows:
I set the antenna up on my driveway on sawhorses. Although the antenna
was only 30" off the blacktop, it had reasonable SWR on all three
bands. I fed the antenna with 100 feet of RG-8/U cable, whose loss
was within spec.
I used a Fluke 80T-150U temperature probe, connected to a Fluke handheld
DVM to measure temperature. All measurements are in degrees Fahrenheit.
During the experiments, I measured temperature at three locations -
the strap connected to the 10 meter capacitor on the front driven
element; the boom, just behind the front driven element; and the PL-259
connecting the feedline to the supplied KLM balun.
I measured the boom temperature as a means of watching for changes in ambient
temperature. I made the measurements starting at 5:30 pm on a nice warm,
sunny day, so the sun was going down and ambient was dropping slowly.
First, I made initial measurements, then gave the antenna a short blast
on 20 meters using my IC-765 driving and AL-1200. The RF applied
was 1200 watts for 5 minutes. Interestingly enough, the temperature on the
outer case of my AL-1200, just over the tube anode, rose from 91 degrees
to 253 degrees!
Boom Trap Balun
Initial 85.4 81.7 85.8
Final 85.3 83.2 126.1
So the conclusion here is that the ambient decreased slightly, the trap
dissipated some heat, and the balun got warm. No surprise here. Next,
I added a piece of foam pipe insulation around the outside of the
10 meter and 15 meter capacitors. This would serve to reduce measurement
errors due to air movement cooling the traps. I could easily measure the
temperature by poking the temperature probe through a small hole in the
insulation. The insulation would allow the heat to build up for a more
accurate measurement.
For the next run, I applied 1000 watts of RF on 10 meters for 5 minutes.
I figure the 10 meter traps, being parallel resonant on 10, should have
some pretty big circulating currents in this mode.
Boom Trap Balun
Initial 82.7 81.8 107.8
Final 81.0 87.2 178.0
So in this mode, the trap temperature rose 5.4 degrees. If you include the
fact that ambient dropped 1.7 degrees, this is a net rise of 7.1 degrees.
I'm not sure the ambient decrease needs to be factored in, as the insulation
on the trap should have prevented it from being cooled; however this will
lead to a higher dissipation estimate so I will let it stand.
Next, in order to quantify the amount of power that 7.1 degrees represents,
I installed a 10 ohm power resistor inside the foam, in physical contact
with the 10 meter capacitor tube. I applied 7 volts DC across the resistor
for 5 minutes, and measured the temperature rise as before:
Trap
Initial 71.2
Final 83.4
This shows a 12.2 degree rise due to the application of 4.9 watts for
5 minutes. If we assume that the ratio of temperature rise to power
dissipated is linear, this means that 2.85 watts were dissipated in the trap.
Now let's extrapolate this measurement of power dissipated in a single
10 meter capacitor to power dissipated as heat in the entire antenna.
I'll do two scenarios - conservative and optimistic. First the conservative:
Assume that I only accurately measured half of the power dissipated in the
trap, i.e. that a similar 2.85 watts was being dissipated in the inductance
portion. Also, assume that the same amount of power was dissipated in
all of the ten traps of the antenna. This results in a total power dissipation
of 57.03 watts. If the antenna was fed with 1000 watts, the efficiency is
94.3%. Or expressed in dB, the resistive losses were 0.25 dB.
Now for the optimistic model: Assume I did measure all of the power
dissipated in the trap. Also, modelling tells us that in this antenna,
the element currents are not all equal. The front driven has the highest
current, the rear driven has somewhat less current, and the parasitic
elements have much less current than the front driven. So instead of
multiplying the power in one trap by the number of traps, we need to
multiply the current in the measured trap by the current ratios given
by modelling to get the current (and power) in the other traps. This
results in a total power dissipated of 12.19 watts; efficiency 98.7%;
resistive losses of .05 dB.
This analysis does not include the losses in the balun - which can be
studied and analyzed by itself. For a great reference, see Jerry Sevick's
books, and WA2SRQ's posts on feedpoint baluns. As a side note, I
considered replacing the KLM balun in one of the earlier rebuilds.
There was nothing wrong with it, other than 'looking old'. I bought
a cheapo 200:50 ohm balun from another source - KLM price was high.
Being the enginurd type I am, I tested the two baluns on a network
analyzer at work. The KLM balun was superior by far, giving a
good 50 ohm match over the entire bandwidth. The aftermarket balun
was way off - like 25-30 ohms! So I put the 'old looking' balun back
up, and returned the replacement unit - no problems!
So in even the most conservative estmates above, the KT-34XA comes out
to have an outstanding net forward gain - one which would far exceed
that of a trapless C3.
-Tony, K1KP, fisher@hp-and2.an.hp.com
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