Tony,
Thanks a lot for sharing this - I appreciate the thoughtful effort you put
into this and your willingness to share with us. Now, I'd like to ask for a
copy of the rebuild treatise.
I recently got back on the air after being a "hired gun" for the last few
contest seasons at K0TV. I've got a KT34XA and had some issues that fellow
YCCCers K2TE and WB1ADR helped me with this fall - I'm getting past the
point of doing meaningful work at 110 feet now that I'm 65. The insulator
that holds the driven elements' crossover straps apart (and off the boom)
must have rotted away, so Ed and Lee slipped a piece of PVC over the lower
strap. The hard part was replacing a boom bolt that had loosened up
allowing part of the boom to rotate. So now the XA up and back "in service"
again.
I had forgotten how good it was! Last weekend I was particularly successful
in the short time I had (after doing 20M at K0TV in the mornings) in my home
effort with low power. I was really happy with how it plays.
I'd been dreaming of a new STEPir, but now I'm thinking that the pile of
aluminum I bought 3 years ago from a guy in Jersey - it's another KT34XA and
2-KT34's - needs to find its way to 60 feet and 30 feet on the tower, for a
3 stack of XA's.
I look forward to see what I should do in the refurbishment area...
-Rex-
K1HI
Rex Lint
Merrimack, NH
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Tony Brock-Fisher
Sent: Wednesday, October 29, 2008 7:04 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Lossy Traps (really long)
The issue of lossy traps is only one of a number of factors that enter into
antenna comparison and selection. The real question is which antenna is best
for you? And of course, there is no simple or single answer. Over the period
of more than 20 years, I have considered this question many times, and I
have consistently come up with a single answer for my station. So before I
continue, let me admit to being a fan of the KT series of antennas - the
original KLMs and the newer M2 version. However, that being said, I'd still
offer the following for consideration.
Lossy Traps
Traps are seen to be lossy for two reasons - one is the issue of limited Q
and thermal losses of the input power. A second issue is that traps
inherently result in shortened elements, which results in slightly reduced
gain.
Heat Loss
Several years ago, when this issue was raised before, I had an opportunity
to make some measurements on the thermal (resistive) losses in the traps of
the KT antennas. Now, the KT antennas are often referred to as having
'linear loading' and not traps - but this is not really true, as on 10 and
15 meters, the combination of linear loading inductance and air-capacitors
does form a tuned circuit. It may be the lowest possible loss for a tuned
circuit - but it's still a trap. I did some careful measurements, which I
will reprint here:
(Republished from 1996)
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 arguments.
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.
[details of the rebuild omitted]
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.
(End of Republished Text)
Gain Loss due to element shortening
When traps are added to an element and operated at a frequency below the
frequency for which they are tuned, they insert an inductive loading into
the element. To compensate for this, the element is typically shortened to
bring it back into resonance. When a half-wave dipole (the basic element of
a beam antenna) is shortened, it's pattern becomes broader and its maximum
gain drops by a few tenths of a db. The overall gain of an array of elements
is the product of the gain of each element times the gain of the overall
array. Therefore the slight reduction in gain from a shortened element
carries over to the array of shortened elements. I haven't done the modeling
lately - but I think I recall that in the case of trapped antennas the gain
reduction is on the order of tenths of a db.
Apples vs Apples comparisons
Many antenna designs exist which achieve multiple band functionality without
traps. One means of achieving this is to simply combine two or three
monoband beams on the same boom. The downside of this approach is that the
elements of the lower frequency antenna tend to disrupt the pattern of the
higher frequency antennas because their long (and often harmonically
resonant) elements act as reflectors in the middle of the higher frequency
antenna. The way around this is to forward-stagger the elements - which
requires more boom length, but allows the different antennas to coexist on
the same boom with less interaction. The C31XL and Bencher Skyhawk are
examples of this. The problem comes when trying to compare these antennas to
conventional trapped tribanders. Without resorting to modeling, most hams
realize that boom length is the primary determinant of gain in a beam
antenna, with the total number of elements being less of a factor. So one
might compare the 32' boom of a KT-36
XA to the 31' boom of the C31XL and conclude they are similar in gain.
However, due to the forward stagger, none of the three monobanders on the
C31XL boom get to use all of the 31' boom length - with a resultant
reduction in gain.
Reliability & Other Factors
Recently I had to do a rebuild on my KT-36XA. (Anyone who has read this far
and is still awake and interested can email me for the write-up of the
rebuild). Because of the expense and time involved in this project, I
seriously considered changing to the nouveau sine qua non antenna, the
SteppIR. Before I made the jump, I asked several local owners about their
experience with the SteppIR. I was especially concerned with reliability,
given that it has moving parts. This was a major concern, right up there
with forward gain. What I found was less than encouraging. Several local
folks had had problems with various parts of the antenna, including
cracking/failing boots, jammed or broken motors and gears, and many had
control box failures due to lightning. So given my high concern with
reliability, I chose to stay with the M2, and did everything I could to
improve it's reliability.
I will state that I think a properly working 4-element SteppIR on a 32 foot
boom probably beats the XA in forward gain, due to slightly lower losses and
full size elements. However I was surprised that the manufacturer is still
experiencing material and vendor problems. I would have expected that they
would be on a campaign to address the number one concern of the SteppIR
design - reliability of moving parts.
The bottom line is that one should not get to wound up about trap losses,
and that there are many other factors that can outweigh the trap issue when
making an antenna selection.
Respectfully Submitted,
Tony, K1KP
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