I submitted this last night but it didn't post, so I have attempted to shorten
it some more. My apologies if it is a double post.
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.
[lengthy measurement details and data omitted; conclusions of measurements
follow]
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-36XA
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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