Traps and advertising claims
L. B. Cebik
Mon, 17 Feb 1997 07:40:13 -0500 (EST)
Trap losses have been a popular subject since the 1960s, when one
manufacturer with an excellent test range (I have been there) measured
some competitive beams. The beams of one competitor showed little, if any
gain, over that of a dipole, but still gave very good front-to-back
ratios. But trap design has advanced (I think) in 30 years.
The loss of gain due to the use of traps or other forms of loading
elements does not necessarily mean loss of power in the sense of
conversion into heat. Hence, a low-Q trap does not turn it into a
resistor. Rather, it turns it into an inefficient trap, which allows
significant power beyond the trap point. Low Q will also mean a higher
resistance, but in relationship to the reactance of the components, and
this may also create a higher power loss, but usually not to the point of
self-destruction. The reduction of gain on 20 meters of a 20-15-10 meter
trap beam is in part due to the fact that at 20 meters, the traps act as
inductive loads in the elements, reducing effective radiation from the
element to the degree that coil loads can be considered to be almost
non-radiating substitutes for what would otherwise have been at that point
a linear radiating element segment.
I have had occasion to do some extensive modeling of various loading
schemes for simple 2-element Yagis. This has included center-coil
loading, linear loading, and capacity hat loading. Without dragging out
all the files, here from memory are some results. For beams about 0.7
full size, the capacity hat models closely matched the full size models in
gain and front-to-back ratio--largely because the hats were located at low
current positions. The small models I used (and built for 10 meters)
showed gains of about 6.1-6.2 dBi and F-B ratios of about 12 dB.
When the same 0.7 full-size beam was center loaded, the gain dropped to
the 5.7-5.8 dBi range, with linear loading having an advantage. In fact,
for this size beam, linear loads from aluminum wire showed an equivalent
coil Q of over 300. In the abstract, it is possible to make solenoid
coils with Qs equalling 300, but in practice, in the weather and
pollution, sustaining a Q of 100 is unlikely without heroic maintenance
and protection methods. Q's of 100 or less dropped the beam gain further.
However, these center-loaded antennas permitted much higher front-to-back
ratios: 18-20+ dB was obtainable (both in models and with point-to-point
tests)--but over a narrow bandwidth.
So what's your point? Simply this: the performance of loading elements
can affect beam performance without occasioning large losses of power by
conversion to heat. The power is simply being radiated somewhere else
outside the pattern that is usually taken at a specific elevation angle of
maximum radiation. It may be up, sideways, or angular, depending on the
design and ground reflections. Compare a full size Moxon rectangle with a
full-size 2-element Yagi for a graphic comparison of full-size antennas
with equivalent radiation efficiencies with very different patterns.
Adequate tests by independent researchers would require an adequate test
range and a tremendous investment of money for beams, work for mounting
and dismounting, and control of all variables. Unfortunately, I do not
hear that Iowa Field of Dreams haunting call: "Build it. They will
come." Until someone does build it, I keep a salt shaker in hand when I
read antenna advertising claims.
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