[TowerTalk] Traps and Losses
Sun, 16 Jul 2000 18:45:36 -1000
Subject has reappeared. Interesting post by LB Cebik
back in 1997 here on Tower Talk; this after someone
(I could not find the note in the achieves) had posted
results of calorimeter tests which showed little or no
temperature rise within carefully measured traps during
tests for power loss. LB's post points out that the energy
is "lost" from the desired pattern, but not necessarily
lost as heat. Further, it is very tough to measure the
true pattern loss, because of the nature of the "beast".
Here is LB's post of Feb. '97:
"L. B. Cebik (email@example.com)
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
equaling 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
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.
73, Jim, KH7M
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Administrative requests: towertalk-REQUEST@contesting.com