[TowerTalk] Traps and Losses

Jim Reid kh7m@hsa-kauai.net
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 (cebik@utkux.utcc.utk.edu)
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 
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.



Interesting reading!

73,  Jim,  KH7M

FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com