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absence of heat can still mean loss. 73 Natan W6XR/2 FORCE 12 East Ithaca, NY force12@qth.com 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. -73- LB, W4RNL" 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 -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact force12@qth.com |
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