On Wed, 29 Oct 2008 13:18:22 -0400, Tom-W3FRG wrote:
>I've always wondered why that when a simple LC tuned circuit is used in our
>transceivers and other electronic gadgets its loss is not mentioned, but
>once one of these is placed in an antenna radiating element, it all of a
>sudden becomes an element of loss.
It is often important in antennas because the resistance of the traps can be
a significant fraction of the radiation resistance. It is important in
receivers to the extent that it determines the Q (bandwidth) of the circuit.
It is important in transmitters to the extent that it affects bandwidth,
AND to the extent that it takes more input power to achieve the output
power, and to the extent that it causes heating.
Example: We feed 250 watts DC to the radio (20A at 12.5V) and get 100 watts
out the coax connector. No problem -- the 150 watts will be burned as heat
in the transistors and the resonant circuit, but we get our 100W out. Now,
that 100w goes into the coax, there's a dB or so of loss in the coax, so we
have 90 watts at the antenna. Let's say that the radiation resistance is 25
ohms and the trap resistance is 25 ohms, so one half of that 90w is burned
in the traps, and we radiate 45 watts. That may be an extreme example, but
it illustrates the problem.
This analysis only talks about loss, and is in addition to any possible
reduction in gain due to non-optimum element spacing for multiple bands,
etc.
Our moderator, K7LXC, has done some excellent work on actually MEASURING the
patterns and relative gain of a selected number of antennas. It took a LOT
of work to do that, so he and his collaborators are selling the reports for
what I consider to be a very nominal fee, considering the hundreds of hours
and investment in materials that it took to pull it off. If you care about
the performance of HF yagis, this report is well worth the cost. They've
also measured a bunch of verticals.
73,
Jim Brown K9YC
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