To: <towertalk@contesting.com>
> Date: Mon, 11 May 1998 02:38:43 -0700
Hi Ed,
> > The lower the feed impedance, the greater the current in the driven
> > element, which results in increased gain. Below 10 Ohms, efficiency
>> may become suspect, but above 10 Ohms or so, the gain seems to be
>>real > and > realizable.
>
> Increase gain by lowering the feed impedance to increase current?
I'm sure N4KG Tom just misstated something he knows. He's in good
company.
The ARRL Antenna book had a similar miscommunication. They claimed
decreased dipole radiation resistance at a certain height is
responsible for increased gain at that certain height. Orr makes the
opposite statements in some antennas, claiming higher feedpoint
resistance means more efficiency. Both are wrong.
> When did they repeal P=I^2 R?
Amateur books, periodicals, and advertisements are full of incorrect
conclusions that feedpoint resistance has something to do with
efficiency.
Feedpoint resistance is only the feedpoint resistance, and tells us
nothing about efficiency without a lot of other data. We have
absolutely no idea what portion of that resistance is due to wasted
radiation (like heat), and what portion delivers useful signal to the
distant receiver.
When you read a claim one system is better or worse than another and
the only supporting data is feedpoint resistance or current
measurements, toss the article it in the trash.
>The high gain which results from decreased spacing,
(snip)
The lowered part of the feedpoint resistance due to radiation
measured at the current loop is caused by destructive radiation from
the additional elements.
Resistance due to radiation becomes less (and current more) because
radiation from the closer spaced elements is more out-of-phase with
the driven element..... even in the favored direction of radiation.
The resistance at the current loop becomes less because if the
antenna "accepts" 100 watts, it ALL has to radiate. What doesn't
radiate as heat radiates as an "EM wave". Only current (charge
acceleration) causes radiation. The tougher you make it to couple
power to space (by forcing the antenna to interfere with it's own
radiation) the higher the current causing the radiation must be for a
given size radiator to get rid of all that power.
Think about this concept:
If we place a dipole in a very large chamber with lossless walls,
all radiation would stop at the walls. Current at the feedpoint
would increase greatly, and become limited only by loss resistance in
the antenna.
It doesn't matter if that chamber is 100 feet radius, or a hundred
miles wide in radius. If radiation does not turn to heat or leave the
chamber, the dipole's impedance will go to near zero (to the value of
its own internal losses)! As a matter of fact this is about the only
way to make a simple direct measurement of the part of an antenna's
feedpoint resistance caused by loss in the antenna!
Weird stuff, isn't it?
73, Tom W8JI
w8ji.tom@MCIONE.com
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