To: <towertalk@contesting.com>
> Date: Mon, 11 May 1998 15:27:15 -0700
> Tom Rauch wrote:
No, I didn't write this. The other Tom did...........
> > > > The lower the feed impedance, the greater the current in the driven
> > > > element, which results in increased gain. Below 10 Ohms, > >
> > > > w8ji.tom@MCIONE.com
What I said was feedpoint resistance and current in the system
often tells us absolutely nothing about efficiency. It's much like
measuring length to determine weight, or temperature to determine
relative humidity.
> let me drop you a bone, not on the same matter, but very close.
I'll take losses for 500 bones please! (sorry, a joke about a TV
game show)
> What is happening with Fresnel zone and antenna efficiency,
> if comparing antennas with different Q? All other parameters
> are constant.
Just so this isn't a trick question...
By Q, I assume you mean stored and returned energy in the
antenna zone and/or nearfield (Fresnel) zone and not in components
of the system.
By antenna efficiency, I assume you mean far field radiation
efficiency, since that is what you and I probably want. (Some people
like a sharp tuning hi-Q antenna with low resistance, even if it
has more loss.)
When you say all other parameters are constant, I assume you
allow nearfield and antenna zone power loss to change.
If you mean what I think you mean, losses increase. Energy storage
(Q) increases, and losses increase accordingly. The feedpoint
resistance decreases from increased energy storage around the
antenna while losses increase because of this increase in stored
energy.
I can point out several articles where authors conclude, without a
single measurement of anything that directly relates to efficiency,
lower resistance due to increased energy storage around the antenna
means higher system efficiency. One example is small resonant
elevated radial systems, where authors conclude lower resistance at
the terminals is always better.
Strange concept, but pretty common since field strength measurements
were banned (unless done on computer with a virtual antenna), and
subjective data allowed as verification of efficiency changes (ie,
PY7SOB worked a new record with this system, and was loud everywhere
for one whole weekend).
73, Tom W8JI
w8ji.tom@MCIONE.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|