On May 4, 2012, at 8:10 PM, Richard Fry wrote:
> Bud, W2RU wrote:
>
>> 3. Using radials that are longer than a vertical (of reasonable
>> electrical length) is tall simply wastes a lot of money (and real estate).
>
> Those tending toward such beliefs should be interested in the clip at the
> link below, as well as the BL&E study linked earlier in this thread.
I think you missed the parenthetical portion of my sentence: "of REASONABLE
electrical length".
> Note the logical conclusions therefrom that the radiation efficiency of
> every vertical monopole system of every electrical height depends on the
> loss of the r-f ground reference against which it is driven.
No argument there. The ground losses and the radiation resistance of the
vertical monopole form a voltage divider. The higher the radiation resistance
of the monopole itself, the greater the percentage of transmitter power that is
radiated, rather than being dissipated in heating up the ground near the
monopole. The "logical conclusion" (to borrow a phrase) to take from this is
that you can spend a whole lot more of your time getting on the air and
actually working far away stations and a lot less time fussing with your radial
field, transmission line connections, etc., if you first put your effort into
making your vertical monopole as close to a quarter-wave in height as you
possibly can.
> These data show that for monopole heights no matter how short in electrical
> wavelength, system radiation efficiency using buried radials is dependent on
> the r-f loss in the circular surface area at/just below the surface of the
> earth within ~1/2-wavelength radius of such monopoles.
Agreed. But as I attempted to explain in my previous posting by using the
examples I took from the two curves you pointed us to, for a REASONABLE
electrical height (say, 75 degrees), the difference between quarter-wave and
half-wave radials is hardly earth-shattering (pardon the pun).
The thrust of the paper you were referencing was that the AM broadcast
industry, which had been fixated on half-wavelength vertical radiators since
its inception, could attain comparable field strengths with vertical radiators
as short as an eighth-wavelength (45 degrees' electrical height) if reasonable
attention was given to the radial fields beneath them.
What was absent from the paper, however, was any concern for losses in the
matching network between the transmitter and the base of the antenna. All the
curves, and all the comparisons in that paper are based on constant power to
the base of the antenna — NOT constant power at the output of the transmitter!
Only at the very end of the paper is it noted that the added cost of low-loss
inductors for the matching network can be paid for out of funds saved by
constructing a shorter tower....:-) AM broadcast stations are governed by a
different set of regulations than amateurs. I'll try your approach provided
I'm free to run as much transmitter power as I need in order to deliver a solid
1500 watts to the base of a very short vertical.
Furthermore, you neglected to mention the curve in that same article that shows
how a 25-degree tall tower, for example, has more than 20 times the peak
voltage across the base insulator than a 75-degree tower does. The difference
in insulator specifications required by those two towers represents a potential
cost increase (or, at the very least, an increase in the mechanical complexity
of the vertical at its base) that is totally unnecessary if appropriate
techniques are used to maximize the electrical length of the vertical portion
of the antenna.
> In fact, the shorter the electrical heights of such monopoles, the more
> important such r-f loss becomes toward defining the radiation efficiency of
> those electrically short monopole systems.
Gosh, that was my point, I thought.
One of the core things you learn in any course on antennas is that there's only
a fraction of a dB difference in the theoretical gain of a half-wave dipole or
quarter-wave monopole versus an infinitesimally short one FOR EQUAL FEEDPOINT
DRIVE, but I'll always choose to put my effort into erecting the full half-wave
dipole or the full quarter-wave monopole. Why? Because I know the difficulty
and the losses I'll encounter trying to match the 50-ohm output of my
transmitter to the extremely low resistive and extremely high reactive input
impedance of an infinitesimally short wire — especially if I'm limited to
AFFORDABLE matching network components. I have a good friend struggling with
that exact issue right now; he has an (expensive) antenna coupler that claims
to be capable of handling 1500 watts. It also claims to be able to handle SWRs
up to such-and-such. What the specs don't say, however, is that it can't do
both simultaneously. Components melt if my friend tries. Antenna matching
units that can deliver 1500 watts of RF to high SWR feedpoints are neither
simple nor inexpensive. By and large, they are far beyond what most amateurs
are willing or able to spend.
Losses in the ground are only one part of the resistance divider equation. Six
ohms' ground loss (as measured by at least one person on here) is far less
important to a vertical that has an input impedance (over perfect ground) of 37
ohms than it is to a very short vertical with an input impedance of 1 ohm.
What the Brown article neglects is that similar or higher losses in the
matching network are equally destructive to the performance of very short
verticals. You can have a very short vertical attached to a perfect ground
extending for miles in all directions, and still get lousy performance because
you can get only a fraction of the transmitter output power into the radiation
resistance of the vertical. The beauty of striving for the maximum electrical
height possible at your specific installation is that you are reducing the
effect of BOTH kinds of losses!
My comments were directed at those of us who are interested in vertical
monopoles as a means to an end — working far away stations. I tend to work
more stations by sitting in front of my rig than I do if I'm spending hours
carting instrumentation (that I can't afford) around the back yard or burying
50-100% more wire for questionable benefit. Is my 160-meter shunt-fed
vertical with 16 radials of various lengths perfect? No. Do I have highly
conductive ground around it? Heck, no! Is it a full 90 degrees' electrical
wavelength? Thanks to the top-loading of my HF Yagis, it's pretty close. Does
it get out well? You better believe it! Could it get out better? Sure...for
that 0.01% of the time I can't get through to the DX.
To repeat: In my opinion, for amateurs to lose a lot of sleep over whether to
have quarter-wavelength or half-wavelength radials beneath a vertical monopole
of REASONABLE electrical length is foolish.
I stand by my earlier comments.
Bud, W2RU
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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