Hello again antenna fans out there in radio land! In our last episode, our
hero was set upon by his nemesis, that dastardly villain from the
deep......sorry, just kidding, it's not like that at all.
In my last post on this subject I was reiterating the findings and
measurements from an extensive study of shortened monipoles. Responding to
dissenting comments about diminishing current in loading coils, I proposed
that obviously the coil is a radiating device, and that a major part of that
radiation is likely to be heat. I preceded my proposition with a couple light
hearted comments that caused W8JI to wax eloquently on basic AC theory and
Gustav Kirchhoff's laws. Tom, try to develop your sense of humor, OK?
Actually, AC theory has always been a weak subject with me, even as I taught
college electronics, so I have restudied it many times in my life, including
yesterday. Thanks Tom, that is a good thing. Nevertheless, the task for me
has been to reconcile the repetitively measured results in the program with
the applicable theory and laws. (So far, Tom, you've not been a great help!)
Let me go over this again and give you another example from the study. But
first, so we're all on the same wavelength (excuse me) I want to review some
antenna basics. Most of you can skip the next 3 paragraphs.
Either the dipole or its half-brother the ground plane is a capacitive two
terminal device to which we connect the output of an AC generator we call a
radio transmitter. The AC (RF) causes electrostatic and electromagnetic
fields to be created between the two elements of the
capacitor/device/antenna. The strength of those fields is proportional to the
current flowing in the elements. Energy in those fields is *lost* from the
system on each phase reversal of the AC current. That lost energy is what we
call radiation.
One way to achieve maximum radiation from these antennas is to cause them to
be resonant. That is when the capacitive reactance equals the inductive
reactance and they cancel, leaving just the radiation resistance plus loss
resistances and therefore maximum current flow for the avialable power. Of
course, this assumes transmitter/line/antenna match.
Applying these basics to the case in point, the "full sized" monipole/ground
plane is one we have "sized" for resonance. As it turns out, at about a
quarter wavelength, depending on cross sectional area, the inherent
capacitive and inductive reactance of the monipole element will be equal and
will cancel. Now, because full-sized monipoles for the lower HF bands are
too ungainly for our cars, some of our back yards, and many of our pocket
books, we often desire to achieve resonance with much smaller than a quarter
wave element. There are several ways to do this.
Of course, shortening the element as much as we often need to seriously
reduces both its capacitance and inductance. To achieve resonance we can
either add a lot of capacitance, a lot of inductance, or a combination of
both. They may be added anywhere along the monipole, but their positions
will determine the radiation resistance, the size of the fields surrounding
the antenna and therefore the amount of radiation that occurs.
I reported earlier that in the Fletcher program we did extensive comparisons
of inductively loaded masts, from base to top, and I gave far-field field
strengths. Added to that, RF current was measured in loading coils, both
HI-Q and LO-Q. Invariably, there was a vast difference in current into the
bottom and current out the top of the coil. The difference was bigger at
lower frequencies, bigger with shorter antennas and bigger with LO-Q coils
than HI-Q coils. A 9 ft. base loaded whip on 75 meters had a current that
was less than expected 9 ft. from the top of a full sized 60 ft. vertical!
At one point, an antenna was assembled with a 92" overall height and 40
radials of a similar length. It was tested on 10.145 MHz. This was an
effort to simulate a 160 vertical about 42 ft. high. The HI-Q loading coil
(2" diameter, #12 airdux) was fitted with RF ammeters. Both HI-Q & LO-Q (
.625" dia. closewound, #18 on phenolic form) coils were tested in base,
center and top loading configurations for field strength, bandwidth,
radiation resistance and so forth. Important to this discussion is the fact
that the RF current was always readjusted to 100 MA. at the bottom of the
coil. This resulted in 75 MA. for base, 60 MA for center, and 52 MA. for top
loading at the top of the coil. By the way, in the final comparisons, 3 top
loaded configurations of this 8 ft. antenna were compared. One used the HI-Q
coil and a small hat, another used the LO-Q coil and the same hat, and the
third had no coil and the hat wires (4) were extended horizontally for
resonance. The field strength at 3 wavelengths was within one-tenth of a dB
for all three.
Tom said:
> "It is a two terminal device Barry. Kirchhoff's laws require it to have the
same >current into one terminal as out the other."
Well, it's a good thing Gustav's not around. He'd have a fit! Actual
measurements do not confirm that statement. The current in either long,
skinny, closewound loading coils or big, open wound, spaced turn loading
coils diminished very quickly from bottom to top when they were used in
shortened vertical monipoles.
It would appear that the term *significant radiation to the outside world*,
is the key phrase in Tom's remarks if you include RF, heat and, heaven
forbid, light. (Maybe you have been some help, Tom) But it's a moot point
because other measurements from these studies show no significant favor to
HI-Q inductive, LO-Q inductive or capacitive top loading in terms of
radiation strength. Bandwidth is something else altogether.
On the other hand, loading coil position in the mast means everything to
radiation strength. We wanted to see just exactly what the advantage was in
field strength if we followed the recommendations and formulae of guys like
W6TWW, W7XC and others. They have done some very extensive and seemingly
well based calculations that lead to proper positions for the loading coil in
the mast.
The Fletcher program concentrated the bulk of measurements on 80 and 20 meter
antennas, with supplementary work on 160 thru 10. In all cases, using both
HI-Q & then LO-Q coils, we could not confirm any available formulae. The
field strength was always higher as the coil was moved up in the mast, with
peak strength when the coil was at the top and only a hat was used for a
series capacitance.
My wife, Joyce, WB9NUL, says that if I write a few more of these posts I'll
have danged little to reveal when I write my book. Who said I want to write
a book?
73's Barry, W9UCW
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