Hi Barry,
Much of this is at:
http://www.w8ji.com/radiation&fields.htm
and at
http://www.w8ji.com/radiation_resistance.htm
> 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!
With any monopole antenna radiation resistance carrying uniform current
radiation resistance is given in textbooks (like Jasik's "Antennas") by:
1580 * (He/WL)^2
With wavelength 250-feet and an antenna 9-ft tall we have a maximum possible
radiation resistance (uniform current case) of 1580*(9/250)^2 or about 2
ohms. With triangular distribution (base loading only) we have 1/4 that
value, or 0.5 ohms. In order for such an antenna (between .5 and 2 ohms
radiation resistance) to have less current than the area 9-ft from the end
of a monopole, the small system would have to have hundreds of ohms of loss
resistance because the short radiator is a virtual short-circuit (someplace
between .5 and 2 ohms).
I= sqrt P/R, so current into the whip to radiate 100 watts of actual
radiated power as EM energy would be sqrt of 100/2 ohms ranging to
0.5ohms....or someplace between 7.07 and 14.14 amperes. If you didn't
measure at least 7-amperes into the whip you were not radiating 100 watts as
EM radiation, and it would be ~14 amperes if the whip was uniform
cross-section with base loading only and 100 watts was radiated.
Even with 1-watt radiated from a short 9-ft whip on 75-meters, current into
the whip would be between 0.7 and 1.4 amperes.
Whip feed current with ONE watt radiated would be exceed current 9-ft from
the end of a 1/4-wl monopole with the 1/4 wl monopole radiating 100 watts!!
(Current is approximately .4 amperes 9-ft from the open end of a 1/4wl thin
monopole with 100 watts actually radiated.)
> 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.
How did you physically manage measuring current in a close-wound 2" coil
without perturbing the system?
> 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.
Looking back over what I said, I made a misleading statement. I'd better
correct it.
In this context neither loss resistance nor any other form of energy
conversion or radiation will cause current taper, unless it occurs in the
presence of large amounts of distributed capacitance. Distributed
capacitance provides a third path for current in the component or radiating
system through displacement currents, so all circuit laws are satisfied.
In a dipole, the distributed capacitance of the long antenna conductor wire
forms this path to the other half of the antenna and objects around the
antenna through the electric field and the resulting displacement currents
in space around the antenna. The taper is greatly reduced in a compact
system (in terms of wavelength) because there is minimal stray capacitance.
The exception would be if we "hung" large objects off the coil, or had a
coil that was physically large compared to the wavelength and the antenna
system it was inserted in.
If current is tapering in a coil, the coil must have large distributed
capacitance to the outside world compared to the system at the untermninated
area above the coil. This would be the case with a helical radiator.
In a small antenna, capacitance from the coil to an area outside the coil
(and antenna) would have to be a large percentage of the effective load
capacitance on the "open" end of the coil, otherwise current would remain
equal. This is a fundamental part of circuit theory, and is applied to
networks and antenna alike. It's at the root of how we size and design RF
components, and how losses in systems occur.
I've never measured a large or significant taper in current throughout a
component when the component was physically small compared to a wavelength
and the rest of the system. This is true in amplifier tank systems, tuners,
and even antennas...and I measure different components and systems several
times a month.
73,
Tom
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