At 12:16 AM 2/1/97 EST, you wrote:
>Fred Hopengarten, K1VR
>On Fri, 31 Jan 1997 10:49:03 -0500 (EST) W8JITom@aol.com writes:
>>Antenna's are the same way. but the only proof is in a measurement.
For years Telex has made field strength measurments of their own
(and now Hy-gain) antennas. One of particular interest to this
discussion is the Hy-Tower: 50+ foot tall, 1/4 high
or odd multiple thereof for 80 up to 10 meters, no traps, but
needs radials as is ground mounted on insulators from ground.
Operates as does a parallel dipole set up: current only flows
at a particular frequency to the open lengthed section that is
at or near 1/4 resonance at the operating frequency, the rest
of the structure appears to be non-existant to the siganal.
Telex measurments and discussion, taken from the Hy-Tower
manual follow. Perhaps this info may help. Note their
disticntion between ground plane and ground mounted verticals.
" Installation of Radials
There is no need to make radials exactly 1/4 wave long for the
Hy-Tower. In fact, the only case where you should have 1/4
wavelength radials(for the lowest desired/planned operating
frequency) would be for approximately 90 radials. This
differs rather dramatically from the case of a Ground-Plane
antenna where resonant radials are installed above ground.
Since the radials of a Ground-Mounted vertical are actually
on, if not in, the ground, they are coupled by capacitance or
conduction to the ground, and thus resonance effects are not
important. Basically, the function of radials is to provide a
low-loss return path for ground currents. The reason that short
radials are sufficient, when few are used, is that at the perimeter
of the circle to which the ground system extends, the radials are
sufficiently spread aparrt, and most of the return currents(in this case)
are already in the ground between the radials rather than in the radials
themselves. As more radials are added, the spaces between them are
reduced and longer radial lengths help to provide (lower resistance)
paths for currents still farther out.
Since the Hy-Tower is a multi-band vertical antenna, the radial system
should be optimized on the lowest frequency you plan to use.
Higher frequencies will benefit equally from the ground
system.
You must decide what is the limiting factor for your installation:
1. Cost of radial wires
2. Land available for radials
3. Efficiency of your antenna"
Telex then provides a large table showing the improvement in
antenna radiating efficiency, in dB pick up over no radials;
the impact on radiation take-off angle in degrees; and
the feed point impedance all vs. the number and length of
ground mounted radials.
In short for sixteen radials, each about 0.1 wavelength long
at the lowest operating frequency desired, 22.5 degree
radial spacing, the power gain over no radials is 3.0 dB.
The angle of max radiation is about 30 degrees over
poor conductivity ground. Over excellent ground conductivity
this angle may drop 5 to 8 degrees. For 16 radials the feed
point impedance is 52 ohms.
Going all the way to 120 ground mounted radials, spaced
3 degrees apart and 0.4 wavelengths long, the operating
numbers improvement to gain over no radials, 6 dB; max
radiation take off angle, 24 degrees(poor ground) down
to 16 degrees over sea water; feed point impedance, 24 ohms.
Also see QST, June, 1985 for a lot more info on radial systems
for verticals.
So, there you have it, from one of the more respected
manufactures of classic ground mounted vertical antennas
as well as beam antennas. Telex/Hy-gain measures 'em.
73, Jim, KH7M
On the Garden Island of Kauai and about to order a Hy-Tower!
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