To: <towertalk@contesting.com>
> Date: Wed, 15 Apr 1998 15:47:35 -1000
> From: Jim Reid <jreid@aloha.net>
> Aloha to all,
>
> Now have up the 160, 80, 20, 15 and 10 meter monoband
<snip>
> 73, Jim, KH7M
> On the Garden Island of Kauai
Hi Jim,
One thing to keep in mind is that with small radial systems,
ESPECIALLY including elevated systems, you can have a bunch of common
mode current on the feedline.
One effect of the undesired common mode current is ANYTHING you do
that perturbs the shield of the cable, or modifies the impedance from
the shield to ground, changes the antenna's characteristics.
In other words, when you mess around with the shield and its path to
ground or length, you are messing with the antenna itself!
I just experienced this in a series of measurements on radials used
with a 70 ft tower. I used a very high impedance RF choke at the base
of the vertical, to isolate the elevated radials from ground. When no
cable shield choke balun was used, changing the grounding of the
cable would greatly affect SWR, resonant frequency, and the far field
signal level.
BASE IMPEDANCE had almost nothing to do with field strength. The
lowest base impedance systems at times produced poorer FS than
higher base resistance ground systems.
By the way, the best system tested here was STILL 60 ground
mounted 1/4 wl radials (the largest system tested). It was (again)
almost 5 dB up from four properly tuned ground isolated elevated
radials! The lowest base impedance came with two elevated loaded
radials, but it had the POOREST efficiency.
In my recent measurements, 16 ground mounted radials were 1 dB up
from four elevated radials, and three dB up from two elevated
radials, even though the base resistance was actually higher with
the sixteen radials.
On the air tests indicated the same thing. With four elevated
radials, there was NEVER a time when a A-B switch to a reference
vertical (500 ft away from all other antennas and ground systems, far
enough to avoid interaction) would produce equal signals. In general
people reported the elevated system 5 to ten dB down from a reference
vertical with 50 radials, and a reference dipole!
When I installed the 60 radials, there was absolutely NO difference,
report after report, when A-B comparing the two vertical systems. As
a matter of fact the 70 ft tower is now competitive with my 120 ft
high dipole on 80 meters at ANY distance beyond a few hundred miles,
and almost always equals the dipole for DX (broadside to the
dipole).
Grounding the common point of the elevated radials produced a
definite reduction is field strength, proving the need for a choke
balun on the feeder to keep the common point ground independent!
73, Tom W8JI
w8ji.tom@MCIONE.com
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