[TowerTalk] Five Gladiators Up (Shorter)

[Tom Rauch]

> 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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