It is true that gain changes a few tenths of a db over the range of
radial heights. Antenna efficiency matters more than gain changes for
verticals near earth.
The height matters a great deal below 10ft high. It is less sensitive
the more radials. My antenna is a T, 77ft of vertical wire and two 36ft
top hats with 8x 125ft radials at 10ft.
So here is a comparison for my antenna R (=Rr+Rg) over average ground
for 2 x 125ft vs 8 x 125ft. Modeled EZNEC Pro/4.2 with radials at
different heights over average ground:
ht-ft 2 rad 8 rad
5 33 25.2
10 33 24.3
15 30 23.5
20 29 22.8
25 28 22
50 19
75 17
100 14
A 2:1 TLT matching transformer shows a 1.1:1 SWR at resonance for my
antenna with radials at 10ft.
The difference in efficiency between the 2 and 8 radials at 10 ft is
1.38db, using K8YC 10log(ohm ratio) formula. For my 8 radial antenna the
Rg loss difference is 0.28db between 10ft and 20ft high.
The difference in efficiency between my 8 radial as built and free space
(100ft proxy) is 2.34 db. The take off angle also changes from 23* to
15*. A plan B vs near/over salt water is put the antenna on top of a 10
story building. ;)
So my conclusion during design (tree supported T in a forest) was the
effort/performance was just fine at 10ft. Plus being in a forest causes
more near field radiation loss - maybe 4 or more db. As one poster
said, "all the trees are cut down around BCB antennas".
The advice to have elevated radials at least as high as 10% of the
wavelength seems fairly practical for higher bands. Likewise, more
radials are better until as Rudy showed, there isn't much benefit >8.
With a coil loaded short radials 2 radial DXpedition antenna we tested
the difference between 4 ft and 12ft was on the order of 10 ohms. Again,
height matters most with few radials, more so with shortened ones.
Grant KZ1W
On 12/3/2022 15:00, Artek Manuals wrote:
If you read N6LF's work closely (see fig16 in 3/2012 QEX) you will see
that radial height above 5' adds a few 10ths of a db improvement at
most. (8ft =.015 wavelength above ground) Given noise and QSB on 160
hardly worth the effort ..IMO. Ditto on the number of radials more than
4 at a reasonable (8') height buys very little as well. What is actually
more important is soil conductivity Rich farm loam vs sandy Florida
(Fig 15) is worth 5DB !!! I cant imagine trying to keep sixteen 134'
radials 20 feet in the air !
Another good read on elevated radials ( non resonant radials) that
doesn't get much play is by K5IU, "Optimal Elevated Radial Vertical
Antennas" , Communication Qrtly, spring 1997. If Google is not your
friend contact me off list and I will send you a copy.
Dave NR1DX
On 12/3/2022 2:17 PM, Jim Brown wrote:
On 12/3/2022 10:20 AM, Chuck Dietz wrote:
Thanks for the info. I think I am going with 60 radials on the ground. I
want to use the 160 vertical tower to support an 80 meter array
around it.
Elevated radials might make adjustments way more complicated.
It's worth studying N6LF's work on elevated radials. He stresses that
keeping radial currents equal reduces loss, that making them slightly
shorter than resonant helps that, and so does having MORE elevated
radials -- for example, 8 is better than four. From N6BT, I learned
that elevated radials for 160M should be at least 16-20 ft high; I
learned that when I asked him why my 4 ft high radials weren't working
well.
I have a
tractor and a good welder guy to weld a bracket and tube to a single
plow
shear to make a radial plow.
Remember that the only virtue of burying radials is to protect them
from damage, or from being a trip hazard. We are NOT trying to couple
the antenna to the earth. The function of radials is to SHIELD the
field produced by the antenna from the lossy earth, and to provide a
low resistance path for the antenna's return current IN PLACE OF THE
LOSSY EARTH.
Radials DO couple to the earth, and the ground loss shows up as series
resistance. But radial current divides by the number of radials, power
loss is I-squared R, so gets smaller in each radial twice as fast as
the number of radials is increased, so more radials reduces loss.
73, Jim K9YC
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