Hi Guys, here my two cents.
Radials are important to increase the power gain. The directivity gain is
the same. More efficiency means more irradiated power, just it. Steve TX
antenna may have 50 % efficiency. It means using a 1 Kw only 500w is
irradiated. The ground type is responsible for low angle irradiation on the
far field.
My back yard is quite small, 100 x 150 ft. and my tower is not in the center
because I need to tilt it over. There are 64 radials, most of them only 50
ft. long. The efficiency may be 80% or a little better. No problems to work
#298 countries on 160m using a pair of 3CX800 legal limit power amplifier.
1K5 in and 1k2 out (80%).
Doug NX4D city lot is 1/5 of acre, no space for radials. Most of them are 20
ft. long. Doug worked #311 on 160m.
For sure more radials is always good, but increase the efficiency for 90% to
95% is only 5% increase, and it means you double the number of radials.
The more important thing most top-bands like to ignore is the re-radiation
of noise.
It is very simple to eliminate it and tune your tower.
The best way is 3 or more wires 1ft around the tower , grounded at the top
of the tower and connected together at the bottom. This is called UNIPOLE,
or Folded Unipole, the impedance is near 200 ohm, just a 1200pf and a 4:1
BALUN can give you over 200 KHz of bandwidth. Very broad band. If your tower
is over 100ft you can get 200KHz below 1.5 SWR.
The second advantage besides the broadband is to detune the tower. If the
skirt is less than 36m, you can add a capacitor to ground and tune for 1.8
MHz
If you use only one wire to shunt feed the tower, you can do the same. If
you use an inverted L you can the same as well, just disconnect the wire
from the cable. A vacuum relay is a must because the speed, you need 10 ms
or less to avoid hot switch during TX.
Detuning your TX antenna the way above can save you several S units of noise
on any RX antenna. I've seen cases of 4 S units decrease in urban areas, 2 S
units or more is the average for most stations.
Using multiple tower it is very important to detune all of them. You can
phase all of them as well to increase gain and directivity. Vertical arrays
around the tower can perform very well if you detune the tower at the center
of the array.
Any EWE, FLAG, K9AY, Hi-z verticals and HWF or VWF will perform better if
you detune your TX antenna. Most of them wont perform if you do not detune
your tower.
No pain no gain!
73's
JC
N4IS
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Steve
Lawrence via Topband
Sent: Saturday, September 15, 2018 3:22 PM
To: Top Band List List <topband@contesting.com>
Subject: Re: Topband: Use shunt fed tower
My shunt fed tower has no radials. Two 8 ft copper ground rods were driven
into the soil when the tower base was installed. Heavy gauge wire was used
to attach the rods to the tower base bolts. There is simply no place for any
radials on my postage stamp size West Coast city lot.
I believe my house plot was part of a golf course which flooded when it
rained. Flood control channels were built just before the housing tract was
developed.
I use the shunt feed on 160 and 80m where I have over 200 DXCC confirmed on
Top Band and into the high 200s on 80.
Obviously I've been fortunate relative to ground conditions. The laws of
physics are still at work. Could the performance improve with radials? It's
just not possible at this QTH.
My point: Even if you can't install radials you could do worse that not even
try a shunt feed to see how it works. It's not a particularly difficult nor
expensive proposition.
Your mileage will vary.
GL - Steve WB6RSE
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|