Topband: Direct fed tower
RadioIR at charter.net
Mon Feb 11 11:40:58 EST 2008
Peter Voelpel wrote:
>I am going to direct feed my eletrically 53m high tower against two elevated
>radials at 15m height.
>So far the two extended radials (53m) have been aligned to resonate on the
>same frequency with the tower alone.
>Connected together and shorted by a variable capacitor I get the SWR down to
>1.2 with 80kHz bandwith for SWR2.
>Now my question:
>What is the best solution to for the coax feed, into the radials or into the
>The feed line will be run inside the tower, do I still need a current choke?
>Do I need to decouple the lower part of the tower with a linear trap in case
>the tower is fed by the coax inner conductor or in either case?
Radiation wise it doesn't matter whether you feed the tower or radials.
However, since the coax will be close to the tower, to minimize the
potential across the coax outer cover, it is best to connect the coax
center to the radials, and the shield to the tower. This is also the
case for all the other cables running down the tower. Doing it the
other way, would require that all the other wires going up the tower
have their shields tied to the tower at this feedpoint.
Running the coax inside the tower is the best way to do it. Try to keep
the radials very symmetrical, and the same length if you want a circular
With an electrical 38 meters above the feedpoint, and the radials tuned
to make the whole thing resonant lower in the band, the system is
slightly off-center fed. In other words, the antenna is not fed exactly
at the maximum current point. That occurs a short distance out on the
radials. The only thing that does is cause the common mode currents on
the coax and in the bottom part of the tower to increase some.
With only 2 radials, there may be considerable common mode current on
all coax lines exiting the tower, especially if any of the lengths
happen to be close to multiples of 1/4 wavelength (depends on end
termination and routing). With more radials this would probably not be
the case. Also the bottom part of the tower will have some current
flowing into the dirt. The more radials you have, the smaller this will
become. With only two radials, I would recommend adding chokes on the
feedlines. This will reduce currents on the feedlines, but won't
eliminate it entirely. Fields from the antenna will put some of the
currents back on the feedlines even if you have a perfect chokes, but
this should be small enough to not worry about.
Where to put the chokes is the next question. If they are placed at the
feedpoint, it may not do much because the capacitance between the cables
and the bottom part of the tower may put the currents back again.
Placing them at the base of the tower seems to be the best choice. This
doesn't do much for the currents going into the tower lightning ground
system, which will be mostly lost power. If you want to try to improve
gain by a small amount, you could also consider decoupling the bottom
part of the tower.
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