[TowerTalk] Topband: topband info

[ersmar@comcast.net]

Ferico:

     I have a small yard for my 64-foot tall tower.  The tower supports a triband Yagi (Bencher Skyhawk) and a rotatable 40M dipole (Cushcraft D40) on a 15 foot mast above the tower top.  I use a shunt-feed system to load the entire tower on 160M.  It works well for me.

     Phil KB9CRY recently posted some photographs of his tower's shunt-feed system which looks a lot like my tower and many others'.  His photos are at:  http://nidxa.org/kb9cry_shunt_fed_tower.htm .  The shunt-feed system is one or more parallel wires held about two feet off the tower.  The wires go from ground level up to perhaps 45 feet on your tower.  At that point the wires are connected to the tower itself.

     At the bottom of the tower, the ends of the shunt wires are connected to one side of a transmitting (HV) variable capacitor of about 500 pF maximum.  The other side of this capacitor is connected to the center conductor of your coaxial cable feedline.  The coax shield is connected to the tower and its ground system (two wires for you.)

     You then feed a bit of power (QRP from your transmitter or from your antenna analyzer) at 1.84 MHz (or your favorite DX spot on 160M) and adjust the capacitor for best SWR.  If you cannot get a good SWR setting, then change the position of the wire on the tower (move it up or down a foot or so.)  Adjust the capacitor again.  This configuration (a single series capacitor) is called a Gamma match, just like feeding a Yagi on HF.

     If this match won't work, you can add a second capacitor from the shunt wire to ground/earth.  These two capacitors make another type of matching system called an Omega match.  You adjust the shunt capacitor for minimum reactance as shown on your analyzer, then adjust the series capacitor for best SWR.  If you don't have an anlayzer, then feed QRP to the antenna and adjust the shunt cap then the series cap, then repeat, until you have a good match.

     I would be pleased to answer any questions you might have about loading your tower on 160M.  I have been very happy with my tower's performance on Topband and have helped several Hams here in the USA to construct a similar system.  

Good Luck and 73 de
Gene Smar  AD3F


 -------------- Original message ----------------------
From: "Federico Zanini IK3UMT" <ik3umt at ir3ip.net>
> Hi all,
> 
> My station is located in a little city lot , let's say about 115 X 65 ft
> Dream of all hams in my condition is to have somewhat "Magic" antenna to
> work topband DX in a so limited space.
> Can you suggest us best antenna(s) to use ??
> Surely a tx one and a rx one will be used.
> My house is on lot center, tower is 53 feet high, with insulated guy wires,
> grounded with two ground rods but i have no way to install any kind of
> radials or other high supports.
> On top there is a hf quadriband yagi with a warc one stacked.
> Preferred way is to have an antenna that doesn't need good ground.
> Other thing to keep in mind : at North, East, South-East and South , there
> are 8mt high , 2 floor houses very close to my garden, will they greatly
> affect patterns of eventually ground-mounted antennas (i.e. transmitting
> verticals or k9ay)???
> Let me know, I hope your answers may be helpful for other operators.
> 
> HNY
> 73 de Federico
> ik3umt
> 
> 
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