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[Towertalk] Shunt Fed Tower 160/80

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Subject: [Towertalk] Shunt Fed Tower 160/80
From: (VR2BrettGraham)
Date: Fri, 06 Sep 2002 02:54:08 +0000
VE7FO asked:

>I'm having difficulty visualizing this.  Do you have a wire running from
>your roof to ground?  Radials on the (big?) roof?  Or do you just
>connect the coax shield to the bottom of the tower and forget about any
>kind of ground?  Could you describe the "bit of wire" which provides
>some top loading?  I expect to soon have a C4-SXL on 9m of Rohn 25 on
>the roof and would be very interested in being able to use the structure
>on 80/160.

 From the base of the tower, I have connected all the radials I used to have
on an HF2 that used to live on another corner of the roof.  These are scaled
up 450-ohm punched twinlead equivalents of the Butternut stub-tuned
radials they offer for their HF9.

But since the radials start out laying on the roof of a reinforced concrete
building, they are no longer reasonant at ~10.5, 7 & 3.5 Mc.  I therefore
have added as many various chunks of wires that I could find & just throw
them around the roof & over, if they are that long.

The tower itself is not connected to anything else.

Feeders from antennas on tower come down to base, then lay on roof &
drop down to window below to enter shack.  Any spare length of feeders is
wound on PVC pipe right at base of tower in an attempt to choke stuff
coming back into shack.

The top loading wire is supported by an fiberglass windsurfer mast which
is jubilee clipped to the mast stick out of the top of the tower.  Going from
"diameter" of tower to a little wire like that probably means wire contributes
more top loading as seen by my eye, instead of the happy LF electrons
I push into it.

Tower is one of those JA erector set things like W3DQ is putting up on
his roof.  It has a small platform just below the top of the tower, sticking
out perhaps 0.5m, from which I drop a wire to the omega match below.

Base of drop wire is held away from tower by two pieces of PVC pipe -
one jublilee clipped to the tower, the other of OD slighly less than ID of the
first & wire is fixed to this pipe.  Result is wire position in relation to 
can be varied to change tuning (for 80, I just move it between two positions
to move from 3.5 to 3.8 Mc, on the rare occasion I can find my mic).

A second drop wire hangs from the other side of the platform & is used
for 160.  PVC pipe is used to push it even further away from tower as it's
a real push to get reasonance at 1.8 Mc with such a short tower.

Omega match is the remains of an MFJ tuner - I use one of the variable
caps with some doorknobs for 80, the other variable caps & more doorknobs
for 160.

Since tower is guyed with rope & the top is not insulated, tuning changes
when wet.

Although the pictures are from five years ago, look on K8CX's Ham Gallery
site for VR97BG snaps - at least that would help you visualize what the
tower was like prior to adding the shunt feed.

 From your description of your roof tower - boy, am I envious! - you will have
no problem getting something to play.  More tower is better & you have far
more top loading than I'll ever be able to fly in typhoon country.

Tie the end elements of your C4 to the boom through appropriate chokes
(mentioned on TT or topband lists recently) & she'll be right, mate.  You
may or may not find that better insulation between tower & staircase
enclosure during times of moisture might be needed.

73, BW2/VR2BrettGraham

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