Topband: Shunt fed tower: PLEASE HELP (long)

[Herbert Schoenbohm]

----- Original Message ----- 
From: "Sante - IK0HBN" <ik0hbn at libero.it>
Subject: Topband: Shunt fed tower: PLEASE HELP (long)


> galvanized, and last but not least, I will be not able to climb up
> and down to find the best match.
>
> I have thought at an OMEGA MATCH, but suggestions are welcomed.
>
> As the hazer will support a 3 El. yagi for 40 mtrs, is there a
> "minimum" safe distance from the yagi to the "shunt", up on the tower?
>

Sante,  a 40 meter full size beam (assuming boom grounded elements) on a 100
foot tower should make your pole appear a bit longer than a 1/4 wave on
1.8Mhz but that is not a problem as the top loading will makes it a even
more efficient radiator..  However the 50 ohm sweet point is not all that
important, IMHO. The inductive reactance and impedance found with a cage can
be reliably compensated in the ATU as long as the cage is shorter than a 1/4
wave..  I would guess, absent any model, and with a good radial system, any
convenient height from 15 to 25 meters should be adequate for the wire cage
to attach to the monopole. (Most run the cage much higher than they actually
need.)  In the tuning box consider a bridge T design for more matching range
rather than the conventional L network if you can't get a direct series cap
to the cage close enough to a reasonable match.  Omega matches a fine but
sometimes they take expensive vacs where as a tapped coil with a fixed mica
works fine and saves a bit.

Feedlines and rotor cables may require decoupling at the tower base and/or
run underground to avoid pattern distortion.  If the cage is problematic in
construction you may consider just dropping aluminum 3/4 inch CATV cable
from the platform and use that in lieu of a cage. Having impedence bridge
when your ready to design the ATU component line up would help as well.

Good Luck

Herb Schoenbohm, KV4FZ