Topband: Shunt fed tower

[Herb Schoenbohm]

Wayne Rogers wrote:
> Herb:  What do you mean "sweet point?"  This has always been a mystery to 
> me.  I have a 90' tower with a TH-& etc on top that I am getting ready to 
> feed.  I have heard "feed it at the top (just under the beam)" and I have 
> heard 55' up.  How does one go about finding the right spot to attach gamma 
> rod to the tower?  I plan on using 1/2" hardline outer conductor for the 
> line.
>
> Thanks to all for the good advice on this reflector and 73, Wayne N1WR
>
>   
Normally the "sweet point" where the connection between the gamma rod 
and the tower will exhibit 50 ohms plus some inductive reactance makes 
the gamma rod easier to match, i.e. less components in the ATU.  This is 
a bit more difficult if using CATV 3/4 inch cable as the material for 
the gamma rod.  One can use a large alligator clip with a outer jacket 
piercing spear.  This allows you to find that point but requires trips 
up and down the tower. After the point is found the jacket can be cut 
away and a good quality hose clamp can make the connection to the 
tower.  At this point a flexible stranded braid or copper wire to the 
tower is OK as long as the connections are well covered with butyl.  
Wrap with tape first then cover the tape with butyl rubber material. 
Avoid connecting aluminum to steel as this will turn to aluminum oxide 
over time and most likely present problems.  Aluminum to copper is OK if 
the connection is prepped properly.  there are also large AL/CU 
connectors available at large electrical outlets.  Most power company 
medium voltage distribution is all Aluminum and somewhere copper to 
aluminum connections must be made such as from the distribution to the 
tops of pole pigs and drop wires to the house.

I think in most cases the sweet point will be around 35 to 45 feet on 
the tower you described.  You don't need to be exact if you have a MFJ 
259 type analyzer which can measure the result and allow you to design 
the ATU accordingly.  You might also pick a spot, and if you are close, 
an Omega cap from the rod to ground can electrically slide the tap point 
up and down.  However this cap must have the voltage rating high enough 
for the power you run.  Some use a bread slicer to find the value 
required and then replace with some G2 Aerovox micas or equal.  Or if 
you don't have the correct value of a high voltage high current mica you 
can put an inductor in series and tap it up and down making the fix cap 
adjustable.  the principle is aplicable to the  anti-reactance cap 
needed in series between the coax feed and the rod itself.

I hope this helps.

Herb Schoenbohm, KV4FZ