[Towertalk] Grounding mast to tower

[Pete Goudreau]

Red wrote:

> Hi;

> A modestly high impedance between the mast and the tower will send
> more of the current down the feedline, thus increasing the voltage 
at
> the lightning arrestor, suppressor, or attenuator (of course, you 
have
> one of those!).

Yeah, wondered about that as well.  Unless the antenna is a log 
periodic with a stub shorting the booms, there's no path from a yagi 
to the feedline other than via the driven element.  But then the 
entire log periodic is insulated from the mast and all the elements 
of most yagis are insulated from the boom, too.

What I haven't figured out yet is how to go about earthing all the 
conductive elements at the top of the tower to reduce the probability 
of charge buildup.  Sure, the buildup will usually just discharge 
across to earth via any surface that presents a creepage path and 
that's likely what happens all the time, but I'd rather all the 
surfaces were earthed to keep the antennae as quiet as possible and 
to minimize the buildup that in a storm that could attract a stepped 
leader.

Maybe it doesn't really matter but it strikes me that if all surfaces 
are tied to earth through the mast by adding RFCs across elements to 
the boom, both static discharge to earth as well as optimization of 
top loading can be achieved as well as avoiding the potential for 
arcing (sorry...) during high power transmitting with the tower being 
used as a vertical.

I'm pretty sure the use of RFCs for this purpose was discussed here 
fairly recently.

> It is desireable to bypass the current through as many paths as
> possible.

Surely won't argue that.

> Each additional path reduces the overall impedance and reduces the
> voltage on the feedline. Model every path with a complex impedance 
and
> calculate the effect of changing impedance. Read Polyphaser's book 
on
> the subject.

Yep, done that already for the overall installation to minimize 
potential and common mode surge currents at the station ground 
window.  Just concerned at this point with minimizing the probability 
of a strike, as well as simultaneously optimizing RF grounding of the 
elements that would form the tower's top loading capacitance.

> I recommend connecting a flexible lead from the mast to each tower
> leg. Pay attention to corrosion prevention in the connections, in
> accordance with the materials used. Inspect such connections at 
least
> annually.

It's a good idea, I think, and with a Harger pipe clamp, at least the 
required 1.5" of contact area for both the clamp to the mast and the 
wire to the clamp would be met.  These clamps only allow for two 
wires though but maybe that's enough, one to the rotator mounting 
plate and the other to one of the legs.  An interesting problem.

> 73 de WO0W

Thanks very much,
Pete, AD5HD