Hello Alan,
Great info there, and I take on board what you are saying, I am moving
away from insulating the tower for reasons stated a moment ago in
another reply. Too dangerous with animals about, and technically with
risk of a fair amount of work not doing what i had hoped. I am about
to post a link to a zip file of photos of how it is now, and I think
optimising this is the way forward, it seems close to what you are
suggesting. Otherwise doing what Rik illustrates with out of phase
current on the two verticals. thanks for the detailed reply, much
appreciated Alan, as always.
Sunday, June 25, 2017, 9:50:21 PM, you wrote:
> Hi Chris, I think you know me and my approach by now :-)).....suggesting you
> asked the wrong question !
> Insulating the mast is possible I am sure as Mark a professional BC engineer
> says..........
> But a different question might be, "how to I use the tower to hold up an
> effective vertical?"
> Rik's tutorial is very good, and one easy way is to pull the bottom of the
> vertical away from the tower structure. I think the easiest way would be to
> combine a couple of ideas, both we know work from experience.
> The loss to the grounded tower will probably not be that big. However the
> loss is a function of the voltage on the vertical wire. Because this wire
> leads from the top of the loading coil the voltage is very high (at high
> powers) coupling current into the tower structure. If you reduce the
> inductance in the base coil(leave sufficient to allow for weather changes)
> and put the balance at the top of the mast you will greatly reduce the
> voltage on the vertical wire thus reducing the current coupled into the mast
> metalwork (Mike, G3XDV pioneered some of this on hs house mounted pole mast)
> You have a sturdy support for the coil and to a great extent the Q or loss
> factor of this coil is irrelevant. this be because the antenna current has
> traversed the radion resistance before it reached the coil (well maybe not a
> very technical explanation but that is the effect)
> Is this better than the loop fed via a vertical wire as as a heavily
> capacitance loaded vertical?? The argument goes like this.....
> The tower supported vertical will have an effective height of half the tower
> height because there is little capacitive loading (unless you string
> horizontal wires from the top, which I believe, the "estate manager" will
> veto :-))
> The loop because of its high capacitance will have an effective height
> almost equal to its physical height. This if the loop is about half the
> height of the tower there will be little difference in the ERP between the
> two systems. In fact the loop may be better, as I keep banging on about it,
> the capacitive load will lead to a much lower ground loss than you will
> probably see from the tower supported wire. This is relatively easy to to
> check by measuring the complex impedance of the the two systems (without the
> loading coil or ant tuning) I think you will findd the "R" in R+jX is much
> higher for the mast and this R is 99% due to ground loss, because the
> radiation resistance is minimal. Matching will not help, you need to
> increase the current in the vertical section, and that can only be achieved
> by reducing the ground loss..
> I hope that makes sense, as it was typed rather quickly.
> Best wishes
> Alan
> G3NYK
--
Best regards,
Chris mailto:[email protected]
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