Hi thanks for a fascinating and instructive discussion, it is surprising at
first just how close Peter and Dick's results are. I suppose on reflection
the methodology is the same in both programs, one might expect similar
answers. It probably says more about the accuracy, rounding errors and
such, in the calculations. I must admit I like simulations, even if they
don't yield the right (practical) answers, they do give you a good handle on
the 'way things move'. I started a long time ago simulating dopant diffusion
in transistors on a hand cranked calculator. The accuracy was about +/- 100%
because as it turned out,the diffusion parameters were not known accurately
enough. If I'd been a bit cleverer at maths I wouldn't have needed to use
the numerical approach....but it was a lot more interesting seeing the
numbers, and getting a 'feel' for their significance.
I am now waiting to hear the practical side of the 'T' antenna story!!
One thing that seems to leap out from Dick's figures is that there is not a
lot to be gained by spacing multiple wires by more than 0.5 m. Interestingly
the improvement in going for 4 top wires of 5m is the same as 2 of 10m
(about 50%). The percentage gain is the same at either 10m or 20m height. So
its the total length of wire you can support in the sky nearly horizontal
that is important. Confirming the descriptions of antennas that note the
total length of wire used.
I have wondered whether using a antenna system like Steve (GW4ALG) has, it
would be of any benefit to build a small capacity load with very light stiff
wire ('piano' wire) say 2m in diameter with a few spokes. (Would this be
the same as a single 6m flat top?) As it is at a voltage point the
resistance of the wire won't matter too much and it would probably also act
as an 'anti corona' device as well. Of course this thin stiff wire would not
yield as high a capacitance to ground. I could see Steve having difficulty
controlling something like this in a wind though. As I read ( and probably
extrapolate incorrectly) Dick's results, the effect of this small a
'capacity hat' on top of a 20m vertical should produce an increase in Rs of
60%!! That's assuming Steve doesn't already use a metalised balloon!
If it works this might be on as a top-load for the verticals made with those
thin whippy fibreglass tubes.
On a slightly different tack, the effects of nearby trees adding loss has
been described. Has anyone any idea how far away one needs to be to reduce
the effect the a manageable level. I assume it is a near field effect so
will fall of fairly rapidly with increased spacing? For instance I have 5m
fruit trees under the proposed 13m high top. Is that a problem or do I need
to do some radical 'pruning' this winter. I presume the vertical section
should be at least its own height away from buildings, masts etc. but is a
smaller spacing workable?
I suppose these questions have been answered before, but I dont mind
standing up as the dunce and asking the silly questions again. These are the
sort of questions the simulations won't answer but our rapidly increasing
fund of practical experience soon will.
Great Stuff....cheers de Alan G3NYK
[email protected]
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