Many thanks for all the replies both on and off the reflector - I have
carefully read them all. Some of them were extremely detailed and would have
involved a lot of work and I am very grateful. Here are some comments:
A number of people said that the tower is too short to use a gamma match. That
is probably true, but it is certainly not too short to use a shunt feed. If the
tower appears to be more than 1/4 wavelength high then a tap will be inductive
and can be made a pure resistance with a capacitor. If the tower is less than
1/4 wave high then the tap will be capacitive and can be made a pure resistance
with an inductor. It had been my intention to use an L network, so that with
the right components I could match either condition.
The tower sits on a base insulator, but that is bridged. I would prefer to keep
it that way for the sake of safety, but also because there are a number of coax
feeds going up the tower. The most important of those is the heliax cable going
to the owner's mobile radio antennas right at the top (and actually sticking
out about 15' above that). I know that it would be possible to choke all those
feeds but it would be a major job. And if I were to upset the owner, then all
this becomes irrelevant!
It would be possible in principle to add top loading - I have a 160m dipole at
the 320' level. However, I would then need to put switching circuitry at the
feed point to connect the legs directly to the tower. I really don't fancy
relays up there - it's a major job to get up there at any time and it is almost
impossible to even get to the tower in the winter. And I care much more about
1.8MHz than I do about 475kHz.
From a Top Band point of view the tower is possibly too tall. As well as the
dipole I have a W4RNL half wave vertical array (a driven element spaced from
the tower and 6 parasitic half wave elements around the tower forming a 3
element equivalent vertical beam). If the tower truly appears to be a 5/8 wave
vertical then the pattern of that array is excellent. If however there is
sufficient top loading to make it appear as a 3/4 wave vertical then the
pattern is truly horrible, mostly going straight up. So I don't really want to
add any more!
In principle I could feed the 475kHz to the 160m vertical element. That is also
not a fun thing to do - that is a high voltage point on Top Band, so I would
need a vacuum relay in a box with excellent rf insulators, as well as a
matching network which would be mostly big inductor.
(The tower has the original BC radial field - 120 x 1000' radials although a
significant number of those are broken. But I think it is still a pretty good
ground.)
I don't know why, but I never seem to think of antenna modelling until somebody
else suggests it and in this case a number of others! I did try and it appeared
to show that the tap needs to be at at least 150' before the feed impedance
becomes reasonable. Hank K7HP did some very detailed modelling showing it
needed to be even higher. (I just made the tower the same diameter as the shunt
wire - Hank did it properly!)
On balance I am inclined to abandon the idea or at least put it into abeyance
until next year. But again, very many thanks to all those who responded.
73 Roger
VE3ZI
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