And one more question -
ON4UN in V5 states that the Spitfire will lose a few db of gain due to
ground losses. How does one calculate near field ground losses? What
software is needed?
A related question is a general one for antennas with near ground
horizontal elements - e.g. Bruce & delta loops. I think Tom has stated
more than once that performance could be improved by a ground screen
under this type of antenna. What would be a good cost/performance
ground screen geometry?
Grant KZ1W
On 11/17/2013 3:54 PM, Tom W8JI wrote:
<<<<In order to detune the elements to allow omnidirectional radiation
from the vertical, authors suggested to ground the elements where they
are switched from director to reflector which will give you aprox 74.5
mts long element grounded at the base.>>>>>
You better have a good HV relay for that!
<<<<
In my case I have a relay installed at the corner of the elements
which when opened leave only 53.6 mts sloping in the space from the
top of the 1/4 wave tower to 40 mts from the base and 3 mts from
ground. My question is, could it be possible instead of grounding it
or leaving it opened, to add an inductor or a capacitor at that 53.6
mts point in series with the remaining element (24.0 mts for the
reflector and 17.6 mts for the director) in order to make them
invisible to the tower?>>
The relay requirements are much more reasonable at the point you are
switching. With 1500 watts you have about 600 volts peak across the
relay contacts and no significant pattern distortion. It is much
better than the other method for everything, and requires no detuning
capacitor or inductor. It has to be a good relay with good insulation.
My model shows the array slightly out of tune with those dimensions.
The director is slightly low in current. But it has good F/B.
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Topband Reflector
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Topband Reflector
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