Topband: Spitfire.......one more question

[Grant Saviers]

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.
>
> _________________
> Topband Reflector
>