Herbert Schoenbohm wrote:
>
>\> To support a Marconi "T" at 90' may be difficult for many of us and a
> lower
> height may be more easily to obtain. Additionally the wing span of the
> "T"
> of 150' to 180' or longer to obtain a reasonable feed impedance may also
> be
> problematic in normal lot sizes. So give some consideration to either a
> square or delta (triangular) top hat supported by corner supports that
> drop
> the vertical wire. Two cross wires (on the square wire top hat) with the
> vertical drop in the center or three intersecting wires for the triangle
> top
> hat should save space, improve efficiency, and allow for a lower vertical
> overall height. The "T" is simple and easy but does require some real
> estate, whereas a smaller profile top hat will do the same thing or better
> (cancelling high angle useless radiation) as described. If someone would
> be
> so kind to model the various shapes you may find the best arrangement that
> yeilds the best efficiency for the size. Early radio 500 Khz and below
> antennas were constructed this way and perhaps still are.
>
> Herb Schoenbohm, KV4FZ
I have a 90 ft vertical with four 30 ft top wires that follow the
guy ropes down at a 45 degree angle, and it resonates around 1.8 MHz.
I have modelled various top hat configurations and the limitation on
top wires that slope down (umbrella configuration) is that the tips
need to be at least 2/3 the height of the vertical section. If you
try to make them go lower than that the radiation resistance starts
to drop. Using more than 4 or putting a skirt on them is somewhat
helpful, but it gets to be a mechanical nightmare especially if you
have various other guy ropes in the area.
Rick N6RK
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