Many years ago I used a 50ft vertical on 160m quite successfully. Top
loading was key to its success. I top-loaded it using 6 equally spaced
wires which came away at 45 degrees from the vertical and formed part of
the guying arrangement. Adding "perimeter wires", which joined up the
ends of these radial hat wires, increased the capacitance considerably.
I found an old 1930s IERE paper which showed that there is an optimum
size for this type of "hat". Too small and there's not enough capacity;
too big and the vertical component of current in the 45 degree radial
wires (which opposes the current in the vertical) begins to lower Rrad
again.
I had some fun slotting the radials in the lawn one-by-one and measuring
the feedpoint R drop at each stage. I gave up with exhaustion once the
graph levelled out :) The whole excersise was published in RSGB's RadCom
magazine.
Another Ham I worked with took a different approach: he top-loaded his
mast with a 160m mobile-whip loading coil and top section. The clever
bit was his mechanical arrangement for auto-tuning the top section. This
was way before the days of "screwdriver" antennas.
73.
Steve G3TXQ
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|