[TowerTalk] The Vertical and the Balun

[Steve Hunt]

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