Topband: Tophat? Does it have to be at the top?

[Donald Chester]

My vertical is 127' of Rohn 25 on a base insulator, with 120 1/4L radials.  
At the 119' level I have an 80m. dipole attached at the top set of guy 
wires, with about 400 ft of Phillystran attached to each leg, running to 
wooden telephone poles set at the property boundaries.  I chose this point 
instead of the top of the tower to prevent possible bending of the top 
section in case a rare ice storm ever hit this area and took down one leg of 
the dipole.

The dipole is fed with open wire line, running up through the centre of the 
tower.  Nowhere is it electrically connected to the tower (except through a  
knife switch that grounds the bottom of the feedline to the tower for 
lightning protection when the antenna system is not in use).  I hadn't 
expected such a large effect on the vertical, but the base impedance of the 
tower measures in the hundreds of ohms instead of less that 40 ohms, as it 
should for a 1/4L vertical.  The dipole evidently produces a tremendous 
amount of top loading, making the antenna closer to the old "vertical tee" 
arrangement than to a simple quarterwave vertical.  The top 8' of tower 
probably contributes little if anything to the antenna, except for lightning 
protection.

One pleasant surprise was the bandwidth of the vertical on 160.  I match the 
base of the tower to 50-ohm coax using a simple L-network, which is tuned to 
1900 kHz with a 1:1 SWR.  At 1800 kHz and at 2000 kHz, the SWR on the  coax 
line is less than 3:1.  I simply locked down the L-network and when I change 
frequencies the only antenna adjustment I make is to the matching network 
between the transmitter and coax line that goes out to the tower.

This antenna has worked well for over 20 years, and the only major lightning 
damage has been to some antenna and guywire insulators.

-k4kyv

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