Topband: Tall tower with large antenna atop

[Donald Chester]

The base impedance may be high due to top loading provided by the antenna, not necessarily due to ground losses.

My tower is 127' tall, insulated at the base.  At the 119' level (top guying point), I have the centre of my 80m dipole attached.  The ends of the dipole droop down to a little over 100' at each end.  The dipole is fed with open wire line that runs up through the interior of the tower sections.  When using the tower as a quarter-wave vertical, the open wire line is disconnected using a DPST knife switch at the base of the tower, leaving the dipole and feeders to float free.  The ground system is 120 quarter-wave buried radials.

The base impedance as measured with a General Radio antenna impedance bridge shows between 100 and 300 ohms plus a considerable j factor all way across the 160m band, depending on the frequency.  With a simple L-network at the base of the tower set to match the transmission line from the transmitter to 1:1 SWR at 1900 kHz, the SWR stays less than 3:1 all the way to the extreme ends of the band.  I just leave it set at 1900 and tune out the reactance of the feedline at the transmitter end with an additional matching network.

The antenna works excellently.  I get very good signal reports from all over N. America, and have worked some DX.  My DX limitations are more receiving than transmitting, with a rotatable indoor shielded loop and one beverage, directional to the northeast from here, I miss a  lot of DX except for what comes out of Europe. The vertical performs very poorly as a receiving antenna.

I don't think the dipole, which is apparently closely coupled to the tower due to the proximity of the feedline to the tower, reduces the performance of the vertical.  But with the top loading provided by the dipole, I  suspect what I have is closer to the old fashioned vertical tee antenna than a true quarter wave vertical.  If anything, the top loading should slightly improve the efficency of the antenna and lower the radiation angle.

Don k4kyv

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