Feeding a Tower Vertical
A friend here in Tulsa purchased a used 70-foot Rohn 25 tower, complete with tri-bander and rotator. He needed fifty feet for his tower and didn’t need the AR 22 rotator. So Don, K5KW, made me a present of two tower sections and the rotator. All I had to do was come and pick it up.
While looking around the place for a spot to erect a tower, we found the only logical place was where a three-band vertical was standing. With regret, we took down the vertical and started digging a three by three foot hole for a cubic yard of cement to anchor the tower.
Thanks to K5KW, we only needed to purchase a concrete base section and top section to complete the thirty-foot tower. A five feet 2-inch pipe was added above the rotator which was mounted in the tower. Into this was fitted a six-foot section of 1-1/4 inch aluminum tubing from the vertical. A Ringo two meter antenna for 2 meter FM was placed at the top. We now had a tower with the total height of about 39 feet. Now a decision was made to use the tower as a shunt-fed vertical for DX on the 75/40/20 meter bands.
The ground system needed some improvement for this project. Eight more radials were added to the ground system, left from the previous vertical, for a total of 11 radials from 20 to 60 feet in length. There were two ground rods at the base of the tower and one at the operating position. Apiece of RG-8 using the shield and center conductor together connected the tower base and ground system at the operating position.
An RF bridge was used to measure the impedance at the base of a 10-foot TV mast mounted a foot away from one leg at the base of the tower. The bridge indicated low reactance, but very low impedance—less than five ohms. This was too low for a good match across the phone band. First attempts at feeding the vertical with coax without any matching network were not too successful. It worked, but for some reason, the received signal was never as good as the reports we received.
Using a T network at the transmitter, the antenna could be loaded up on 75, 40 and 20. It didn’t work all that well on 75, but did work on 40 and 20. A 45-foot section of RG-8 was being used for feedline and that length of line presented a very low impedance to the tuner on 75 meters.
Series Condenser
Next, the 20-foot section of aluminum tubing, which had been in the other vertical, was used in place of the 10-foot mast. I don’t remember what the exact impedance was, but it was around 300 ohms. Placing a series condenser at the bottom of the feed rod, we were able to tune it until the bridge read 50 ohms with very low reactance. The SWR didn’t go above 1.2:1 across the phone band.
I called it the “unipole feed” and it did perform. Out to about the 600 mile area, it did not equal a low dipole. But, from there out, it was no contest. Schedules with Frank WB5AHS at Midland, Texas gave me better reports than W5KQD here in Tulsa who was using a 135-foot center-fed antenna. I couldn’t raise a VK of ZL on the low wire antennas, but worked them whenever I heard them on 40 meters on the vertical. By this time, the little SBE-33 was pushing a pair of 813s to about 400 watts output.
Originally posted on the AntennaX Online Magazine by Glenn Thomas, W5INU
Last Updated : 9th March 2024