How many radials?
Quite a few years ago I tried to find out how many radials I would
have to *plant* to get a decent ground system for a new T-antenna to
be put into service on 160m....
The height of the antenna was 23m (70 ft) with two toploading wires
to resonate the system around 1750 kHz (for easier matching). Both
toploading wires were horizontal and 40 ft long (12m).
The feedpoint impedance of the T-antenna was measured with a rf impe-
dance bridge. I began laying 50m long radials on the ground connecting
more and more of them to the common ground connection at the rf bridge.
The result can be seen in the following table:
no. of radials: 2 5 9 14 20 30 50
feedpoint (ohms) 122 66 48 39 35 32 29
The 9 radials would almost look like a perfect match to 50 ohm coax.
But your system efficiency in that case is way down below 50 percent!
I stopped after having added 50 radials and looking at the diagram I
decided that I would have to add at least 50 more radials to see a
major improvement.
A few years later I learned that I was within 2 db of the optimum.
The system worked very well for many years. Unfortunately I always had
to remove the radials when the grass started to grow in spring and to
*re-plant* them every fall....But it was worth all the effort and after
some practice and building a terminal plate I was able to install the
whole system (including the matching capacitor) within two hours.
Matching? Yes, I simply had to connect a 1800 pico-Farad capacitor bet-
ween feedpoint and ground to match the system to a 50 ohm coax feedline.
You do not have to bury radials, just lay them on the ground !
Peter, DJ8WL <dj8wl @ uugate.wa7slg.ampr.org>
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