TopBand: Top Hat Loaded Verticals
Wed, 30 Apr 97 17:21:37 -0400
>John, W1FV, has been using ~41 foot tall verticals for many years on
>80M. They use the top set of guys as the loading and are arranged in a
>triangle array with 1/8 wave spacing. John has one of the best 80M
>signals on the East Coast.
Thanks for the plug, Carl! Actually in its first incarnation, I did
use 40 foot verticals, but they are now 61 feet tall. This is still
somewhat short electrically on 80, so I do use the top set of guys
to obtain some top loading to resonate the system on 80.
>He has also developed a feed method to drive all three verticals as one
>single "fat" vertical on 160M. His signal on 160M is also outstanding.
>I haven't a clue as to efficiency, feed impedences, radiation
>resistances, ground losses, etc.
I do indeed drive all three verticals in phase on 160 and switch in base
loading coils to resonate them on 160. Each vertical by itself has a very
low feedpoint resistance. With a laboratory grade General Radio impedance
bridge I have measured the feed resistance (with loading coils in place)
to be about 7 ohms. Compared to theoretical modeling predictions for my
antenna over perfect ground with no resistive losses, I infer a loss resistance
(primarily ground and and some loading coil) of around 2-3 ohms. The radial
system consists of over 100 ground mounted radials per vertical, with most of
the radials in the range of 60-100 feet long. However, when all 3 verticals
(spaced 35 feet) are driven in phase, the mutual coupling between the elements
drives the radiation resistance at each vertical up to around 15-20 ohms (which
is a calculated number and is hard to measure directly). This improves the
efficiency by making the actual antenna gain less sensitive to loss resistance.
The 2:1 SWR bandwidth on 160 is only about 60 kHz, but almost all my
around the 1830 window, so this is not a problem. Using top loading on 160
of base loading would theoretically be even better, but would complicate the
dual band 160/80 operation capability, since switching the loading is harder to
do at the top of the antenna than at the bottom. In any event, it is possible
to make fairly efficient short vertical systems, but much more attention must
be paid to reducing losses than for "full size" antennas.
>So, while the technical discussion rages on, it is interesting to note
>that another ham has excellent results with short, top loaded antennas on
73, John W1FV
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