Hi Ted.
> Can you Towertalkians tell me the pros and cons of each antenna?
A J-pole is not as ground independent as one might think or read. It is an
end-fed 1/2 wl antenna , fed by a 1/4 wl stub. Because the end impedance of
a 1/2 wl is not infinite, but rather some finite impedance between a few
hundred and a few thousand ohms (depending on conductor diameter), the
terminal current of the balanced line driving the actual radiator are not
balanced. This causes the balanced feedline section of the J-pole to have
common mode current adding destructive radiation to the pattern that raises
the wave angle and lowers gain.
There was an article in CQ about this topic, written by Danny K6MHE. (By
the way an end fed zepp has the same problem, and that problems shows as
feeder radiation IF the antenna is modeled correctly or measured in the
real world.)
A ground plane antenna has better unbalance, making it is easier to
minimize feedline radiation. Any groundplane using a small or modest number
of radials will benefit from a feedline choke, to prevent the feedline from
radiating. A 1/4 groundplane and a 3/4 wl tall J-pole have about the same
gain and pattern, because the 1/4 wl groundplane does not have feedline or
stub radiation problems. Given a choice, I'd use a 1/4 wl groundplane.
73 Tom
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