On 10/20/20 1:30 PM, ve4xt@mymts.net wrote:
Hi Jim,
The one thought that occurred to me — given the K7LXC need to avoid horizontal wires
— was in remembering K9YC's work at using a choke to make a vertical dipole by putting
the choke 1/4-wl away from the end of the coax (at which point the centre conductor is
connected to the skyward half of the dipole).
Sure.. I think there's an antenna like that in one of the ARRL
compendia. And, I'm pretty sure, in some recent QST. For a "wire"
antenna, that's not a bad approach. In mobile antennas, you'll sometimes
see a "sleeve dipole" built that way - the outside of the coax radiates,
the inside is part of the feedline.
What I wondered about, given the 143 feet of height for a full-size (with
AB7E's 10 feet of elevation), is whether it would be possible to load the
shield so it, and perhaps the upper portion, could be physically shorter, such
as running a conductor connected to the shield at the choke back along the coax
like a linear-loading conductor (maybe not necessarily snug against the coax,
of course).
One can do that. One can also use ferrite loading material that
increases the "inductance per unit length" - you'd want to choose a
material with low loss at your frequency of interest. It would affect
only the currents on the outside of the element.
You can also do top hats/T-bars like the Force 12 vertical dipoles
Could one break the shield at some advantageous point to insert a coil?
(Without breaking the centre conductor, of course.)
Just spit-ballin' here.
There are tons of ways to do this - most antenna designs are driven by
mechanical constraints not electrical ones. Just look at all the
schemes for loading horizontal dipoles and yagis.
There are people who have built antennas out of conductive liquids.
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