In article ,
Richard Harrison wrote:

I apologize for causing confusion. The Isotron is only a large capacitor
with a trimmer?

Well, from what I can see of it (based on the diagrams on the Web), it
looks rather as if it's intended to behave like a very short dipole.
It's center-loaded and resonated with a large coil, and has a really
humongous capacity hat (the plates). The center coil, and the short
conductors leading from the top and bottom of the coil to the two
capacity-hat plates, would do the radiating.

If this is a correct description, then I'd expect the Isotron to
suffer from the same limitations as any short, loaded dipole - a low
radiation resistance (perhaps only an ohm or two) and a relatively
high loss resistance. Used in isolation (e.g. on an ungrounded mast,
with a choke on the feedline) its efficiency would be quite poor.

The fact that it's stated that it _must_ be grounded to a metal mast
for proper operation raises a big red flag (in my own humble view, at
least). This suggests that other folks are correct in asserting that
the mast, and the outside of the feedline are doing a lot of the
radiating.

It's even possible that in this configuration, the antenna system does
its best radiating when the Isotron itself is tuned _away_ from a good
match with the feedline, thus forcing significant currents onto the
feedline and the mast. One might (in this case) even view the Isotron
assembly as being something akin to a transmatch, with its reactivity
matching the impedance of the "antenna" (the mast and feedline) to the
feedline.

It'd be quite interesting to have somebody set up an Isotron as
directed, tune it to get the best signal in and out, and then run an
RF current meter over the mast, mast-grounding wire, and feedline when
transmitting. If there are relatively high currents present on
these conductors, it'd certainly be diagnostic!

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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