[Antennas] Isotron antenna <was> Re: 40mmobileantennae.r.p.compared

Sun May 1 15:45:07 EDT 2005

yes, the vertical will have a low Z but a base matching coil eliminates this 
problem.
The Isotron, as far as i can tell from the photos, is just a coil, some short 
length of metal, and capacitor plates extended from each end of the coil.
That is a tuned circuit. Maybe they really can get the bandwidth from this.
The Z to feedline is easily adjusted by transformer ratio off this coil, so
the impedance thing versus verticals is a non-issue. Actually, any short
vertical HAS to get out better than the Isotron, because the vertical
has to include some kind of ground plane which YOU must supply,
so the total antenna is quite a bit larger than the Isotron.
As for purely helical wound antennas, those rely on a total length
of wire 2x the actual wavelength, wound on the antenna itself. So
a 1/4w antenna for 40m would have a 1/2w of wire wound on it. It
does NOT base the wire length on the inductance needed to cancel
out the mast's capacitance to ground. I "think" the base Z is then
higher than the usual short vertical, but i don't remember how close
it comes to 35 or 50 ohms. CQ mag had an article on building those
back in its monochrome magazine days (sometime in the late 1950s
i think.) Unfortunately it didn't as i recall go into bandwidth, but just
based on total length of wire used i would assume wider. I will have to
locate that article. I thought their idea of this kind of helical antenna
wound on something like a PVC pipe and with center takeoff for coax,
might be a pretty decent balcony antenna. Radio Shack sells some 
kind of tripod, i can't recall the dia. pipe it takes, 1 1/4 inch i think it
was, which would make a base for the antenna, at least until the 
winds came. -Hue Miller