K7ITM wrote in news:1191602989.414910.321210
@r29g2000hsg.googlegroups.com:

....
results, Mark. And thanks to Roy for pointing out that the GP doesn't
fully isolate the feedline from antenna currents. I should say
"pointing out once again" as I know he's posted it before, now that
I'm reminded about it. I don't have time at the moment, but put it on
the list to play with in simulations when I have some time.

It is an interesting topic for modelling.

I wrote some notes up after modelling an unloaded elevated vertical as a
multi-band HF antenna. I was particularly interested in the effectiveness
of decoupling of the mast / feedline and the loss implications of mast
current flowing to ground. The article is at
http://www.vk1od.net/multibandunload...ical/13mEV.htm . The spikes in
loss in Fig 3 and 4 are mainly due to loss in the simulated resistance of
the earth connection at the lower end of the mast.

So, the issue is not just about pattern distortion, efficiency may be
impacted significantly.

One of the popular antennas amongst our new six hour hams is to use their
StationMaster on 40m and 80m The StationMaster is a half wave vertical
for 27MHz base fed with a parallel tuned circuit and the 50 ohm coax is
tapped onto the coil of the tuned circuit. (A 27MHz version of the once
popular ham ringo... but the coil is multi turn.)

When modelled on a 10m high mast on 40m and 80m, most of the current
moment contribution is from the support mast and the efficiency is low
due to losses in mast current flowing to lossy ground. Additionally,
there a huge transmission line losses. It is a top fed dummy load... but
with the magic of an ATU, it has a VSWR of 1:1!

But, when their investment in ham radio is 6 hours of learning, they
aren't well equipped to appreciate that the StationMaster isn't an
efficient antenna on 40m and 80m.

Owen