Thanks for the insight. I will try the EZNEC suggestion.
"stub" was a bad choice of words, I've got two approaches in
contemplation/analysis mode, both wire loading schemes:
1. Four relay switched vertical wires at the center bottom voltage
node. The antenna has about a 25 KHz 2:1 bandwidth so a "binary" ladder
of 4 wires can cover the whole band if the natural loop is resonant at
about 2Mhz. Of course, the wires are not 8:4:2:1 in ratio, so I've been
trying a lot of length combinations to get full, no gap coverage and
have a compromise set that "works". I've also got a continuous tuning
controller scoped out to sequence the relays.
2. a metal spool and contact system to feed out hi-flex Cu bare wire and
dacron pull up line on a plastic spool on small (screwdriver) motor with
rotation count output (both spools on a single delrin shaft). Wire out
one side, line in the other to a pulley hung from the apex that lowers
with the antenna. An MFJ screwdriver controller provides 10 preset
turns counts, established at 20KHz steps by manually measuring swr. I
use the MFJ controller on my Tornado variable inductor tuned 80m
rotatable dipole (rebuilt EF-180C) for full band coverage and it has
been solid.
Option #2 is gaining on #1 in my mind. I could buy a SteppIR 40m sized
vertical to do the variable loading, but that is a bit pricey!
Anything like this by other topbanders?
My tree climber peaked out at 87' for the apex pulley support. It seems
to me that the vertically polarized delta loop is the best single
support choice for 160m < 30 degree gain with a little directivity,
and a no extensive radial system. Comments?
Happy Turkey Day!
Grant KZ1W
On 11/22/2012 5:27 AM, Tom W8JI wrote:
It is often mentioned that "very high RF voltages" are present at
ends of dipoles, voltage nodes on loops, etc. but I haven't been able
to find any guidance in the form of numbers. How high is "very high"?
An antenna is also a transmission line, and so the voltage depends on
losses along that line, the line surge impedance, and the current
along the line.
This is why thin wires in antennas (antennaE are on insects, antennaS
are on radios) with low radiation resistance have very high voltages,
especially when loss is low. Since radiation resistance of a full wave
loop is fairly high, voltages will not be extreme. A delta is a little
worse than an expanded area loop with a little less gain, but not
much. All are about equal to a regular dipole.
http://www.w8ji.com/delta_loop.htm
Assuming QRO power into a full size apex up delta loop, corner fed,
vertically polarized, what will be the voltage at the apex? I
haven't found a way as yet to have EZNEC give me a value.
Connect a second current source to a small wire running away
vertically from the corner for 1/8th wave, and set the source to zero
current. The voltage given in the Source Data button tells you the
voltage.
If I wanted to put a relay at a voltage point to switch a stub, how
would I float the 12v DC coil from the RF present?
Why would you switch in a stub? Switching in a "hanging" wire at the
apex or at the center of the bottom horizontal wire (at a voltage
maximum) will lower the resonant frequency. Or are you switching in a
long stub, to make it an entirely different mode of operation?
73 Tom
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