First, I want to apologize for taking so long to respond to everyone who
put time and thought into helping me with my question posted Jun 1/05.
It is very much appreciated. June was pretty much a lost month for me
(just busy, busy, busy) and it's taken most of July to catch up on stuff
(such as the 1700 TT posts still unread).
What I was wanting to do was suspend a half wave vert from a balloon
with the idea that I'd have a nice low take off angle. In retrospect,
I see that I should have mentioned the balloon as lift capacity pretty
well precludes the coax dipole solution.
The reason for wanting a broadband xfmr was that I would maybe be able
to reel in or let out wire as required to have a half wave on 80 or 40
(and maybe 160) without having to retune anything. The xfmr would clip
onto the wire (uninsulated) where it left the reel. So, it was the
matching device for which I wanted a 2:1 or, better yet, 4:1 frequency
range. I really appreciate the various statements pointing out the
difficulties with the broadband idea and have accepted the suggestions
that the parallel tuned circuit with a tapped coil is the way to go.
It did occur to me that I could add a 1/4 wave of wire to transform the
impedance down to something easy to match so I modelled a 3/4 wave
vert. Oooohhh.... yuckkk.. Not a low angle antenna.
Thanks to those who recommended ON4UN's book. What a treasure!! His
statement that 1/2 wave vert performance still depends on ground
characteristics, but not those right at the base, does make intuitive
sense, given that the current max is 1/4 wave above ground. Still,
when I modelled an 80m 1/2 wave vert made of #22 AWG Cu in NEC Win+
with no radials of any kind and the the average .005/13 ground
characteristics I got, at the resonant frequency, a take-off angle of
17 deg and a gain of about 0 dBi (I think NWP shows gain/loss in dBi -
the manual and the help files don't seem to say). Still, my modelling
skills are quite rudimentary. Didn't bother modelling a ground screen
as there's no possibility of providing one.
I received a few direct posts. One which referenced my musings about
stubs said, "Congratulations..... you've just invented the J-Pole.."
Broke me right up.
Another, who has an array of 1/2 wave verts on 160, pointed out the
difficulties in using stubs when he said, "I did look at using a
quarter wave coaxial stub to lower the feed impedance to something
manageable. Problem with that was that even with 7/8" hard-line at 160m
the loss was some 10dB. Good match though!!!!"
Another suggested a 1/4 wave xfmr using 450 ohm ladder line. I had
thought of this but it seems to me that the currents in the line would
be very unbalanced, leading to messing up of the pattern due to high
angle radiation from the line. This assumes that the line is running
parallel to the ground and not very high above it. If the line is
running vertical and connects to the 1/2 wave vert 1/4 wave above
ground it seems to me to be the same situation as with just a 1/4 wave
length of wire, given the current unbalance in the ladder line.
Another proposed a 3/8 wave vert as being easier to feed and working
just FB (310 countries on 75 from Ct). My model says - 3 dBi at a T/O
angle of 21 deg which is 4 degrees worse than the 1/2 wave.
I'm going to have to play with NECWin+ some more to make sure I actually
know how to use it (given that I seem to be disagreeing with ON4UN)
before I pursue this any further.
Once again, thanks very much for your comments and guidance. I have
learned a lot from them.
73 de Jim Smith VE7FO
Jim Smith wrote:
>I want to end-feed a half wave vertical so I can get a nice low angle of
>radiation. The base of the antenna would be about 0.05 to 0.10
>wavelengths above ground. It looks like the antenna impedance will be
>around 4,000 ohms. I would prefer not to use tuned circuits or stubs as
>I would like the device to work over a 2:1 frequency range. (No, I'm
>not expecting the antenna to be a half wave over a 2:1 frequency range!!
>I would have 2 different antennas to choose from.) Power level is 200W.
>
>I was thinking that an auto-transformer with a 9:1 turns ratio would do
>the job and provide a reasonably low SWR for 50 ohm coax. I figure on
>making the primary winding reactance about 500 Ohms at the lowest
>frequency of interest and adding 8 times as many turns for the
>secondary, presumably all on a suitable toroid. The wire insulation
>would have to be suitable to withstand several hundred volts.
>
>The auto-transformer would also nicely solve the static drain problem.
>
>Am I nuts or is this actually feasible?
>
>If it isn't, my next step would be to look into stubs.
>
>73 de Jim Smith VE7FO
>
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