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Re: [TowerTalk] End feeding a half-wave vetical

To: "" <>
Subject: Re: [TowerTalk] End feeding a half-wave vetical
From: Jim Smith <>
Date: Mon, 01 Aug 2005 14:58:55 -0700
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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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