[AMRadio] wanted antenna analyzer


John Coleman ARS WA5BXO wa5bxo2005 at pctechref.com
Sun Apr 16 22:28:56 EDT 2006


This is an edited revision of the email I sent earlier because the first
one did not read properly due to improper spelling and grammar.  I was
in a hurry! Sorry


Brad,
The sum of one ant leg and the feed line needs to be about 120 ft or
doubled for 240 ft so as to come out with a high Z for the Johnson match
box.  Your antenna at 120 - 150 ft is a low Z center fed doublet and
with a half wave of feed line  (135 to 110 ft) it will come out as a lo
Z feed point at the bottom.  The Match Box will not work well on 80
meters feeding a low Z.  

Here are some examples of what you should have.

These all assume you are using open wire feed line with a very high
velocity factor.  I did all my experiment using feed line made #12 wire
spaced at 6 inches about every 2 ft. using phenolic spacers.  Later I
found the 5.25 in drive bay plastic covers from computers works great
for spacers as well, but they do get brittle in the heat and sun light. 

1. - 60 ft per leg on the doublet (120 ft antenna) and 60 ft of feed
line or optional 180 ft of feed line (that's an extra 120 ft -.5
wavelengths option in each example)

2. - 75 ft per leg on the doublet (150 ft antenna) and 45 ft of feed
line or optional 165 ft of feed line

3. - 45 ft per leg on the doublet (90 ft antenna) and 75 ft of feed line
or optional 195 ft of feed line  

4. - 120 ft per leg on the doublet (240 ft antenna) and no feed line
(Puts the Match box up at the center of the antenna, not practical) or
120 ft of feed line.

Example number 5 is cool because the full wave antenna will represent a
very high feed Z at its center and if you wanted to put the Match Box up
in the air and run coax to it you could, as I said "not practical" but I
have, at a field day site, ran the ends of the antenna down to the Match
Box sitting on the picnic table and it works fine.  It was some what
like a large rabbit ear system. HIHI, At any rate if you can use 120 ft
of feed line down to the Match Box it will work very well and it is
usable on 160 meters as well if you wanted to construct a home brewed
high Z tuner for it because the Match doesn't do 160 meters. 

Your lengths of one ant leg + feed line, are summing to about 180 ft
which is .75 wave lengths with a low Z at the feed point and Johnson
Match Box's won't like that. There are other tuners that are made for
low Z loads that will work with it.

For high Z, your shortest lengths need to sum out at .5 wave length.
(One leg of ant + down line length on 75 meters this is 120 ft.). After
you have decided on the antenna and feed line length you can add feed
line at a rate of about .5 wave lengths at a time (again 120 ft on 75
meters).  These lengths are not real critical as the tuner will
compensate as long as you don't get more than a tenth of a wave length
off and maintain a balance. If you start getting close to the .75
wavelength or 180 ft on the sum, then you will have trouble on 80 meters


Now as you go higher in frequency (40 meters or 20 meters) the Match Box
can handle lower and lower impedances but on 80 meters it tends to be a
little persnickety due to the limitations of the capacitors.

At the bottom of this web page (address below) you will see some other
stuff similar to what I been saying here.  
http://www.qsl.net/wa5bxo/swr/swr.htm

Hope this helps!
Good Luck and keep up the experimenting.

Let us know what you come up with.

John, WA5BXO



-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of uvcm inc.
Sent: Friday, April 14, 2006 10:48 AM
To: AMRadio at mailman.qth.net
Cc: antennas at mailman.qth.net
Subject: [AMRadio] wanted antenna analyzer


Any other ideas about matching my extended zepp to 75 meters,
Tried changing feed line length, added 300 to .001 caps, changed ant
length
>From 135 feet to 110 feet on each side, changed from 120 feet to 150
feet ladder line, Match is 3:1 after Johnson Kw1 matchbox


Thanks for the help,

 

Brad KB7FQR

uvc1 at verizon.net






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