I believe the problem here is that the vertical will not perform
adequately when fed with 50 ohm coax unless it is an odd multiple of a
quarter wave in length (to show something approaching 50 ohms at the
feedpoint of the vertical) If a tuner is used at the radio end of the
coax to successfully reduce the SWR at non resonant frequencies,
performance will be compromised as the coax lying along the ground now
becomes a radiating part of the aerial system.
Adjusting the length of coax feeder may well allow the radio to 'see' a
reasonable impedance, but is this not just achieving the same effect as
in my paragraph above with the associated problems? I would argue that
a tuning unit directly at the base of the vertical converting the feed
impedance to 50 ohm unbalanced for the coax is the best solution.
Trev G3ZYY
In message <[EMAIL PROTECTED]>,
[EMAIL PROTECTED] writes
Pretty good explaination. I'll only add that the formula of 468/f
already has a "correction factor" involved as the real formula is 492/f
(for lenth in feet of a half wavelenth with frequency given in MHz).
So, using 492/f and the proper velocity factor depending on type of
dielectric you should be able to get pretty close to a physical half
wavelength of coax.
If you have an antenna analyzer (pick your favorite, mine is an
MFJ-269), you should be able to tweak it slightly as well. Actually,
1/4 length stubs are really slick as well, again providing a
transformer effect. This is where it gets really cool. Cut a length
of coax slightly longer than your calculation (based on the proper
velocity factor, also divide by 2 if you want a 1/4 wavelenth rather
than a 1/2 wavelength) and install a proper RF connector of your choice
on one end. Connect the connectorized end to the analyzer. You know
you are at a quarter wavelength when the open circuit becomes a short
circuit (with RF using the analyzer, not at DC). Then, for fun and to
prove to you that you just have a 1/4 wavelength line at that
frequency, short the loose end and you should see that the analyzer
measures an open circuit. The reason for this can be explained with
some nice formulas, but also graphically what happens is you rotate 1/2
way around the Smith Chart, from either an open or a short circuit,
whichever case you have, to the opposite side of the Smith Chart. If
you are doing this with 75 ohm feedline instead of 50 ohm feedline, it
will act as a transformer from 100 ohms to 50 ohms also. So a 100 ohm
load if applied directly to your antenna analyzer of choice would show
a 2:1 SWR (assuming Zo = 50 ohms). Put that 100 ohm load at the end of
your 1/2 wavelength 50 ohm feedline and see that it is still a 2:1 SWR.
Now attach it to the end of your 1/4 wavelength 75 ohm feedline and
whatch the SWR magically become 1:1! COOL! Since Field Day is coming
up, use this tip to minimize your SWR (for loads that are slightly high
impedance) very easily without raising and lowering the antenna to trim
it "just right" and without using an antenna tuner. BTW, I always keep
a couple of lengths of 1/4 wavelength 75 ohm "Q-lines" handy and
labelled for what band they are tuned. I saw an old RF engineer do
this at my first Field Day at K8DAC in Saginaw, MI back in about 1983
and I was amazed at the simplicity. Now I'm an RF engineer and I'm still amazed at the simplicity!
73,
Mark, NK8Q
K2 4786
--
Trevor Day
UKSMG #217
www.uksmg.org
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
