In a message dated 10/19/01 3:51:42 PM Pacific Daylight Time,
ve7hcb@rac.ca
writes: <<
From my readings of things and listening to things, the idea of
tuning
near, at, or in the antenna appears to be the best thing to do for most
operation. I have been looking at the screwdriver types of antennas,
some
even offer a dipole arrangement. I think that the only drawback would
be
the lower radiation resistance. But I have not been able to figure
that
out or the efficiency of tuning in the shack versus tuning at the
antenna
for the same size antenna. Then of course for smaller antenna tuning
therein.
Chris opr VE7HCB
>>
Chris: A tuner at the antenna has some merit for complex loads but coax
is now the major feedline to the shack. A remote tuner has to be tuned and
I've done it from the shack to the tuner with 2 selsyns. You also need a
band changing switch. You are suggesting something that is very inconvenient
and slow. Using a tuner at the antenna admits you don't know how to create
50 ohms with a matching system like a "T Match, L Network or a Series Xc" for
use in Verticals or L's which the tuner is now required to do. Transfer the
task of impedance matching regardless how complex it is to the shack end of
very low loss (that never changes) controlled open wire line length--in the
shack. I have used a remote tuner at the bottom of top loaded vertical on
the LF's as it was voltage fed there and coax loss is low on LF.
In case there are any who have repeatedly missed the message, there are very
simple techniques from the Stone Age of the last Century where you can match
complex Z's to 50 ohms in very low loss, a repeatable and recordable form.
Nobody has found a better way to do it--that has shown it. I will personally
reward anyone that does it--with a reward of my choice, naturally. Hopefully
this will inspire someone to give it a try.
One of the main reasons for a tuner is to match the Z at the end of open
wire line preferably of a certain length--to 50 ohms. You let your fingers
do the matching in the shack. The antenna load can be resistive in the
middle of the band or it can be complex on the band edges at the antenna or
end of the feedline. Maximum power can transferred to either load with the
least loss over the entire band(s) by matching the Z at the end of the open
wire line feedline in the shack which a MB will do. The open wire line is
the easiest to repair if damaged (coax is discarded), lasts for a lifetime,
costs the least of any feedline in particular if you make it yourself. After
the settings are determined and recorded across the band they can be quickly
selected without checking the SWR each time. Remote tuning has a few
applications.
Heathkit had a box with a "Motor Driven Series Xc Variable" that was
installed at the feedpoint in one leg of a dipole or inverted vee for broad
banding purposes. Even though it was just in one side it worked just great.
It's "just another example" of where a "Series Xc" has been used to increase
bandwidth over the band "Very Successfully." It can be also used to obtain a
higher Rr value for verticals or L antennas in addition to increasing the
bandwidth--with a single reactance component remotely tuned. Many seem
brainwashed they have to use a 1/4 wave vertical length when in most cases
the Rt is around 40 ohms. So make it a bit longer and add a fixed series Xc
such that it gives 50 ohms. Or add a motor or selsyn driven Xc to obtain the
lowest SWR over the entire band. This should be standard practice for the
progressive in this Century.
I keep getting E-Mails with this basic thread--"I've read your
recomendations and finally decided to try the Series Xc in vericals or L's or
the MB and open wire line on and on and on despite what the Poo Pooers say or
those who never try it and--it works. Why do they do that?" I suggested
"they ask them!" Progress is tough unless the "Professional Poo Pooers"
supposedly generate an idea which is of course very rare. Progress is not
their state of the mind.
Regardless how far you look you will never find a more useful technique
than a "Series Xc." Those who "Slam It Repeatedly" do not give any other
technique that is more useful or what they use. I'm still waiting. I'll
give credit where it's due to anyone. I'm looking for new ideas all the time
regardless who they come from. k7gco
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|