Re: Experiment of reproducible results PART II

---------

From: George, W5YR (w5yr@att.net)
Date: Sun Jul 14 2002 - 14:27:38 CDT


Dave, let me "me, too" on the end of your excellent posting on just one
aspect.

There is still a lot of confusion in the ranks about whether or not the
antenna tuner actually "tunes" the antenna. There need not be.

Back in the 40's George Grammer, then the Technical Director of ARRL, wrote
a classic paper in QST titled "My Feedline Tunes My Antenna!" It was highly
controversial then since it dealt with issues about transmission lines,
etc. that Joe Ham just knew nothing about. Sometimes I wonder if we have
made much progress in 60 years! <:}

One has to start with asking" What does 'tune' the antenna really mean?"

The generally accepted answer is that tuning is the process of allowing or
causing the antenna to take and radiate all the power that is applied to
it. We sometimes refer to this as a matched condition where the antenna Z
matches the feedline Zo, there are no standing waves, and all the power
that reaches the antenna is radiated or turned into heat by antenna losses.
We "tune" our dipoles, for example, by pruning their length for minimum SWR
at a particular frequency

Now, this tuning can take really two different forms: the conventional
meaning given above which is termed a "Zo match" which most of us are
familiar with and sorta understand. But there is another form: the
conjugate match which Dave references and which the excellent book
"Reflections" by Walt Maxwell W2DU explains in exquisite detail.

Briefly, though, the idea is that when the entire antenna system is
conjugately matched all available power from the source (transmitter) less
any internal losses is conveyed to the antenna itself. The antenna may not
be Zo-matched in which case there will be standing waves on the line and
attendant loss, but setting that aside, the conjugate match condition
assures that all the power that can be taken from the transmitter will be
accepted by the antenna system, if not by the antenna itself.

The test of a conjugate match at a junction, say where the feedline
attaches to the antenna, is to just cut the feedline at the antenna and
looking at the antenna, measure the impedance. Then, looking back at the
feedline, the impedance will be the conjugate of the antenna impedance. If
the antenna is 25+j10 ohms then the feedline output, looking back in, will
present the impedance 25-j10 ohms. Under this condition, and only under
this condition, will maximum power applied to the line be transferred to
the load, the antenna. It happens that under these conditions, you can
break any other line at any point and look both ways and see conjugate
impedances.

What most of us do when we "tune" our antennas *at the antenna feedpoint*,
as with a beta match at the feedpoint of a Yagi, is seek a Zo match to the
feedline. Or when we put a remote automatic tuner at the feedpoint and let
it do whatever is needed to transform the antenna Z to that of the coax
back to the shack.

What we do when we tune our "antenna tuner" in the shack for minimum SWR in
the coax between the tuner and the rig is to adjust the tuner components
such that at the *input* of the tuner a conjugate match condition is setup
which will cause the reflected wave from the antenna/feedline mismatch to
see the conjugate of the impedance seen looking through the tuner to its
output terminals and the input to the feedline. This causes a total
reflection of the reflected wave at the so-called match point (tuner input)
and conserves the power in that wave as it is reflected to join with and
become a component of the forward traveling wave.

So, bottom line, tuning means at least two different things. We are
concerned with Zo matches and with conjugate matches. Sometimes they are
they same - sometimes they are different.

"Read all about it!" in Walt's new book "Reflections II."

73/72/oo, George W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121 Kachina 505 DSP #91900556 Icom IC-765 #02437

Dave Hottell wrote:
>
> Dave,
>
> Whether you believe it or not, it is true. You can see for yourself by
> borrowing another watt meter and installing it between the antenna and the
> tuner. Watch the power as you tune. It will rise as you approach the
> point of proper tuning and it will fall as you move away from there. The
> higher the SWR, the more of an increase in forward and reflected power you
> will see.

<snip>


Search QRP-L Archives

[ QRP-L Archive | ]
[ 1993 | 1994 | 1995 | 1996 | 1997 | 1998 | 1999 | 2000 | 2001 | 2002 ]

---------

This archive was generated by hypermail 2b29 on Wed Aug 14 2002 - 12:29:37 CDT

kd4ab@kd4ab.org