To: <towertalk@contesting.com>
> Date: Wed, 10 Feb 1999 21:28:51 -0500
At great risk we tread on the edge of the thin conjugate match ice!
> > I use 3/4 inch CATV and a run of about 180 feet to my
> > tri-bander. The worst case I have is 2.2 to 1 on 15 cw. I
> > look at it as 2-2.5 to 1 is negligible on HF, whats it
> > going to do, reduce your delivered to the antenna power a couple
> > of watts? The other end will never know..
>
> Um, well...a 2:1 SWR will eat up about 10% of your power, so 2.2:1
> will be marginally worse.
14% is the "% reflected power" at 2.2 : 1 SWR. But those numbers are
meaningless in our applications. Let me explain why....
My FT-1000 runs 200 watts out, into a 2:1 SWR it folds back to 50
watts of actual forward power (power forward minus power
reflected) delivered to the line. That means I have at least 6 dB
power reduction without a tuner from a paltry 2:1 SWR.
I also have a solid state rig that puts out MORE true power into a
1.4 : 1 SWR of the correct impedance (it does fold back with even a
very slight mismatch in the WRONG impedance direction). That isn't
unusual because rigs are seldom exactly 50 ohms source Zo unless they
are adjusted for maximum power transfer or efficiency with a 50 ohm
load. Most solid state SSB rigs are optimized into loads higher than
50 ohms. That's to prevent flat-topping when the load is mismatched,
since a high impedance load always causes flat topping to appear
sooner than a low impedance direction mismatch.
If I run the tuner in the FT-1000, the power reduction is
eliminated and the only loss, other than the loss in the tuner, is a
very small additional loss in the transmission line (assuming the
line is a fairly low-loss line).
An amplifier with an adjustable network that can match the line's
impedance will not suffer the mismatch power reduction of a untuned
PA stage, so in that case the 14% figure is totally meaningless. The
only SWR caused power loss would be the small additional loss in the
transmission line primarily caused by increased current in sections
of the line, but that loss would have absolutely nothing to do with
the SWR measured with a 50 ohm instrument connected to a 75 ohm line.
A high SWR does not always increase system loss. In a typical
pi-network PA loss in the tank decreases with higher impedance
loads, and increases with lower impedance loads! It's a very small
change, but it's there.
Also, loss on a fractional wavelength transmission line can be
reduced by operation with a high SWR, if the impedance movement is in
a direction that reduces line current integrated over the entire
length of the line. That's because line loss is primarily
due to conductor (I^2R) loss at the frequencies where we operate and
with the lines we use. Once the line is 1/4 wl long, we can't reduce
current because any increase in SWR always results in an increase in
current somewhere on the line, so for longer lines an increase in
SWR always causes an increase in line loss.
The true SWR on the 75 ohm line, measured with a 50 ohm bridge as 2.2
to one, could be as high as 3.3 to one in a lossless line and higher
when line loss is counted!
We can not accurately measure the 75 ohm line SWR with a 50 ohm
bridge inserted at one place on the line. The 50 ohm bridge measures
the SWR normalized to 50 ohms, not normalized to 75 ohms as it
should be measured on a 75 ohm line! A 2.2 : 1 reading only
tells us that the line presents an impedance somewhere between 22.7
(50/2.2) and 110 ohms (50*2.2) at point where the 50 ohm-design-
meter is inserted. That means the worse case impedance presented to
the 75 ohm line could be more than 75/22.7 ohms, or 3.3 to one!!!
When things are measured with the wrong equipment, and the
incorrect data is used to estimate the results of the incorrect
measurements, and we then misunderstand what some fancy words
really describe, the final conclusions get pretty far from the truth.
What does this all mean??? It means we often have no idea what
the heck is actually going on in our systems when we mix in odd cable
impedances. We measure the 75 ohm cable's SWR with the wrong
impedance instrument at the wrong point in the line, and have little
or no idea how our equipment ore the cable behaves with the higher
than normal SWR.
Fortunately if we are happy, that's all that really matters. As with
the best antenna, the best SWR is the SWR that makes you
happy.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|