<< In a message dated 10/4/01 9:19:49 AM Pacific Daylight Time,
firstname.lastname@example.org writes: <<
After thinking about it, you are correct. Obviously, power reflected from
the antenna, will travel back down the line. Unless the source is purely
dissipative as you say, some power will then be reflected from the source,
travel up the coax to be reradiated again.
So yeah, you are correct. The percentage numbers are useful, but they
tell the overall efficiency of the system. It is correct that in a 2:1
match that 11.1% of the incident power at the antenna is reflected back to
the source. That's all it says. Some of that reflected power will get
to the antenna where it will then be delivered to it and be radiated.
John your thoughts are good. With 2:1 SWR on coax and the 11.1% reflected
power, it alters the 50 ohms resistive 1:1 load to some R value above or
below 50 and adds a reactive component also. If the generator matches it
like a pi and reflects an optimum Rp load back to the tube, you have maximum
transfer of power (say it's 100W) to the input of the feedline--regardless of
the 11.1% reflected.
However, the 100W input will be attenuated by the loss in the feedline (say
1 dB for 100' at 10M) and 79.4W will arrive at the antenna. That's 79.4%.
There might be tad less due to the coax losses due to the higher SWR above
1:1 which should be ignorable.
Since most transceivers use the broadband 50 ohm output that's not tunable,
it's output will be less with other than a 50 ohm resistive load. With
increasing SWR, I've had rigs where the fan came on sooner and I could
literally predict the SWR. Finals can only take so much additional
dissipation due to the final load other than the desired created by the SWR,
have protective circuits to start limiting output and finally cut them off.
My IC-720 cut off at 1.5:1. The "final mismatch power reduction" can be
greater than the loss in the coax due to SWR. That's why the lowest average
SWR over the bad is obviously desirable.
I had a 20M horizontal antenna (40M horizontal quad loop) with a W8JK low
angle pattern pointing into the shack and it affected the cut off circuit to
shut the rig down to 20W output even with a 1:1 SWR load into both IC-720's.
ICOM locally had no explanation either of how the cut off circuit was
affected or a fix. It didn't affect other rigs. With SWR less than 1.5:1 it
is always a good idea and switch in the rigs tuner (which it didn't have) for
any SWR above 1.5:1.
There is no automatic sensitively cut off circuit for receive with higher
SWR's or the need for it that I know of. In some rigs the tuner is not in
the circuit on receive and I've never been told why. Strong fundamental
overload on receive (like LA in a contest) even with 1:1 SWR could
desensitize a front end of 75A4's. I was told that by someone he slipped a
10 dB pad in his friends 75A4 receive coax (in LA) and he actually heard the
weak ones better in the contest. Receive circuits now have 10-20 dB
attenuation that can be switched in.
Reflected power is important if you can't match the Z it creates at the end
of the feedline. Lets take the example of a 70 ohm dipole fed with a 1/2
wave of 600 ohm open wire line into Balanced L like I use. The tuner sees 70
ohms. The SWR is 9:1 and if I remember right that's 63% reflected power
(don't have my charts here). When the jump is made to open wire line,
dielectric losses due to SWR can be ignored for all practical purposes. I
use 2 rotary coils silver plated .175" diameter wire and 2 variables of
fairly close spacing in this Balanced L. With 1.5 KW on 75M they run "Stone
Cold." The tuner input 50 ohm RF Ammeter reading is 5.48A for 1500W. There
was about 4.6A in each output ammeter and should be about 4.63A with no
losses in the tuner and a 70 ohm load. Now that's .055 dB or about 19W loss.
This is subject to error of the input and output RF ammeters and could be
better or worse. It's so good who cares? I used to measure at the antenna
also with the same 2 RF ammeters and read them with a binocular and couldn't
see any difference for all practical purposes. That felt awfully good.
In 1953 I worked with W0AJL on a similar 20M set up with a different tuner,
450 ohm open wire line and even 2 balanced 150&56 ohm coaxial stubs matching
450 to a balanced 16 ohms at the DE of a retuned Workshop beam. The rig was
a Collins KW1 with 750W out. We ended up with 700W in the antenna for a 50W
.3 dB loss. Over the years I've seen data from several similar set ups which
shows it can be done using the right techniques. There is a W6 on TT with an
80M vertical and 500' of open wire line. With baluns it had a very low
measured loss of less than .5 dB if I remember right. Some Stone Age
Techniques of the last Century are very difficult to grasp for some even with
many years experience. Coax is on the way out with the up coming Major
Recession--you won't be able to afford it.
Here is a sneaky Stone Age Trick of the Last Century to measure SWR on open
wire line. If you shunt the terminals of a standard spacing RF ammeter using
3/8" washers on #12 open wire line, it reads 1/2 value. It saves cutting the
wire to insert it for a quick measurement. So if you attach the RF ammeter
to the wire and clamp it down for a measurement, then move it 1/4 wave and it
doesn't change its reading--you have 1:1 SWR which happened in this case.
Since the same RF ammeter is used for the 2 measurements and even if there is
any error of the actual I value--it's irrelevant. Another version of this
was used in the 50 ohm coax input feedline to the tuner. The KW-1 had an RF
ammeter in the output. 1/4 wave of coax was used to the tuner with another
RF ammeter in the link circuit. If they both read the same (3.87A) there was
1:1 SWR. The Jones Micro Match came out about then.
Open wire line is often terminated for a flat line as in this case and it's
resonant length is even far less important. The IE&Z is then constant alone
the way. RF voltage is more difficult to measure along the line compared to
the relative current with RF ammeters. If you can't find any RF ammeters at
the flea markets--I probably have most of them. So you can use open wire
line either way--with or without SWR. For 750W the current in a flat 450 ohm
open wire line is 1.29A with virtually no RF dielectric losses compared to
3.87A in the coax center wire of about the same diameter.
The Delta Match was often used on yagis using open wire line. It ran right
into the link coupling with a series Xc in the final tank. The Delta Match
did radiate some due to the nature of its fanned feeders but was still a
great system. A T Match needed some bars and 2 more connections but didn't
require spreading or fanning of the feedline and is the most efficient system
there ever was. So called "Modern Technology" keeps going away from high
efficiency systems for convenience and there is no shortage of those
defending it the practice without ever trying these systems?? I'm going to
duplicate this again and make power measurements at the beam for 300'
So here is 63% reflected power for 9:1 SWR in 600 ohm open wire line
connected to a dipole and maximum transfer of power where 11% reflected for
2:1 SWR could have more loss in coax even properly matched at the rig. Of
course one is on 75M and the other is 10M but you get the main idea. Open
wire really pays off on the long runs on any band and you can construct it
yourself. The porcelain insulators can still be found in the flee markets.
I still have the originals. And no I didn't use Teflon insulators back then
as it didn't arrive until after WWII. It and Delrin work great for spacers.
Before coax in the Stone Age Days of the last Century we never heard the term
"Reflected Power" or worried about it. We didn't have SWR bridges for the
open wire line other than a modified (wider) 300 Ohm Twin Bulb SWR Indicator
and it worked great. We used RF ammeters or shunted pilot light bulbs to
check balance, relative output and SWR. You don't need to know the exact
values all the time, just whether it's going up or down, the ratio or no
change. Matching the Z at the "end of the feedline" is often the most
All SWR is not evil. 1/4 wave matching stubs use SWR to change the Rload to
another more desirable R for longer runs say of 50 ohm coax. Whatever
additional loss due to a higher SWR is usually small if the power level is
reasonable and it's only for a 1/4 wave. You can make SWR work for you this
way. Make sure the rig output sees a low SWR on coax from any antenna with
the built in tuner if necessary and that's all you can do. Would you believe
I'm actually getting inquires and great reports of the use of these
techniques? There are some hams who like to try new things that work even
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