Re: Antennas, SWR, feed-ling length, & tuners

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From: KE3FL@delphi.com
Date: Tue Dec 19 1995 - 12:16:50 EST


Again I want to thank everyone that responded. All comments further
explanations and questions were welcome and good.

A number of folks took me to task for "Apparent SWR" and for "fooling
the rig". Sorry folks. You are correct. There is NO "Apparent SWR"
and one can not fool an inanimate object. ;-)

Still, let's not lose the points which are:

1. An SWR meter reads differently when placed along a coax run at
    different distances. That is, adding coax to your system WILL
    change the SWR reading. (So, what do YOU call this? It cann't be
    called SWR because it is not the SWR at the feed-point. So,
    again, what do you call it?) It's the old point of knowing which
    tool to use and how to use it. An SWR meter is designed to read
    SWR at an electrical half wavelength (EHWL) of feed-line at the
    wavelength (WL) of interest. Nothing more, nothing less. It
    does NOT give correct readings when used at an arbirary distance
    from the feed-point.
 NOTE: This feature can be used to check the SWR of your feed-point
    in the following way: Once your believe you are at the EHWL and
    you have tuned the antenna to 1:1 (NOT with a tuner, but by
    changing shape, length, height above ground, etc) add a length of
    coax equal to 1/4 WL of the WL of interest and measure the SWR
    again. This is the worst position for the SWR meter (its at the
    90 degree point from the EHWL) and if there is ANY reflected
    energy coming back (you might NOT have been at a true EHWL) the
    meter will be able to measure it. If you were in fact at an EHWL
    and the SWR is truely 1:1 then you will once again read 1:1 since
    there is no reflected energy.

2. Adding a few feet of coax does NOT change the SWR. By definition
    the Standing Wave Ratio is a ratio of stuff going out to stuff
    coming back and simply lengthening the caox can not change this.
    I like to think of it as simply the RF energy going out to the RF
    energy coming back. This is NOT correct, because if an SWR meter
    actually read the RF E out vs RF E back it would read the SWR no
    matter where along the coax you measured. So in this case you do
    need to be carefull of what you think. Perhaps this is why some
    think that an SWR meter reading of 1:1 at a non-EHWL makes them
    safe. Perhaps I am doing a dis-service using that concept?
    Suggestions here are welcome. Is there an easy concept to use?
    What we're dealing with is the REAL stuff vs what the real-life
    meter does.

3. Adding coax does NOT protect your transistor finals from a
    returning RF hit, even if your SWR meter says the SWR is 1:1
    because you added coax to get it to say so.

4. Use an antenna tuner which does change the SWR the rig "sees"
    and protects the rig by re-reflecting the RF back to the antenna,
    yes and losing some of it too. (I know, I know, a rig doesn't have
    eyes. I got words from Rick WZ2T- BA & MA Psychology, to mention
    only one, for my use of "fooling the rig", next I'll get them
    from biologists. Please forgive this humble physicist.) ;-)

I see I'll have to pay more attention to my words since you are
getting hung up there rather than in the content. ;-)

QUESTION: With a given feedline and antenna you cut the system to a
          perfect (?) match, and lets assume you did this with 60 ft.
          of feed line to the antenna.

          If you change the feedline to...say 100 ft. to the same
          antenna are you suggesting that you will still have a
          perfect match?

ANSWERE: I believe my explanation above covers this, but...
          1) This is the typical situation. It's how I started too.
             Build an antenna, and test it with an SWR meter at some
             length of coax. Tune the antenna until you get 1:1, or
             the best you can get. Now you're safe & on the air!
             Wrong & Right, at least for awhile. Your rig may not be
             safe. If the 60 feet was an electrical quarter WL the
             true SWR will be considerably different than that being
             read by the meter at 100 ft (if 100 ft is NOT an
             electrical quarter WL.)
             From experience: I did exactly this, I built a 2-meter
             J-pole and tuned it with 50 ft of coax (vf = .78) Center
             WL of 146 MHz = 2.08 meters = 82.02 in = 6.84 ft. A half
             WL = 3.42 ft. (50/.78)/3.42 = 16.74 This is NOT an
             EHWL, NOTE an EHWL = 3.42 * .78 = 2.67 ft 19 of these
             gives: 50.68 ft off by .68 feet = .68/2.67 about 25%.
             When I added a few more feet the SWR meter was no longer
             at the same electrical position and the SWR changed. I
             had to think about it for some time before I understood
             what had happened. (I make J-pole kits for ARES/RACES
             groups. I now supply enough coax and explanation so that
             people make the coax an EHWL long.)
  INTERESTING PROJECT: Tune an antenna for 1:1 SWR while using and
             electrical quarter WL of coax. Add enough coax to now
             make it an EHWL. What would the change in SWR reading be?
             Is the real SWR dangerous? Perhaps someone (W4RNL) can
             model this on the computer for us and let us know?

          2) IF the 60ft was an EHWL for the WL of interest, and you
             tuned for 1:1 then changing to 100 ft will NOT affect
             the SWR reading AT THAT FREQUENCY ONLY. If it was NOT,
             then yes, you will get a different reading. Also, all
             other non-matched frequencies will show different SWRs.

Thanks to Paul/NA5N for the data sheet on coax. Missed RG-8X ?

73 de KE3FL/Phil
:-)


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