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Not to start an arguement just some thin s Ive learned over time.

Discussion in 'CB Antennas' started by Grim Reaper, Sep 29, 2011.

  1. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    Hang on a minute, how does a parallel voltage divider circuit (two 100 ohm loads) act as a series circuit at 27mhz:confused:

     

  2. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    I don't understand circuits very well, but where do you come up with two 100 ohm loads?

    I thought this topic was about co-phasing two mobile antennas?
     
  3. W5LZ

    W5LZ Crotchety Old Bastard

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    The antenna input impedance plus the impedance of the matching section for that antenna are approximately 100 ohms impedance. Parallel two of those antennas and the resulting impedance is approximately the desired 50 ohms impedance. That '100 ohms' and the resulting '50 ohms' are -only- approximations. There can be quite a variation in those impedances.
    - 'Doc
     
  4. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    Fine, 98 ohms, 106.371 ohms, I don't really care if it's exact, I just don't understand how 4 watts can be divided by 2 but remain 4 watts into both parallel loads, now equaling 8 watts?

    If that's the case, then let's set up 128 pairs and keep piggy-back-splitting them by using 75ohm coax phasing harnesses and then the original 4 watts will now provide over 1 kilowatt? :unsure:
     
  5. office888

    office888 Active Member

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    I'd rather have a 1/4 wave vertical at 10,000 ft than a 10 element yagi at 10 ft.
     
  6. if you're using coax for the feed line on both the 1/4 and the yagi id rather have the beam at 10 ft. .
    80 db of loss from 2 miles of RG8 LOL :laugh:
     
    1 person likes this.
  7. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    Having just checked the price of tea in China, I've decided I'd rather have a bottle in front of me than a frontal lobotomy.
    I hear the skiing is excellent this time of year.
    Can you even imagine a supercharged V-8 mid-engine?
    How 'bout them Knicks!
    Never run with scissors.
     
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  8. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Below I hooked two 1/2 wave 50 ohm jumpers to two 50 ohm dummy loads. I connected the jumpers to a T-connector and hooked my Autek VA1 to the third port on the T and I saw 28 ohms.

    I hooked one jumper direct to the VA1 and got 49 ohms, the other image is also 49 ohms with the VA1 direct into the dummy load.

    I don't have a 75 ohm co-phase harness, but if I did and hooked it up in this setup then I would expect the TX'r end of the T-connector to show close to 50 ohms. Am I wrong?

    The following is not a transformational harness so we should not see any transformation by the harness, but we do see an approximate halving of the impedance at the transmitter end of the harness.

    50 ohm co-phase setup into two 50 ohm dummy loads.
    Co-Phase harness (640x480).jpg

    VA1 into T-connector at the end of the harness.
    VA1 Z value on Co-Phase (640x480).jpg

    VA1 direct into a 50 ohm non-reactive dummy load.
    VA1 direct to dummy load (640x480).jpg

    VA1 direct into one of the jumpers and a dummy load.
    VA1 thru one line into dummy load (640x480).jpg

    Sorry for the meter response no showing up well, this meter is old and worn somewhat.

    If I'm not wrong, then where are ya'll getting 100 ohms? Is the discussion about a mobile antenna or some other antenna like a quad or maybe a 5/8 wave with an end impedance somewhere around a 100 ohms with a lot of reactance to boot?

    NB, you're getting all incredulous again, like I called your momma a bad name. I asked a civil question, and it was not a trick question about the accuracy of the impedance.

    BTW, I agree with you not seeing how 4 watts....however you said it.
     
    #38 Marconi, Feb 21, 2012
    Last edited: Feb 21, 2012
  9. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    Hi Marconi,

    You turkey!

    Sometimes you take things personally that are never meant that way, not by me anyway. You're the most enthusiastic user on this forum and the only one who addressed my question on topic. (y)

    ...but stop calling my momma bad names! :tongue:

    OK, the 100 ohms is what you should get at the other end of a 75ohm electrical 1/4 wave matching transformer coming from a 50ohm load. (Don't even begin to try to make me think you don't already know this)

    Two of them, each from a 50ohm antenna into a T connector should show right about 50 ohms at the single input, like your test, except double the impedance back to 50ohms.

    5 out of 4 Doctors recommend wine over Chicken McNuggets.
     
  10. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Boy it don't take much to get you boys all bent out of shape! ^can not stand it^

    Booty is right, if old 888 could get it up at 10000' feet his signal from his CB would probably peter out with about 200' - 300' feet up the coax. Plus if he was that high above everyone around him for about 200> miles, and could make a signal, nobody within that range would even know he was out there.
     
  11. bob85

    bob85 Supporting Member

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    eddie,
    if you used odd multiple electrical 1/4waves of 75ohm coax terminated with 50ohm pure resistance input impedance seen looking into each line becomes

    Zin = Zo^2
    ---------
    Zl

    the characteristic impedance of the line (75ohm) squared, divided by (50ohm X=0) load

    if the loads are not 50ohm X=0 the phase relationship between current and voltage along the 75ohm lines shifts and all bets are off.
     
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  12. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    I'll have to dig around in the shop and see if I can find one of my old harnesses, and see how it acts hooked up to my dummy loads.

    You may be right, but I have never seen a 50 ohm complex load from a 1/4 wave radiator attached to a mobile. Sometimes it's even difficult for me to get such a load on a real 1/4 wave base radiator, with slanted down radials unless I have real good control over the slant of the radials below 50* degrees.

    Check out this 1/4 wave with 50* degree radials. This is the Z I usually see and worse when reading the match at the feed point on a mobile. When I check through the feed line, this match does often get better due to transformation. This is why I think guys typically tell us they have a perfect match in their mobiles with a flat SWR that is low and hardly moves the needle. IMO many mobiles can't come close to this angle and provide the necessary ground plane. We found that installing these co-phase setup closer to the ground, on the bumper of pickup trucks seem to improve performance just using old Mother Earth on the roadway. This was the reason for my work with the 102" whip over the years, long before I could make a model.

    View attachment 6665
     
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  13. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Good morning Bob. I use this test formula that I've used for years, and it is base on the feed point impedance for a single installed radiator. Of course both antennas should be the same of very similar for this to prove out and work right the way I understand it.

    I don't know if it's good or not, and I don't know the source. Could be my old antenna Mentor buddy gave it to me when we were building co-phase harnesses years ago.

    Z = SQR (load impedance ohm x transformer line impedance)

    Example of 37 ohm 102" antennas using RG6, 75 ohm, or RG71U, 93 ohm.

    37x75=2775 SQR = 52.7 ohms results at the TX end of harness. So, if using a 50 ohm feed line in system, then further transformation can occur, and the following is supposed to calculate the new impedance value using the results from the first calculation.
    52.7x50=2635 SQR = 51.3 ohms.

    Then we trimmed the pig tail ends of our harness to the lowest SWR and I figured, if everything was good, maybe the Z value for the TX end of the match should be close to 51.3 ohms.

    I never questioned the idea, and I really don't understand all the nuances of this business. So, if you find this in error or totally off base...then let me know.

    Do you see anything wrong with the images I posted above? Is that pretty much what I should be seeing with such a setup?
     
  14. bob85

    bob85 Supporting Member

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    im not familiar with that formula eddie,

    the only thing i see wrong with your pics of the 50ohm lines are the rubber and glass where your lcd display connects needs cleaning with alchohol to get the missing segments working.
     
  15. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Well I see my parallel 50 ohm harness shows about half of the two parallel 50 ohm loads in my example above, so I guess 'Doc and NB were correct. I stand corrected, a transformer should show 100 ohms if the load is 50 ohms, and when run in parallel the net load results will be 50 ohms.

    Sounds reasonable to me, but I always imagined this thing backward I suppose.

    What is a great tip Bob, thanks. No wonder Autek is so willing to fix it for a nominal charge, over $50 dollars plus shipping both ways. I'll try to clean mine.

    Are you telling me that this part of the meter is rubber, like latex?

    Thanks for the tip and the advice.
     

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