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SWR vs Field Strength?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W5DXP, Dec 12, 2019.

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  1. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I think you are on to something. The best SWR looking in at the transmitter end indicates that you are tuning into the losses in the system not necessarily the best match at the feed point. I think this goes back to the article on tuning the T network that Zak mentioned. And I have seen this when measuring the current leaving the transmatch. The lowest SWR happens when you are also matching into the losses in the transmatch not necessarily when there is max current leaving the transmatch. I measured the RL looking into the transmatch (got it down to some very high RL > 40 dB and it doesn't agree with max output current at the output of the transmatch for this reason. The difference was very small however between lowest SWR and max current. You can get max current when totally mismatched and I was going to investigate this but other things got in the way,
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    I just did a simple test: my IC7300, nominal 100W solid-state rig terminated into a Bird model 8251 50.0Ohm Termaline draws a Drain current of 17A.

    If I put my Palstar AT1500CV T-match between the rig and the Termaline, and then adjust the T-match for maximum Drain current, I can get it up to 19A. I can observe a corresponding 10% increase in the RF voltage across the Termaline load on my RF monitor scope.

    That tells me that IC7300 output impedance is not a perfect 50+j0 Ohms, and that there is some setting of the transmatch that transforms the 50.0 Ohm Termaline resistance to the complex conjugate of the actual IC7300 complex output impedance.
     
    N0TZU, WB5YUZ, K0UO and 1 other person like this.
  3. AI3V

    AI3V Ham Member QRZ Page

    Now the 64 dollar question is wether or not the finals can withstand 19 amps.

    If they can, why not run 'em that way? :)

    Of course there may be reasons (read imd) that you might not want to....

    Rege
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    My normal procedure is to adjust the T-match for lowest indicated SWR; not max Drain current. I just did the test in post #22 to show Cecil a possible explanation for his Field Strength indication.
     
  5. AA5MT

    AA5MT Ham Member QRZ Page

    At minimum swr, you are tuned to 50 ohms, but the X value is not zero. You can see this in an swr analyzer. At low swr, you have XL or XC values attenuating the signal slightly. At X=0 you are at resonance, with less XL and XC active. It's not much of a difference, but it is there. In most cases at hf, the frequency difference between swr=1 and X=0 is about 4 KHz.

    Tom
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    This entire thread is quibbling about a few% increase in rig output power as a matcher is adjusted (as indicted by an increase in Field Strength rather than adjusting the matcher to lowest indicated SWR). Even if few% was 20% (likely not that much), that is still less than 1db. Does anybody care?
     
    Last edited: Dec 13, 2019
    SWL37632, N0TZU, K0UO and 1 other person like this.
  7. KF5LJW

    KF5LJW Ham Member QRZ Page

    No amplifier is. In Train Driver School the first thing they teach you on day one is:

    We measure with a micrometer.
    Mark the spot with chalk.
    Cut it with an axe.
     
  8. KK5JY

    KK5JY Premium Subscriber QRZ Page

    I see tons of car reviews where people argue and quibble over 1MPG difference in the mileage predicted by the "EPA sticker" on the window of a new car. I then see "owner reviews" where people further complain because their new car gets a few MPG different from that same sticker when the vehicle is operated under their lead foot.

    Yes, some people care about fractions of a dB. They probably shouldn't, but they do. :)

    And in the interest of full disclosure: I'm one of those people who will tinker with the tuning of a monoband antenna to get it as close to 1:1 match as I can. And I do it because, well, because I can. :)

    The fact that nearly all my antennas perform just as well at 2:1 is irrelevant to me. :D
     
    N0TZU likes this.
  9. KF5LJW

    KF5LJW Ham Member QRZ Page

    An honest man with to much time on his hands and not enough to do. :D
     
    N0TZU and KK5JY like this.
  10. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Everybody's talking about tuners but on the original post it didn't have any indication of a tuner just to the network in the transmitter,
    If you havd a bad mismatched, you might want to put your Field Street meter just outside your tuner and see what it looks like in your Shack ! LOL
     
    Last edited: Dec 13, 2019

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