How to measure SWR of ladder line?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K3RW, Feb 20, 2020.

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  1. K3RW

    K3RW Ham Member QRZ Page

    Sorry if this is a dumb question. I couldn't find any guidance on this one...

    I picked up a Dentron tuner without a meter for experimenting with balanced/ladder lines.

    Since it has coax and balanced lines, do I need a special type of SWR meter for the balanced line outputs? Or put it after the amplifier and before the tuner for a balanced line measurement?

    I see dozens of posts asking about ammeters and amplitude metering. Assuming both legs are balanced (probably not perfectly), if I just want to spin the knobs and inductor switch for a better match for SWR... how do I place the meter? Or do I need a special one for balanced line?

    I'm not interested in the power out (unless that max power is the only way to tell me it is the lowest reflected), but I would like to know if I am matching better or worse when I spin knobs.

    Meter will be a AWM-30 Ameritron cross-needle, but it has a coax connection. I haven't seen a meter (yet) that has balanced connections.

    Please reply direct to the thread so I see the notifications. 73 de Greg, K3RW
     
  2. KP4SX

    KP4SX Premium Subscriber QRZ Page

    I'm assuming that it has a coaxial input. That's where the SWR meter would go anyway. Always before the tuner. That's how you tune the tuner!
     
    KL7SG, N0TZU, K7TRF and 1 other person like this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    I would use the SWR indicator built-into my '7300. If you have to use a standard 50 Ohm SWR bridge, place between the transmitter and the input to the tuner.
     
    K3RW likes this.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ^Exactly as above, last two posts.

    No reason to try to meter SWR on the balanced line.
     
    WA8FOZ and K3RW like this.
  5. K3RW

    K3RW Ham Member QRZ Page

    Thanks! It does have a coax input. Depending on the model of Dentron, there are NO switches between the coax and balanced outputs (Super, Super Super, MT2000). So I suppose if I want the balanced line, then don't connect the coax too (!). Other Dentrons (some Monitor variants, the MT-3000) have coax/balanced line selectors.

    As for ammeters and all that, I'm assuming one wants that to adjust the individual legs of balanced line? Can this even be done with a Dentron (or any other balanced line tuner), or does it have to be a 'true balanced line tuner'?
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    If the antenna/line is intrinsically unbalanced, there is nothing you can do to at the tuner end to fix it, regardless of what kind of tuner you have...

    An unbalanced line just causes less problems if the tuner is link-coupled or floating differential.
     
    K3RW likes this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If the balanced line is cut to the same length for both conductors and terminates in a balanced load, the line will be balanced. Slight imbalances might occur if the line "routing" is imperfect, but I always considered that to be a "who cares."

    The only objective of the tuner is to pump as much current into the line as possible, and if you can do that (by adjusting the tuner for 1:1 SWR at the transmitter port), you're done.
     
    KA0HCP and K3RW like this.
  8. KL7AJ

    KL7AJ Ham Member QRZ Page

    Here ya go http://www.rfcafe.com/references/qst/twin-lamp-qst-october-1947.htm
     
  9. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Not necessary. In fact I've never seen it done. I suspect it would require individual tuners and bi-directional ammeters in each line. And there's really nothing practical to gain.
    Enjoy that AWM-30 :)
     
    WA8FOZ and K3RW like this.
  10. KL7AJ

    KL7AJ Ham Member QRZ Page

    Back in the ancient of days it was standard practice to have a separate RF ammeter for each leg.
     
    K0UO and K3RW like this.

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