AA-600 Antenna Analyzer with 600 ohm ladder line

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7F, Apr 5, 2018.

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  1. WA7F

    WA7F Premium Subscriber QRZ Page

    So I was doing some final tests on my 600 ohm ladder line fed 80m doublet at 90' last Sunday and I was getting some erroneous SWR readings on the analyzer. This led me to question the way I'm using it.

    When using an analyzer like the AA-600 with an N connector, what is your set-up for 600 ohm ladder line measurements? I've seen double banana plugs with two leads and I'm using a short length (24") of 50 ohm coax with an alligator clip on the center conductor and the shield.

    Maybe the AA-600 just needs a calibration and I'll check that as soon as I can. I would welcome any advise on the coax to ladder line adapting subject.

    Dave, Ki7AAR
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I have a set of very short alligator clip leads soldered directly to a male connector that plugs into the analyzer. The clip leads are about three inches long each. I wouldn't run coax between the analyzer and the clip leads as that will shift your measurements a bit with greater impact at higher frequencies.

    If you don't run a 1:1 balun between the analyzer and the clip leads (I usually don't) then it's best to set the analyzer on some non conductive surface or object (like an upside down plastic bucket) while making the measurements so body capacitance doesn't influence your readings. IOW, it's best to avoid handholding the analyzer while making measurements though at lower frequencies this usually isn't much of an issue.

    Similarly it's best to run straight off the batteries instead of any kind of AC adapter. I've never tried powering my RigExpert off of anything but batteries but when I've run older MFJ analyzers off of a wall wart it helped to wrap the power cord a number of times through a ferrite core (e.g. mix 31, mix 43) to minimize any common mode coupling on the supply line, battery powered operation eliminates that concern.

    It's easy enough to check analyzer calibration with a set of known loads like a 25, 50, 100 and 200 ohm non inductive resistors and perhaps a few known value capacitors and inductors, measuring at frequencies where their reactance is somewhere between about 20 and 200 ohms or so.
     
    W5DXP likes this.
  3. WA7F

    WA7F Premium Subscriber QRZ Page

    Dave, thanks for the pointers. That all makes sense to and I'll ditch the coax. I was handholding but, the primary interest was 80m for this measurement so that may not have been a big factor. I need to find a set of non-inductive resistors for calibration. I have a hunch that it's way off. Are 1% non inductive resistors suitable for the calibration or do I need to find a NIST calibrated resistor set?
     
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Just take the resistor tolerance and ideally actual value measured on a good DVM into consideration when evaluating your measurements. IOW, if you have a resistor that actually measures 47.5 ohms on a good DVM and your analyzer reads it as 45 or 50 ohms then you have a pretty good idea how accurate your pure resistive measurments are near 1:1 SWR. Similarly if your 200 ohm test load measures say 220 ohms (10% off) on a good DVM then take that into consideration when measuring the same load with your analyzer. I suspect the kind of errors that you're concerned about are a lot more than 5% or 10% errors in impedance values being read.

    Ideally you'll have set of non inductive resistors of known value and a set of reactive components like capacitors and inductors also of known value. But if you're just doing a sanity check on your analyzer's accuracy you can get by with looser standards. IOW, a spiral cut, laser tuned metal film resistor will have some inductance but if you're checking for gross analyzer errors it's not a big deal to use such a resistor as a test load knowing that the inductance will introduce some error that will increase as frequency increases. But if you put say a 30 ohm metal film resistor with a bit of unknown inductance across your analyzer terminals and even at low frequencies you see something crazy like 90 - j40 ohms then you know your analyzer is out to lunch.

    Testing into a known good dummy load is another good sanity check for your analyzer and paralleling a couple of dummy loads if you have access to them with coaxial T connectors is a good way to asses how your analyzer responds to a known 25 ohm load (2:1 SWR).
     
  5. K4SAV

    K4SAV Ham Member QRZ Page

    TNC to banana plug adapter with screw terminals.

    IMG_4008a.jpg
     
    K2CAJ likes this.
  6. W1VT

    W1VT Ham Member QRZ Page

    Antenna analyzers are typically designed to measure low SWRs near 50 ohms, since that is primarily what hams are interested in. If you want to measure high SWRs, it is likely that you can get better accuracy by measuring the voltage or current as a function of distance along the 600 ohm line.

    Alternately, you can add in some transmission line to move the impedance closer to 50 ohms resistive.

    Zak W1VT
     
  7. WA7F

    WA7F Premium Subscriber QRZ Page

    That's all good information. Thank you. I've got a few Bird dummy load resistors and I'll start there with some sanity checks. It's probably time to upgrade the DVM. My old Fluke 77 has been through a lot and it hasn't been calibrated in maybe 20 years.
     
  8. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    For measuring balanced lines I made a balun with very high common mode isolation. It is a coax line wrapped around a form and then I added a variable cap across from shield to shield to tune the assy to parallel resonance. It is only for one frequency at a time but that is how I need to use the old Z bridges anyway. The Q of the coax on the form is > 200 so the common mode Z is very high compared to a ferrite core balun or non-tuned coax-wound balun. I determine the electrical length of the balun and interconnecting coax and take it into consideration for all results. (to determine the elect length I short the output terminals and measure the Z looking in and then plot this on the Smith Chart). TLW is a useful tool for this as well. 73, Pete
     
  9. VE3WI

    VE3WI Ham Member QRZ Page

    I asked RigExpert for advice on using my AA-600 with 450 ohm window line. They said they had no information or advice for me. I should have asked here instead. Thanks to all the posters for being much more helpful.
    73
    Dave, VE3WI
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    The calibration procedure for my AIM4170 automatically compensates for a balun (or anything else) in the circuit and measures the impedance at the balun output terminals looking into the ladder line.
     
    WB2UAQ likes this.

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