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. N3DT

    N3DT Ham Member QRZ Page

    I've had my AA600 for a few years now and have been very happy with it. In a 50Ω system. I would be very skeptical of using it at 600Ω without some verification. First I would think a transformer to get it to 600Ω would be very problematical unless you have some non inductive loads to verify the results. Mine tests just fine (<1.2:1 through 600MHz) with some decent commercial 50Ω loads and I've even managed to make some loads/opens and shorts with N, BNC and UHF. The UHF are the worst, they're actually only good through maybe 250MHz at best with an SWR of about 1.3:1. The BNC and N are good through the range. But adding cable or connectors will show up in the plots. I've been using mine with the PC all the time. I only use it on battery when I'm testing something like beams outside. You can get high freq resistors from Mouser. C10A50Z4 is a 50Ω 10W 2% SMD good through 3GHz and is less than $3. Mounting and heat sinking non inductively is the big issue. Sometimes you can pick up commercial 10/25W dummy loads in N at reasonable prices used. Lead length is always the bugaboo, and the higher in frequency you go the worse it gets. Through 10M is not much of an issue unless it's getting to 6" or more. I do most of my measuring with the OSL method and it's much easier if you can use the PC. Using it stand alone is a bear that way.

    As far as support from the manufacturers it's not very satisfactory. May be a language issue, I don't know.
     
  2. WA7F

    WA7F Premium Subscriber QRZ Page

    Thanks, Dave. That information will come in handy as I will soon need the AA-600 for some 2m/70cm antenna setup.

    John, I like to your idea of measuring current on each ladder line leg to assure that the system is truely balanced. Here's an interesting article by W8WWW for building current probes and performing balance measurements with an O-scope.

    http://www.seed-solutions.com/gregordy/Amateur Radio/Experimentation/RFProbe.htm
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Of course the SWR is infinite at the ends of the dipole but what is it at the feedpoint? With the data I provided, we have four equations and four unknowns so we can solve for Vforward and Vreflected. Vfor = 650 volts and Vref = 550 volts so rho at the feedpoint is 550/650=0.846. Then SWR = (1+rho)/(1-rho) and that is 12:1.

    They are not on the transmission line - there is no transmission line. They are on the antenna itself. There is a forward traveling wave from the feedpoint to the open end of the dipole and a reflected traveling wave from that open end back toward the feedpoint. Those two traveling waves superpose to form a standing wave. The current envelope displayed by EZNEC (above) is a standing wave - a dipole is a standing wave antenna.

    The feedpoint impedance of the dipole is equal to (Vfor-Vref)/(Ifor+Iref) = 50 ohms.
     
    Last edited: Apr 6, 2018
  4. AI3V

    AI3V Ham Member QRZ Page

    Wow.

    Let's do a little math

    Halfwave center fed dipole antenna.


    What's the difference between $pending 600 us dollars to measure vswr

    Which your rig does anyway.

    And $spending 600 us dollars on a second hand sb220 that provides 10 db or so gain?

    Rege
     
  5. WA7F

    WA7F Premium Subscriber QRZ Page

    Hi Rege,
    I didn't buy the AA-600 for one antenna or just SWR measurements. It's a tool that I hope will cover most of my antenna analyzer needs. Just learning the in and outs of using it correctly and that what started this thread. :)

    By the way the SWR at the rig/amp is typically minimal when using a remote ATU configuration. An SWR reading over 1.2 to 1.5 would indicate the ATU is having difficulty finding a good match. If I didn't use a remote ATU most of the gain from my amp would go to worm warming in the long coax run. Also, I have no desire to listen to XYL whine about a ladder line run across the meadow to the shack so coax it is.

    Dave
     
  6. N3DT

    N3DT Ham Member QRZ Page

    My AA600 agrees nearly exactly with my Transceiver SWR meter and also the LP100A. But they all will see small differences in different connections to the antenna. I can see the variation of coax VF and swr of antennas with the AA600 depending on the weather too. I find it very sensitive.
     
  7. W5DXP

    W5DXP Ham Member QRZ Page

    Here's what a balun can do to the measured impedance. The violet trace is with the AIM4170 calibrated to read the impedance at the balun output (looking into the ladder line). The green trace is with it calibrated to read the impedance at the balun input (looking into the balun). This illustrates the advantage of being able to calibrate the antenna analyzer to measure impedance at different points in the antenna system including at the balanced impedance points. There's 44 ft of RG400 in the balun wrapped around two FT240-K toroids. Note that the two impedances are virtually the same at the system resonant frequencies where the two curves intersect (red dots) where the measured impedance is close to the Z0=50 ohms of the balun. The two SWR curves for the two impedance plots come very close to overlaying each other just as they should.

    XMTR----coax-----balun========40 ft ladder line=====92 ft dipole

    ZS6BalunIOZ.png
     
    WB2UAQ likes this.
  8. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Thanks Cecil. Still doing the tests as if it was the 60's and further back. Short the bridge, null it out (gets rid of the strays associated with the connections on the bridge), attach the balun with the short attached, null the bridge and read off the R and X, plot it on the Smith chart, attach the load to be checked, null and read dials again. Rotate this reading by the elect length determined above and that is the Z at the balun's output. If I was smart enough I could to an open short load cal and use the data from that to correct the reading. This could take into account the transmission line better than me plotting it on the Smith Chart. The shorted balun measurement is a reactance and a small resistance. I am just taking into account the electrical length and ignoring the small resistance due to losses in the balun's coax. Taking into account the coax loss I might improve the accuracy a little. Anyway, it is a lot of fun doing it the old way with instruments that were only $20-50 at hamfests.
     
  9. W5DXP

    W5DXP Ham Member QRZ Page

    Peter, I hope it was obvious above when I said there was 44 ft of RG-400 in my balun, I really meant 44 inches.

    The calibration process of my AIM4170 is one of the things I really like about it. I can calibrate out the impedance between the AIM4170 and the DUT. I was amazed at the graph above. Now I understand why a balun has an associated Z0.
     
  10. IK3UMT

    IK3UMT Ham Member QRZ Page

    Any suggestion for a balun to use in doing such a measure ?
    There is no need for a large transmitting balun, a small binocular core should be enough, turns, core material, any idea ?
     

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