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Where to connect an antenna analyzer ...

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4NYN, Jun 3, 2021.

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  1. K4NYN

    K4NYN Ham Member QRZ Page

    Okay, dumb newb question that I should know the answer to (and I think I do), but I am getting conflicting information. When hooking up an antenna analyzer, do you want it as close as possible to the antenna, or at the other (transmitter) end of the feed line before you trim it to length and tune it? Thanks.
     
  2. AG6QR

    AG6QR Premium Subscriber QRZ Page

    Short answer: It depends on what you want to analyze. You can usually learn what you need to know at either end of the coax.

    Assuming lossless coax, the SWR won't change by putting coax between the analyzer and the antenna. But the impedance definitely will change. So if you take a measurement at a single frequency, your meter may show a capacitive impedance when the antenna itself is inductive at that frequency. So don't put a meter at the radio end of the coax and rely on a single measurement at a single frequency to tell you how to adjust the antenna.

    With real world lossy coax, the SWR will decrease as you add coax between the meter and the antenna. But the frequency at which the SWR is at its lowest will remain the same.

    You can learn a lot by sweeping to find the frequency of lowest SWR, and then you can shorten or lengthen the antenna to bring that lowest SWR point near the frequency where you want to operate.

    It may be impractical to get the meter up to the antenna feed point, and if you do manage to get it there, presumably your body will be up there, too. The antenna sees you as a "bag of saltwater" that couples with the antenna and changes its characteristics. So while it might be nice in theory to put the meter at the feed point, it may not work so well in practice.

    If you use a VNA, you'll be able to calibrate it using open/short/load at the feed point, to make it take measurements as though it's at the feed point end of the coax, even while the meter is really at the radio end. This offers the best of both worlds in many ways. A nanoVNA is inexpensive, with a whole lot of capabilities beyond that of an antenna analyzer.
     
    Last edited: Jun 4, 2021
    WA9UAA, K4NYN and WA7ARK like this.
  3. WD5GWY

    WD5GWY Premium Subscriber QRZ Page

    Connect your analyzer to your coax that will be connected to your radio.
    You need to adjust things according to what your radio will actually be transmitting into.
    If you are using an external antenna tuner, then the analyzer needs to
    be connected to the transmitter connection of the tuner, so you can adjust the controls on the tuner to adjust for best SWR reading on your analyzer.
    Your antenna analyzer should have instructions with it. Or a website you can download the manual. That will help you the most to understand tuning procedures for your particular analyzer.
    James
    WD5GWY
     
    K4NYN likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    The most useful way of using your VNA is to calibrate it into the test loads (open, short, 50 Ohms) that came with it with no coax between the VNA S0 port and the test loads.

    Then connect it to your coax, but also do the following: get a short clip lead (<5 inches long for HF) and connect the clip lead from the barrel of the coax connector (while it is connected to the VNA) to the chassis grounding screw on your transmitter. Now sweep SWR using the S0 port for each band you use.

    While sweeping each band, alternately connect/disconnect the clip lead. If your SWR plot changes, you have a lot of Common-Mode current on the coax feedline (the antenna is seeking a connection to earth ground through your radio and its power supply).

    Note, without the clip lead in place, if you have CM on the feedline, the SWR shown by the VNA is not what your transmitter has to contend with with the feedline connected to the rig.

    If you have CM current on the feedline, you should be thinking about how to mitigate it... Most likely culprit, random wire with 1:4 or 1:9 balun. Second most likely culprit, vertical antenna, radials or not. Third most likely, dipole without balun at feedpoint. Fourth most likely, OCF dipole or EFHW antenna without CM choke on the feedline, etc....
     
    K4NYN likes this.
  5. K4NYN

    K4NYN Ham Member QRZ Page

    Learning a lot here. Thank you all for the help. I will study these responses and keep analyzing and see if I can reign it in. Some of the info in these responses are confirming what I thought I knew, while some of it is broadening my HF horizons considerably. Thanks for the help!
     
  6. W1VT

    W1VT Ham Member QRZ Page

    What do you want to know, the impedance of the antenna itself or the antenna and the feedline and ground system of the station? Both are equally valid questions.

    If you want to know everything you may need to conduct a lot of measurements to sort it out.

    Zak W1VT
     
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  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd ask a psychiatrist, as they analyze everything.
     
    PU2OZT likes this.
  8. K4NYN

    K4NYN Ham Member QRZ Page

    Hey Zak,

    Thanks for the response. I am really just trying to figure out the best way to tune the antenna, and to match it to the transmitter. I was told to do the tuning and trimming with the analyzer connected at the feed point, but I would expect some change in the SWRs and impedance when adding my 50' of feed line, grounded polyphaser, etc. Just want to make sure that I don't trim it too short or something.
     
  9. K6CLS

    K6CLS Ham Member QRZ Page

    All that should not affect the antenna. You can confirm minimum SWR, measure it with the analyzer on one end, and a dummy load on the other end, for example where the coax ends to connect to the dipole.
     
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  10. AC6LA

    AC6LA Ham Member QRZ Page

    Off-topic but kinda interesting. Kai Siwiak, KE4PT and current QEX editor, co-authored this book:

    upload_2021-6-3_19-17-24.png
    On pg 335 there is a description of a model for the human body. The model is affectionately nicknamed SALTY.
    upload_2021-6-3_19-21-43.png
    His focus at the time was evaluating pagers back in the day when they were in common use and typically placed close to the body. The SALTY model is still being used for other RF exposure studies.

    Dan, AC6LA
     
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