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mobile antenna analyzer question

Discussion in 'Mobile Radio Systems' started by GROUNDWIRE, Nov 4, 2018.

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  1. GROUNDWIRE

    GROUNDWIRE QRZ Member

    ok guys i have a few questions i cant find answers too,
    1. when using my antenna analyzer to tune my mobile antenna, should i use a really short piece of coax to tune the antenna or should i tune the antenna through the full length of coax i operate with?
    2. should i make my operating coax a resonant half wave at op frequency using the analyzer?
    3. should i tune the antenna out in the open like a field etc. away from houses, trees, poles etc for an accurate result? or does it not matter?
     
  2. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    First thing you should do, is forget about the SWR reading!!

    Assuming you are speaking of HF?

    You use a short piece of coax, with an appropriate common mode choke. When adjusting the antenna, stay as clear of it as you can. The coax can be any length, as it doesn't matter "IF" you do the tune correctly.

    As for the location... It is best away from driveways, trees and power lines. And remember, every single surface you drive over will cause a shift in the impedance.

    If you still can't get it to work, go to my web site: http://www.k0bg.com/coil.html
     
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    That band are you operating ?

    I have only heard of coax length being important for 11 meters.

    Have Fun.
     
    KU4X likes this.
  4. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Perfect, Don!
     
  5. AA7QQ

    AA7QQ Ham Member QRZ Page

    The OP asked the Right questions & yet, you mock...
    Yeah, coax length can be important in transformers, stubs, phase delays, etc....
    But, you already know that.
    Feel better about yourself?

    To the OP, Alan's advice is correct. I would also suggest having all doors/trunk/hood closed on the car. Personally, I always use a large parking lot for tuning. Again, as Alan stated, your match may vary with different surfaces.

    Ed
     
    KU3X likes this.
  6. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Let's revisit the coax length issue. The only time length really matters (at least in a mobile), is when there is a large value of load reactance (high ±j value). In days gone by, antenna manufactures told customers to cut the coax to a given length in order to make the antenna's impedance "match" that of the transceiver. Great! But those transceivers had final tanks which could be adjusted. Solid-state ones don't have that feature, unless they have a built in tuner. But...! In a mobile scenario, using a built in tuner in conjunction with a screwdriver antenna, will cause you no end of issues, especially so, with tiny antennas like the Lil Tarheel.

    As I said before, if you do the match correctly, the coax length doesn't matter. Unless maybe it is 200 feet long!
     

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