Antenna coil adjustment

Discussion in 'Mobile Radio Systems' started by W6TAB, Jan 15, 2019.

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  1. W6TAB

    W6TAB Ham Member QRZ Page

    I have a Hi-Q 4/160 antenna on a breedlove puck mount in the middle of my trunk. The problem is that in order to connect an antenna anylyzer with the trunk closed I need to use a 22" cable. My understanding is that the cable should be 18" or shorter to get an accurate reading. Will the matching be that far off with the longer cable?
  2. KU3X

    KU3X Ham Member QRZ Page

    If you are checking 160 meters, the short section of coax probably won't have much of an impact on your readings. If you really want an accurate reading, borrow an antenna analyzer that can run OSL Calibration. That cancels out the transmission line so you can read the feed point impedance on a given length of coax.
    W6TAB likes this.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    As Barry says above the impact of the cable length will vary with band and the lower the frequency the less impact a few extra inches of cable will have on any measurements.

    The OSL method is a good way to use an arbitrary length of coax and back out feed point measurements, another approach is to use a piece of coax that's an integral number of half wavelength's long electrically but that gets cumbersome on the lower frequency ham bands.

    Also this only matters to make the initial un-matched impedance measurements if you want to precisely calculate the appropriate shunt matching inductor. For the final tuning you may as well run the same length of coax you'll use to connect to your rig in the vehicle as that's the match the rig will actually see. Normally shunt matching inductors in mobile applications are wound open air with no permanent coil form so as long as you get close in your initial measurements you'll fine tune the Beta match by stretching or squeezing the open form coil to get your final match. IOW, you don't need to precisely calculate the coil value, just get ballpark close and then stretch or squeeze the coil during final tuning.

    If your SWR gets better as you stretch and the coil begins to lose all shape by being stretched out then wind another coil with fewer turns or small diameter. If the SWR gets better until the coil turns short out against each other then wind another coil with a few more turns or a larger diameter. Since these coils take about a minute to wind (just wrap fairly stiff wire around something like a 1" wooden dowel) it's pretty easy to just start with something close to what you think you need, test in place, and if you can't get the match you want with some stretching or squeezing then wind another coil with more or less inductance as needed.
    KU3X and W6TAB like this.

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