How do you go about properly loading an antenna?

Discussion in 'Wireless & RF Design' started by BlackCow, Apr 11, 2012.

  1. BlackCow

    Thread Starter Active Member

    May 11, 2009
    65
    1
    I'm having trouble finding information on this.

    What I have so far is a hearty oscillator for the commercial FM band (just for testing it out). I have no amplification yet but I am working on that.

    To listen to the signal I just simply stuck a resonant length piece of wire on the output of the oscillator. It worked well enough to hear my signal on the radio sitting on my bench but I know that isn't really how you load an antenna.

    So say I am eventually feeding an antenna with a 50 ohm coax line for the 2m amateur band using my home made transmitter circuit, how do you go about making the output of the transmitter match that?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,152
    1,794
    You can start by measuring the impedance of the antenna at the frequency of interest. Once you know what you are working with you can add components in series and parallel to make the impedance of the feedline/antenna system look like an impedance of 50 + j0. You can make this measurement with an antenna analyzer or you can get yourself a mini-VNA which turns your PC into a Vector Network Analyzer.

    http://miniradiosolutions.com/
     
  3. K7GUH

    Member

    Jan 28, 2011
    191
    23
    Before you go any further, check to see if you can legally transmit in the band of interest. If you can't, go no further.

    For simpler antenna matching problems, I start with the formula 5540 / f_in-MHz, to get the length of a 1/2 wave dipole in inches. That will be close enough to match 50 ohm coax from the 50 ohm output of your transmitter.
    Anything more elaborate gets into all kinds of considerations which take any solution way out of the "simple" category.
     
  4. BlackCow

    Thread Starter Active Member

    May 11, 2009
    65
    1
    I'll be sure to identify my call :D

    Well I understand how to calculate and make a dipole but how do you attach the antenna?

    Do you drive it with a coil or something or do you just simply solder the coax to the last stage of the RF amplifier?

    Do you have to do anything special to feed a 50Ω coax line?
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,152
    1,794
    You can use an antenna tuner which consists of variable inductors and variable capacitors. You still need some way to judge the quality of the match by using and antenna analyzer or RF Power meter or well there are lots of alternatives. Most low power 2M rigs don't really care much about matching. They'll transmit into a coat hanger without much complaint. If you measure the rubber duck that comes with most HT's you'll find the resonant frequency anywhere except the 2M band.

    KD8APB BTW
     
    Last edited: Apr 12, 2012
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