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Franklin-Spider Antenna (for advanced users)

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  • Franklin-Spider Antenna (for advanced users)

    SIMULATION RESULTS
    Gain = 3.7 dBi
    SWR = 3.1
    Impedance = 155 - j 8.18 ohms
    HIGH IMPEDANCE/SWR - WILL NOT GIVE GOOD RESULTS WITHOUT IMPEDANCE MATCHING




    IMPEDANCE MATCHING BY "TRANSMISSION LINE+CAPACITOR" TECHNIQUE
    Using Smith Chart:
    Capacitor to be inserted = 3.8 pF (4 pF will do)
    Coax length (from antenna) where Capacitor should be inserted = n x λ/2 + 80.8mm Coax of VF=0.85,
    where n is any integer.





    ACTUAL IMPLEMENTATION

    This is a two step SWR improvement process.

    STEP (1): Trim RG6 feed coax at the pigtail till value of R becomes close to 50 ohms.

    STEP (2): Add a 4pF capacitor in series with core wire of coax to nullify +X component of impedance. Location of capacitor: between pigtail and filter.


    By doing the STEP (1) ONLY, impedance changes from 155-j8.2 (TP1 on Smith Chart) to 50+j38.3 (TP2 on Smith Chart). This results in changing SWR from 3.1 to 2.1, which gives a big improvement in performance.

    Doing STEP (2) after doing STEP (1) results in impedance becoming 50+j0, and SWR=1, a perfect match.


    Note: Unfortunately I have miss-calibrated my antenna analyzer using a 75 ohm resistor. Now it is not resetting to factory calibration of 50 ohms. I have ordered a 50 ohm dummy load resistor. Waiting for deliery to re-calibrate the Analyzer with 50 ohms load. I therefore could not actually do the impedance matching. Following only show the steps I will do when my Analyzer is re-calibrated.





    Last edited by abcd567; 2017-03-11, 17:08.

  • #2
    In my first post, the antenna Analyzer was miscalibrated, and showed 45 ohms reading where it should have shown much higher (around 100 ohms) reading. I was therefore not able to do the impedance matching. I ordered a 50 ohm load resistance to be able to calibrate the Antenna Analyzer.

    Now I have received 50 ohm load resistor, and calibrated the Antenna Analyzer.
    Conducted impedance matching (feed coax trimming+insertion of capacitor) after calibrating the Antenna Analyzer



    INSERTION OF CAPACITOR AFTER TRIMMING THE COAX TO A POINT WHERE R = 50 OHMS (OR CLOSE TO 50 OHMS) AND X IS +VE




    RESULTS AFTER INSERTION OF CAPACITOR:
    VSWR = 1.16
    R=50.2 OHMS X= -7.5 OHMS



    CAPACITOR INSERTED = 2.7 pF
    AN EMPTIED TV SPLITTER HOUSING IS USED TO INSERT THE CAPACITOR
    Last edited by abcd567; 2017-03-22, 18:07.

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    • #3
      Hi...in my case a serviceable Antenna Analyzer, that works good enough for a new Ham like me to cut dipoles for 40 meters, which I did, and comparing it to a very popular MFJ model, borrowed from one of the old timers who is know in the area as ‘the' antenna man. You should be in good company if you only build the original build and depend on it to do what it said it would do.

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