How to check an antenna trap

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by GM0OBX, Apr 8, 2013.

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  1. GM0OBX

    GM0OBX Ham Member QRZ Page

    Well as the title says. I have a rigexpert aa-54 antenna analyzer and I would like to test some antenna traps that ill be making. I've saw plenty of videos using an mfj analyzer with a dip meter adapter however can anyone help me with how to check a trap using the rigexpert analyzer?

    I know how to work the analyzer it's just the physical setting up and attaching the trap to the analyzer that I'm struggling with.

    Thanks
    Billy
     
  2. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Billy,
    I am thinking that you can place a 50 ohm resistor across the trap and the antenna analyzer (parallel circuit)and tune up and down looking for the lowest SWR.
    The trap becomes a very hi impedance at parallel resonance so the analyzer will see only 50 ohms at resonance and therefore the lowest SWR.
    As far as the physical connections, I can not visualize the trap well enough to suggest a method. It might take some hose clamps or clips.
    The 50 ohm resistor could be a dummy load of the type built inside a connector or connected with a 50 ohm coax cable and a "T".
    73
    Pete, WB2UAQ
     
  3. KF5FEI

    KF5FEI Ham Member QRZ Page

    Not sure about the connections to the trap, but on the AA-54, if you set it to do an SWR graph on a wide range of frequencies, you will see a pronounced dip at resonance. use the < or > key to center the cursor (and center frequency) at the dip, then decrease the width of the sweep and re-graph to zero in on the exact spot.
     
  4. GM0OBX

    GM0OBX Ham Member QRZ Page

    hi pete thanks for the reply something like this should do then

    copygy.jpg
     
  5. W9GB

    W9GB Ham Member QRZ Page

    Attached Files:

  6. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Billy
    Looks good to me:)
    Please let us know how it worked out.
    I just finished building a roller inductor and now have it in my homebrewed transmatch. The roller inductor allows me to do more experiments on balanced line transmatches. I finished just in time as the wx is getting warmer and the outside work has started. Only the cold and the short days keeps me inside.
    Keep up the experimenting!
    73, Pete
     
  7. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Not sure the above methods of putting a resistor in parallel will work well. If you instead put the resistor at one end of the trap
    and the other end of the resistor going to the shield of the analyzers input. Then the center output of the analyzer goes to the
    non-resistor end of the trap. This puts the trap in series just like it would be in a normal antenna. There will be a bit of shift in
    the resonance of the trap in a downward frequency because the analyzer is adding a bit of capacitance.
    Now, instead of looking for a low SWR you are now looking for the spot the SWR goes to maximum. Above that frequency the
    analyzer should read just the capacitive reactance of the traps tuning capacitor. Below that the analyzer will read the inductive
    reactance of the traps coil.
    Hope this helps
    73
    Gary
     
  8. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    With most traps you SHOULD be able to get adequate magnetic coupling from the probe by placing it at the end of the trap so the coil of the probe is COAXIAL with tthe trap coil. If not, you can build an auxiliary coupling link by taking an alligator clip lead and winding it a few times around the GDO probe and clipping the free ends to the trap. However, you need to account for this inductance in parallel with the trap inductance.

    Eric
     
  9. GM0OBX

    GM0OBX Ham Member QRZ Page

    Thanks for all the reply gents. I'll try the 50 ohm resistor across the trap connectors and in series with them and see how the test results go. I'll post up the results.

    ta
    billy
     
  10. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Gary
    I looked up the specs on the AA-54. With the resistor in series and at the trap's parallel resonant freq, the Z will go so high that the AA-54 can't deal with it. When determining 1/4 wave (shorted) and 1/2wave (open) length frequencies for a length of coax I use the parallel 50 ohm method looking for the lowest SWR in the same way Billy can find where his trap resonates. Other SWR analyzers have even less measurement range such as the MFJ-259 and 369. The Palomar Noise Bridge is limited even more (and what a dud that unit turned out to be until I re-configured it with a K2BT transformer). In addition, most of these analyzers and bridges are optimized for making measurements close to 50 ohms.
    73, Pete
     
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