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

    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.

  2. WB2UAQ

    WB2UAQ Ham Member

    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".
    Pete, WB2UAQ
  3. KF5FEI

    KF5FEI Ham Member

    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

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

  5. W9GB

    W9GB Ham Member

    Attached Files:

  6. WB2UAQ

    WB2UAQ Ham Member

    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

    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
  8. KL7AJ

    KL7AJ Ham Member

    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.

  9. GM0OBX

    GM0OBX Ham Member

    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.

  10. WB2UAQ

    WB2UAQ Ham Member

    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
  11. KO6WB

    KO6WB Premium Subscriber

    Yeah Pete I was afraid of that. The use of a parallel resistor is just going to flatten the response across the entire tuning range.
    A GDO is a good thing to have. Another approach requires a RF voltmeter or an oscilloscope. You would tune across the range
    and watch for the voltage to peak.
    The actual impedance of the trap to frequencies away from the resonant frequency should still be low but perhaps not low
    enough for the AA-54. Still, if you put enough signal to it the voltage should peak to a point of easy detection.
    Even a clunky old signal generator would work as long as you used an attenuation pad to isolate the low impedance of the generator
    from the trap itself. What can be a problem is the stray reactance of the measuring devices.
    As an example; an oscilloscope has an input impedance of 1 megaohm but that's at DC once you start putting an AC signal to it
    then the internal capacitance, probe capacitance and lead capacitance take their toll. Your reading could be way off at 14-30MHz.
    My DSO has a input capacitance of 30 pfd. So at 14 MHz that's 379 ohms being put onto the circuit. At 30 MHz it drops down to
    about 177 ohms. That's something else to consider when setting up a test. The use of 10X probes on a scope help but there's
    still some effect on the circuit.
    So, seems like your approach may work well as long as it manages the isolation that will not skew the results.
    Have fun
  12. WB2UAQ

    WB2UAQ Ham Member

    Hi Gary

    I understand your concern about attaching the tuned circuit across a scope. In the case of an SWR analyzer, network analyzer, bridge, it is out of the picture when it is attached to 50 ohms (pure resistance) and the circuit under test across it reaches parallel resonance. As soon as you move the freq just a little above or below resonance, the analyzer sees the reactance (L or C) and the SWR immediately rises up. That is the beauty of it. I also use a sig gen driving a 50 ohm return loss bridge for the same kind of testing. I wanted to see where my homebrewed roller inductor reached self resonance with the contact wheel in various places along the length of the coil (i.e. at various tap points). In this case I attached the coupled output (reflected sig) to and oscilloscope and swept the generator in sync with the scope (HP 8601A Sweep gen, its RF to the bridge and its sweep put put to the scope set for X-Y). Utilizing the 50 ohm load is a great method.
    73, Pete
  13. WB2UAQ

    WB2UAQ Ham Member

    I don't agree. Billy is going at it correctly. The trap must go parallel resonant and therefore Hi Z at a well defined frequency regardless of what is hanging off of it. Think of an RF current at the trap's Fres moving down the radiating wires. When that current reaches the high Z of the trap, it is diminished beyond the trap so what is hanging off beyond the trap has negligible impact on it. As an over all system, yes, the height above ground will impact the now isolated dipole in the usual way. Also, just think of how we'd be chasing our tails if we had to keep tweaking the trap to compensate for ground reflections.
    73, Pete
  14. VK2AVR

    VK2AVR Ham Member

    ^^ nope, at resonance what Pete WB2UAQ has said is correct, see Fig 5:

    nothing in life is perfect but the current magnitude is negligible after the trap, at resonance.

    on its non-resonant frequencies, the trap will most certainly modify the electrical length of the antenna. This is why trapped multiband antennas end up shorter than their single-band cousins, because of the centre loading provided by the trap inductor.

    Also, thanks for the SWR trap measurement method Pete. Very handy for those who don't have a GDO :)

  15. WB2UAQ

    WB2UAQ Ham Member

    With respect, I disagree again. Right out of my old 1974 ARRL handbook, page 185, two traps using 8.2 uH in parallel with 60 pF resonate at 7.176 MHz isolating the inner 64 feet of wire to create a 1/2 wave dipole for that frequency on 40 meters. 64 ft is slightly short based on 468/7.176 but the trap it self might be adding a bit of length to make it pretty close:) The trap will be inductive below 40 meters which will shorten the length of the antenna for 80 meter operation and the trap will be capacitive above 40 meters which will make the antenna a but long for the other bands. Maybe this is what you are thinking about.....impacting the length above and below where the trap resonates?
    Apparently W3DZZ was able to take advantage of the inductive and capacitve loading and create a useful antenna for 80 thru 10 meters using only the 7 MHz traps. ARRL Ant Book # 21 from 2008 continues the tradition on page 7-10 with the exact design.
    73, Pete

    And Thanks to VK2MIA for the support:)
    Last edited: Apr 11, 2013
  16. G3TXQ

    G3TXQ Ham Member


    It makes good sense to check that the traps are close to their design values before installing them in the antenna.

    You got some useful advice on how to do it. My own preferred method is to put a small coupling coil on my tracking generator output, and another one on my spectrum analyser input; then simply lie the trap between the coupling coils and look for the peak on the analyser. That way there is no direct connection to the trap.

    Of course, inserting the trap into the dipole doesn't change its parallel resonant frequency.

    Be aware that for some applications - such as a W3DZZ antenna - it's not sufficient to have the trap resonant at the correct frequency; you also need the correct L/C ratio so that the trap provides the required series reactances on bands other than 40m.

    Steve G3TXQ
  17. WB2UAQ

    WB2UAQ Ham Member

    Billy is determining the resonant frequency of a trap with a particular instrument that can make direct connection to the trap. Others and I have suggested methods to do the same with different techniques. I am sorry if I offended anyone. 73, Pete
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