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Thread: How to check an antenna trap

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  1. #11
    Join Date
    Feb 2004
    Location
    Bakersfield, CA
    Posts
    4,599

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    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
    73
    Gary

  2. #12

    Default

    Quote Originally Posted by KO6WB View Post
    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
    73
    Gary
    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

  3. #13

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    Quote Originally Posted by WA8KJP View Post
    You are going to do it completely wrong. The way you test a trap(s) is to install it(them) with the actual antenna wire lengths you plan to use (which were calculated correctly hopefully ) and read the impedance at the feed point of the trap dipole while its hung at the height you are going to use it. You can't get any meaningful information by trying to measure the resonant frequency of the trap itself. The wires connected to the trap enter the picture of how a trap actually works.

    ---------------
    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

  4. #14

    Default

    ^^ nope, at resonance what Pete WB2UAQ has said is correct, see Fig 5:
    http://www.antennex.com/w4rnl/col0302/amod49.htm

    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

    Geoff

  5. #15

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    Quote Originally Posted by WA8KJP View Post
    When a parallel circuit is resonant at a frequency, that resonant frequency will change because the antenna wires when connected will detune the trap. And a trap doesn't behave like a simple switch to disconnect the outer length of wire. The outer length does radiate. The purpose of the trap is to modify the electrical length of the whole antenna. It doesn't just make the outer part disappear to RF current.



    -------------------
    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 by WB2UAQ; 04-11-2013 at 01:15 AM. Reason: Addition

  6. #16

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    Billy,

    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.

    73,
    Steve G3TXQ

  7. #17

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    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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