HOW TO measure the resonant frequency of a trap?

Discussion in 'General Technical Questions and Answers' started by CT2FZI, Dec 29, 2009.

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  1. CT2FZI

    CT2FZI Ham Member QRZ Page

    Hello to all,

    I am a owner of a MFJ-259B and I would like to use it to test my home made antennas, and my question is regarding the resonant

    I have found this info from a known manufacturer and attached 2 images for your appreciation...

    Basically I would like to know if you have tried to measure the ressonant frequency of a trap using this method and your MFJ-2*9.

    I know that MFJ sells an accessory to make our MFJ a dip meter, but if this method works I will be very happy :) and I will not need to buy it.

    The more technical question - and the most important - is why does the trap ressonates at that particular frequency without the "rest" of the antenna, ie telescopic whip or the wires after and before the trap.

    I've read the user manual but it doesn't explain why, just how, and googling made me lost.

    I also attached a print screen of a youtube video where a new antenna analyzer is used to do it the same way.

    Thanks in advanced,

    Luis, CT2FZI

    (It also acts like a "grid-dip" meter, but easier to read, and with digital frequency readout !)

    Bonus:Not in all Analyst Instructions. A simpler way to measure the resonant frequency of a trap, or any tuned circuit, is to connect a piece of wire between the Analyst output and the Analyst ground (the ground screw next to the coax connector, or the outside of the coax connector.) The wire can be a few inches to many feet long.(Note:You must disconnect the trap from any antenna to measure its resonant frequency.)
    Put the Analyst in the Z mode. You will read a small Z which simply represents the inductance of the wire. Now put the wire near the trap. As you tune the Analyst frequency, Z will increase dramatically at the trap resonant frequency! The frequency of peak Z is the trap resonant frequency.
    If the trap has high Q and is large you can probably see the jump in Z with the wire several inches from the coil. For a small coil, you might need to wrap the wire into a small loop at the end and bring it near the coil. No fancy plug-in coils are needed such as a grid-dip meter requires...just a few inches of wire! And the Analyst's digital frequency and Z readout pinpoint the resonant frequency exactly.
    The same wire can usually be used over the entire frequency range of the Analyst, so you can also forget about the many plug-in coils that a grid-dip meter requires.
    Last edited: Dec 29, 2009
  2. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Hi Luis,

    A trap is a parallel resonant (tuned ) circuit as you pointed out. At resonance the impedance across it becomes relatively high, again, as you pointed out, which effectively disconnects any wire or tubing beyond it (outside the feed point end). The high impedance minimizes the current in the outer wires or tubing so they have little impact on the resonant freq of the trap.

    73, Pete, WB2UAQ
    Last edited: Dec 29, 2009
  3. G3TXQ

    G3TXQ Ham Member QRZ Page

    Just be aware that these add-ons for analysers are poor substitutes for a "proper" GDO. Take a look at W8JI's posting in this archived thread:

    I gave up trying to use my MFJ259B that way and went back to my trusty heathkit GDO.

    Steve G3TXQ
  4. CT2FZI

    CT2FZI Ham Member QRZ Page

    Now I understand :)


    Now I understand :)

    The resonant frequency on the trap is somewhat independent from the rest of the antenna system, meaning that it is possible to measure the resonant frequency for a particular trap.

    The rest of the antenna will of course optimize the trap function, on other words instead of a dummy load resonant in a particular frequency we get a full antenna... (well, 1/4 or so ;) )

    Nevertheless I still don't know if I can use my MFJ with the method described on my first post :eek:

    Big thanks!

    Luis, CT2FZI
  5. CT2FZI

    CT2FZI Ham Member QRZ Page

    Steve thanks!

    I am reading the article in this moment.

    Luis, CT2FZI
    Last edited: Dec 29, 2009
  6. CT2FZI

    CT2FZI Ham Member QRZ Page

    Just for the fun, check this item at epay: 220529770405

    Grid DIP Meter Product ID: DM-4061

    It is the same model that MFJ sold a few years ago...

  7. CT2FZI

    CT2FZI Ham Member QRZ Page

    I am sorry about the "bump" but can anyone tell me if the MFJ-259B will allow me to measure the resonant frequency of a coil like in the method described?

    Thanks in advanced,

    Luis, CT2FZI
  8. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Yes, but if you have just a coil then you have to add a Capacitor of known value to make a complete reasonant circuit. Then you measure the reasonant frequency, and solve the formula to calculate the value of the coil.

    With a dip meter you could measure the coil value directly without going through this complexity.
  9. CT2FZI

    CT2FZI Ham Member QRZ Page

    I see, is it possible for you to show me how to do it?

    I do have some capacitors of a known value, I don't know how to set this scenario up...

    Like what connects where :)

    Thanks in advanced,

    Luis, CT2FZI
  10. VK2TIL

    VK2TIL Ham Member QRZ Page

  11. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    Yep, I have the MFJ GDO kit for my analyzer and it lacks the sensitivity to make most measurements. It works for large High-Q tuned circuits like the kind in an SW-3 but a GDO is a far better measuring device in 99.9% of applications.
  12. VK2TIL

    VK2TIL Ham Member QRZ Page

    It's not clear whether the OP is using a '259 or if he bought a "proper" GDO.

    GDOs are often cheap these days; few seem to understand or want them.

    Luis; get a GDO if you can. A basic one should be cheap; search "dip meter" in all ebay and you will find lots.

    As ever on ebay there are many "dreamers" so far as price is concerned but here is a very nice one;

    Even a "Rolls Royce" might be affordable;

    Not a "Boonton" although often called that; these were made by Measurements Inc in Boonton New Jersey. They are one of the "greats". They are not battery-operated so are not truly "portable".

    Millen is another "great"; both mains and battery models were made.

    But the cheaper Heath, Lodestar, Trio etc GDOs are very good.

    N4XY has a wonderful webpage on GDOs;
  13. W5DXP

    W5DXP Ham Member QRZ Page

    Maybe a stupid question but: If the trap coil and capacitor were put in series, the resonant frequency could be detected with the MFJ-259B. Theoretically, when they are put in parallel, the resonant frequency would be close to the same as the series frequency. Would that work?

    I don't have to worry. My Knightkit G-30 GDO still works. :)
    Last edited: Jan 14, 2010
  14. AE1PT

    AE1PT Ham Member QRZ Page

    Of course, the issue with trap antennas is that they can become notorious harmonic problems--as the input frequency "jumps" the trap and travels to other elements. Some say that two traps are the practical maximum for such an antenna. Others will say one. I say none--as this sort of antenna is a shortcut in the same league as a G5RV. Yes it will work, not the most efficiently as it could, and can cause unintended issues.

    Trapped antennas were all the rage in the late 60s and early 70s. Gotta have a 40/20/15 trapped dipole. Just about everyone I knew had one. Now, look around and check how many you see. There is a reason.
    Last edited: Jan 15, 2010
  15. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Hi Luis,
    If a parallel LC circuit becomes a high impedance a resonance (which it does), you can place a resistor across the trap so that when the signal applied across the trap in parallel with the resistor reaches the resonant freq, you will see about the value of the resistor you placed across the trap. If the MFJ unit has a max impedance of 500 ohms, I would use a 470 ohm carbon is a standard type. I have used this idea sucessfully to adjust the electrical length of a shorted piece of coax to a quarter wave length at a particular freq using the Millen GDO / Antenna bridge. The GDO # 90651 is just a signal source for the Antenna Bridge # 90672. Its max impedance is 500 ohms as well. The idea is the same - about 470 ohms is the impedance at resonance whether it is a lumped LC (trap) or a transmission line. Sorry if this idea was brought to your attention already. 73, Pete, WB2UAQ
  16. CT2FZI

    CT2FZI Ham Member QRZ Page

    Hello and thanks for your message, this makes sense all right :)

    The MFJ 259B has only one impedance available [250 ohm] therefore according to your description I should use a +/-235 ohm resistor in my setup, correct?

    I am sorry to ask you this, but is it possible for you to write in other words how to place the resistor across the trap? My technical English is not that good _yet_ :) or an image :D

    I understand that the resistance in the trap will match the resistance in the resistor placed across the trap in the resonant frequency therefore will tell me the DIP [is this the right term?].

    Thanks in advanced,

    Luis, CT2FZI

  17. CT2FZI

    CT2FZI Ham Member QRZ Page

    I do thank you your message, your point was taken.

    Nevertheless I find that the subject is off topic.

    Like the suggestions about using a DIP meter... I only own the MFJ 256B and I would like to use it as much as I can, not just to measure SWR in one antenna :)

    I would like to learn from the other experienced amateur radios how this can be done, since its something that I have available at the moment...

    Secondly regarding the trap antennas, sometimes we just can't get what we want for our desires, so a trap antenna could be a short term solution or for a less space environment...

    In my particular case, I would like to check the trap/coil in an home made buddystick, and I find it very hard to find a vertical antenna this short [max 13 feet] and so cheap to build and tune it to a swr lower than 1.7:1 .

    I have managed to get this, but like I said before, I want to learn more, and if that means that I have to experience the knowledge of the 60's just to get to the 22 century I will :)

    So, my antenna works in 40 meters with low swr. I want to know what the coil does, and where it resonates without the tuning, like a standalone coil.


    Luis, CT2FZI

  18. CT2FZI

    CT2FZI Ham Member QRZ Page

    Thanks, I will look into it!

    Luis, CT2FZI
  19. CT2FZI

    CT2FZI Ham Member QRZ Page


    Could you please be kind enough to tell me more about this idea?

    Luis, CT2FZI

  20. W5DXP

    W5DXP Ham Member QRZ Page

    I asked the question because I don't know if it would work. For series resonance, XL = XC, and the series impedance is very low. For parallel resonance, XL = XC, and the parallel impedance is very high. Traps work in the parallel impedance mode to inhibit current flow at the resonant frequency.

    The MFJ-259B, as is, will detect series resonance but not parallel resonance.

    My question is: Can the MFJ-259B be used with the coil and cap in series resonance to determine the resonant frequency - then the coil and cap be connected in parallel with the resulting parallel resonant frequency being close enough to the series resonant frequency for the trap to work properly?

    Again, it is just a thought and I don't know the answer. It is not mentioned in my MFJ-259B manual.
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