# 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. ### CT2FZIHam MemberQRZ 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.

Luis, CT2FZI

TRAP FREQUENCY MEASUREMENT
(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. ### WB2UAQHam MemberQRZ 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. ### G3TXQHam MemberQRZ 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:

http://forums.qrz.com/archive/index.php/t-190145.html

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

73,
Steve G3TXQ

4. ### CT2FZIHam MemberQRZ Page

Now I understand

Thanks!

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

Big thanks!

Luis, CT2FZI

5. ### CT2FZIHam MemberQRZ Page

Steve thanks!

I am reading the article in this moment.

Luis, CT2FZI

Last edited: Dec 29, 2009
6. ### CT2FZIHam MemberQRZ 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...

CT2FZI

7. ### CT2FZIHam MemberQRZ 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?

Luis, CT2FZI

8. ### KB4QAAXML SubscriberQRZ 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. ### CT2FZIHam MemberQRZ 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