Could use some measurement help

[Scott M]

This message is mostly directed to Ben and Mike but if anyone else can help, that would be great…

After our fox hunt before the club meeting earlier this month, we were talking about how to determine “left” and “right” direction using a switched dipole setup.  In the simple switched arrangement, the operator listens to the loudness of the tone and points for minimum.  The issue is that you cannot tell if the null is in front or behind unless you move to get another triangulation fix.  Since that meeting, I’ve put together a detector simulation that shows how direction angle of a fox can be measured relative to the antenna bore sight. I would be happy to show it if anyone is interested at the next meeting. 

My request for help is if someone could set up a fox/ switched antenna/ radio and put a scope on the radio audio output and the capture an image of the waveform.  Specifically the waveform when the switched antennas are pointing away from the RF source and generating a tone. Even better would be a dual trace showing the audio signal as well as the antenna switching signal.  In short, I’d like to verify the shape of the audio signal. 

Because the antennas are switched and not pointing at the source, the switched RF output will be square-wave phase modulated waveform.  An FM detector response to a phase modulated signal, if the descriptions I’ve read are accurate, is only to the transitions when an antenna change occurs. I’d like to verify this. 

Alternatively, if anyone is up for it, we could make the measurement at the next meeting - but someone will need to bring a fox, a switched directional antenna, and a radio with an audio output along with a way to connect the audio output to the clips of a scope probe.  I can bring a portable scope.

Thoughts?

Scott.  WB0OOD

[Mike Burk]

I have all of that (Fox, radio & scope) and will bring them.)

I also intend to hide 2 foxes prior to the next meeting, the one I have used before, but with a 20 db attenuator and another one with a 50mw output.

Mike KD0JXE

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[Scott M]

Mike,

Great!  We can then take the measurements I was requesting at the next meeting.  I assume you will set up about 30 minutes before the meeting start?

What kind of connection does you radio have to look at the audio output on a scope?  Something like a 3.5mm jack?

Scott.

[Mike Burk]

All of the HT's I have  are the same.

The audio comes out of the 2.5mm jack between the tip and the sleeve.

I have an adapter that changes the 2.5mm jack to 3.5mm and have a 3.5 mm jack with clip leads to get the audio so we can look at it with the scope.

Mike KD0JXE

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[Ben Sanders]

Sorry I'm late to the party, but sounds like Mike has everything in hand.

Just to make sure I understand the setup, I assume the audio from the radio will go into one scope channel and the switching signal will go into the other, so we can see the phase change as we rotate the antenna?

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[Mike Burk]

That is about it.

Mike KD0JXE

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[Harold Johnson]

Just a small request. If it's convenient, maybe you could take a couple of pics of the scope with a cell camera or something -- maybe even a vid. 

Thanks and 73,

On Wed, May 27, 2026 at 6:20 PM Scott M <uppe...@gmail.com> wrote:

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[Mike Burk]

There are a couple circuits involved.

One that switches between 2 antennas at a 1khz rate (adjustable) and sends that signal to the radio.  If the signal hits both antennas at the same time the radio sounds normal. If the antennas are at different distances from the source there is a short 1khz break in the signal that generates a 1khz audio tone

 

The second circuit is a mixer which is made up of a push pull amplifier on top of a current source.   The timing signal that controls the antennas is fed into one side of the mixer and the audio from the radio is fed into the other.  The theory is that the mixed signal should offer clues as to which of the antennas is closer to the signal source.

 

I do not know how to interpret the result.

 

My answer to your request for screen captures is I do not know what to send you.

 

I am planning to plant a fox on 146.565 mhz about ½ hr before the meeting,  Bring your HT and see if you can find it.  The signal should be 24 db weaker than last month.

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[Scott M]

Harry and Mike,

A screen capture is what I would also like.  A photo or video should be enough to show how phase modulation (from the switching of antennas) converts to an audio signal after an FM discriminator and any audio filtering.

Mike, your description is spot on to what I've simulated.  Let's capture photos on Saturday. Looking forward to the hunt.

Scott WB0OOD.