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I'm trying to locate the transmitting antenna on a wireless mic, in particular a shure SLX2 J3. I'm unable to find any antenna circuit, although I suspect an RF switch on side 1, a circular part with 414 marking.

All I can figure out so far is that it is using one of the battery ports, or perhaps mic ground as some type of antenna, however these mic modules are somewhat universal across non wireless mics.

Symptoms are noisy movement dependent noise coming from the mic with no mic attached at all. I've tried this on a couple frequencies, using the demodulation feature of my rigol DSA815.

Where might the designers have they hidden the antenna on this? There is no 50R trace anywhere I can see.

https://youtube.com/shorts/L0L9oVp8A80?feature=share

Side 1

Side 2

IR Port

Mic port

MIC

End

Assembly side 1

Assembly side 2

See yellow trace here Proposed path

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  • \$\begingroup\$ I would guess the antenna to be a monopole housed in the plastic end cup that also covers the batteries. That flexible PC strip might be it, but I agree the connector to the PC board doesn't look to be for RF, and there's no obvious RF transmission line on the PCB. And I can't identify the component at the end of the flex-PCB strip from your photo. Can you post pix to show that plastic assembly, in its normal position with the batteries in place, from both sides and end view? \$\endgroup\$ Dec 19, 2023 at 21:40
  • \$\begingroup\$ I added pictures. I think maybe its using the battery connections for antennas? However, I really suspect that something in the signal chain is broken, and without a schematic its probably a hopeless cause. \$\endgroup\$ Dec 19, 2023 at 23:36

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I found a service manual, which is quite thorough, available online at: https://www.manualslib.com/manual/917665/Shure-Slx2.html

Although it doesn't have a full schematic, there are block diagrams and detailed descriptions. As you suspected, the battery is used as the antenna. It is fed by a long chain of passive components, likely the ones you've highlighted in yellow.

Here's a quote from the manual that provides the clue:

The low pass filter output couples to the battery antenna via C641 and L607. Connector CON640 and C613 are only usedfor Japanese (JB) units. Coupling capacitor C610 is used to ensure that both batteries are driven equally

One would think a battery, with a negative end at PCB ground and a positive end feeding all the circuitry, is a poor choice for an antenna. I'd guess they use RF chokes to isolate the battery assembly from the audio and digital circuitry at UHF frequencies, then feed it with a capacitor.

Your "movement-dependent noise" could originate in components in the audio chain (e.g. a "microphonic" component) or in the RF chain. Tough to troubleshoot without a schematic. Good luck!

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  • \$\begingroup\$ Good find thank you. \$\endgroup\$ Dec 20, 2023 at 13:00
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The first picture shows a LMX2335 under the metal can.

From the above datasheet there is the following Typical Application Example:

enter image description here

Can you trace the traces on the LMX2335 to see if similar to the Typical Application Example use of RF1out and RF2out?

From the pictures currently in the question I'm not sure where the antenna(s) are.

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