I got these today in the mail. Notice the temporary wire I poked into the connector. I ordered some adapters but I haven’t received them yet. I bought a frequency counter with the rubber ducky antenna but it doesn’t have the sensitivity that this cheap one has. I got two of these for $28, free shipping.
These have made it a lot easier to adjust the oscillators to the right frequency. I no longer have to tune the receiver to the carrier to find its frequency. I just hold the wire close to the oscillator and I get the frequency to within a hundred Hz. I no longer have to find the oscillator’s carrier among other strong carriers, hoping I got the right one.
When I received the package, each counter was sealed in a plastic package. But the corner of the package was cut off, and the battery connector had been pulled out. This leads me to believe that the counters were opened up to test and/or calibrate them to indicate the correct frequency. I had read that some consumers had received these and they indicated the wrong frequency. Apparently the makers retested them to find any problems.
I also bought a Gooit frequency counter. This wasn’t very much, about fifteen dollars from an eBay seller. This is also sensitive enough to pick up a small low power transmitter. It has a short pull up antenna but no input jack.