N1GY- The simple Approach to Ham Radio
and My Model Railroad Hobby
As I was browsing around the web this morning I saw a video by another ham in which he used what he called his coax tester. He did not explain the circuit for his tester but it gave me an idea. Obviously what he had was a form of continuity tester with specific design elements for attaching it to a newly installed coax connector. It took only a moment to come up with a workable circuit.
The functionality of the tester is obvious. I have installed many PL-259s over the years and testing for a short with a regular digital meter sometimes seems to require three hands. This tester allows one to simply screw the PL-259 onto the tester to get an immediate “GO/NO GO” indication. If the LED lights up- you have a short circuit either in the coax or the connector. Start over. If the LED does not light up, the installation of the connector has no short circuit.
I mounted the components in a small plastic box left over from a previous project and labeled the box with instructions on its use. I included a test button in the circuit so that the user could make sure that the unit would give a proper indication when used. The button momentarily short circuits the tester so the LED will light up. If it does not then it is probably time to replace the two 1.5 Volt AAA batteries that power the LED. The actual size of the batteries is not critical, AA or AAA will work just fine. The important thing is to power the unit with 3 volts DC. That way, no dropping resistor is required.
If you want to use a 9 volt battery just add the appropriate dropping resistor to the circuit. A 330 Ohm 1/8 watt resistor will do the trick just fine for 9 volt power. I suggest the use of a battery holder to make exchanging the batteries as easy as possible.
A protective ring wall was installed around the test push button to prevent inadvertent short circuits while it is stored in a tool box, pocket, or go-kit. The ring wall is nothing more than a section of a PVC tubing connector, hot glued around the push button. It has to be higher than the button, but only by a fraction since you want to be easily able to operate the button when using the tester.
1 SO-239 connector
1 Red LED
1 Momentary Push Button (Normally Open)
1 Battery Holder for battery(ies) of choice
1 330 Ohm resistor (only needed if using a 9 volt battery for power)
1 Suitable enclosure
1 protective ring to go around push button (section of PVC connector)
Assorted hookup wire, hot glue, etc.
As you can see, the circuit is very simple. If there is a short circuit in the cable under test then voltage will be passed to the LED and it will light up. If the push button is activated then the circuit is also completed and the LED will light up.