Three Hours on a Roof With an RFS GDT
In August 2023, I drove out to a site that had an RFS Smart Communications setup. The active path in the drawings was labeled 6300, and the spare path was 8110. The 6300 path had triggered a VSWR alarm after two rainy nights, so I was there to check the RFS GDT before anyone suggested swapping an antenna.
I didn't do it right the first time.
I put my multimeter on the connected GDT—black lead on the outer body, red lead on the center pin. The reading drifted around 0.8 ohms. I told myself that was a dead short. So we took out the RFS GDT and replaced it. The new part read the same way: around 0.8 ohms. A second bad RFS GDT was possible, but not likely. I kept going anyway.
That is the part that still bothers me. The first reading was not stable. It moved between 0.6 and 0.9, which should have told me that I was measuring through something other than a clean fault. Instead, I let the expected answer decide what the meter was telling me.
What an RFS GDT Actually Should Look Like On a Multimeter
For the record, a gas discharge tube is not a fuse and it is not a piece of wire. In normal condition it has very high resistance. When an overvoltage hits, the gas inside ionizes and the tube becomes a low-impedance path to ground. After the surge passes, it should go back to its high-resistance state.
So an isolated RFS GDT, measured with a low-voltage multimeter, should not read 0.8 ohms. It should look open—or over range—on the resistance scale. If it reads low ohms after you remove it from the circuit, then it may be shorted and should be replaced.
Why The 6300 Reading Lied to Me
Here is the part I didn't think about at the time. The 6300 path was connected to an RFS Smart Communications interface. That means the RF line was not just an antenna and a radio. There were bias tees, remote control paths, and DC-powered monitoring devices in the same circuit. Some of those devices have a DC return path on the center conductor. When my multimeter probes were on the RFS GDT terminals, the meter was reading that whole network in parallel with the GDT.
(I should add: the 8110 spare path was disconnected at the radio end. That is why it looked open. It was a true spare, not proof that 6300 was worse.)
The RFS GDT on 6300 could have been fine the whole time. The problem was my test setup, not the component.
How to Use a Multimeter For An RFS GDT Check
The procedure that finally worked for me goes like this:
- Use a safe meter. Outdoor telecom paths can have DC voltage you don't expect. I use a multimeter rated CAT III 600V or higher. That rating is part of IEC 61010-1, not a marketing badge.
- Disable transmitters and remove DC power. If the RFS Smart Communications feed uses a bias tee, take the DC feed out of the path. Then set the meter to DC volts and confirm there is no voltage across the connector before you touch the internal parts of the GDT protector.
- Isolate the GDT. Disconnect the jumper on both ends or take the RFS GDT out of its holder. Do not try to measure it while it is still connected to the rest of an RFS Smart Communications path. I cannot overstate this one.
- Switch the meter to resistance mode, not just continuity. A continuity beep only tells you that the resistance is low. It won't tell you whether the GDT is open, which is the condition you want to see on a healthy tube.
- Measure from center to ground. On an isolated RFS GDT, a reading in the megohm range or an over-range display is normal. A low reading means you have a problem: either the tube is damaged or you still have a parallel path to ground.
- Check the ground bond. Put one probe on the RFS GDT ground lug or mounting block and the other probe on the station ground bar. This should be close to 0 ohms. If you see more than about 1 ohm, fix the ground connection before you replace any RF component.
One more limit: a normal multimeter will not verify the breakdown voltage of an RFS GDT. It doesn't send enough voltage to make the tube fire. If you need to prove that the GDT still triggers at its rated voltage, you need a proper surge test set or the manufacturer's test procedure. My multimeter test only tells me whether a GDT is shorted or still looks like an open circuit in a low-voltage state.
My experience with these RFS checks comes from outdoor tower sites, not indoor DAS or lab test benches. If your RFS GDT is in a different housing or your Smart Communications path uses another wiring plan, use the manufacturer drawing for the part. The meter-reading principle stays the same, but the isolation points can change.
What Fixed the 6300 Site
After I isolated the RFS GDT on 6300, it read open. The 8110 spare also read open. The GDTs were not the reason for the alarm. The real issue was water in the connector boot on the 6300 jumper. The corrosion changed the RF connection when it got wet, but it still passed a simple DC resistance check because the DC path was less sensitive than the RF path.
We changed the jumper, cleaned and re-greased the connector boot, and the transmitter came back up. Total time wasted because of my first measurement: about two hours. Total cost of the spare RFS GDT we removed and did not need: not huge, but the extra truck roll was expensive.
Bottom Line
If you're using a multimeter on RFS equipment, the meter is not the unreliable part. The unreliable part is the question you ask. Testing an RFS GDT while it is still connected to an RFS Smart Communications path does not test the GDT. It tests the whole path.
Isolate the component, use resistance mode, and check the ground connection. If you do that, a good multimeter can save you a very expensive mistake. If you don't, you might spend the afternoon replacing a part that was never broken. I've got the August 2023 truck roll to prove it.
