During my years as a radio technician I have seen many problems that were simple but difficult to find. There were times when a mobile radio would not be operating properly but when it was tested it was within well in specs. Putting a directional wattmeter on it might show the reflected power to be well in acceptable levels but the unit still does not function well. Here is a tool I found to be very helpful in several cases like that.
The tool is very simple. Use an antenna with a metal Motorola antenna mount and a male compression N connector for RG8U. Cut the antenna so it will reach through the N connector where the center pin would normally go and move the coax side of the connector down flush on the metal nut for the antenna mount. Use a hack saw or flush cutting pliers (if you use pliers be careful not to nick or break them these antennas are very hard to cut) and nick the antenna where it will be flush with the end of the connector. Remove the connector and cut the antenna at the mark. Shape the end of the antenna with a file. I said shape not sharpen because you don’t want it to come to a sharp point but you do want it to easily insert in the mating connector. Place the connector body back over the stub left from the antenna until it is snug against the metal nut and solder the two together. This is a little tricky and takes a lot of heat but as you can see from the photo it can be done. Throw away the center pin, the inter parts used to hole the coax, and the compressing nut.
Now use a double female barrel or a coax jumper with a female N connector on one end and connect it through a directional watt meter to a dummy load. This way you can see if there is any power reaching the watt-meter and if there is compare the output of the transmitter to the power on the watt-meter. If you are having problems matching the antenna then place the watt-meter on the transmitter side of the feed line and run the jumper from the antenna mount to a dummy load.
If the coax is good and the connector is properly installed on the coax then the reflected power should be near zero and the power on the transmitter side and the power on the antenna side should be close to the same. If this is true then you know the antenna is either not touching or making good contact with the mount or it is cut to the wrong length.
Some times the problem is caused by old coax having a very high loss. Other times coax will be cut, usually where it has been routed over a sharp edge and the vibration has cut it through. Sometimes it is a screw that has been run through the coax. I have had a few where everything looked good, dc wise they checked good, the connector was on good but the unit did not work and it was with this tool I was able to positively identify the problem.
I would say about 95% of the mobile installation failures will turn out to be a bad antenna connector but a bad antenna connector will normally show up as a high SWR when checking the power out on the transmitter end.
You may wonder about my earlier statement in the first paragraph, “…it might show the reflected power to be well in acceptable levels.” You might think that is not possible but it is very possible, I have seen it several times. The coax can act as an antenna depending on where and how the coax is damaged. If this is the case the reflected power will change as the length of the coax jumper between the radio and the watt-meter is changed. These do not work well as an antenna. The other thing that can happen is the coax is old and has a very high loss thus it acts as a dummy load.
This is not a tool every ham should have because it is very rare you will need it. It would be a good project for a club to build and then make it available to members. It would be good then for all the members with mobiles to use it to test their installation making sure their ham shack on wheels is operating at its peak performance. When working through a repeater it is surprising how bad transmission and reception can be and not be noticed.