HAM’S LIFE

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 well within specs. Putting a directional wattmeter on it might show the reflected power to be very low or non existent but the unit still does not function well. Here is a tool I found to be very helpful in cases like that.

The tool is very simple. Use an antenna with a metal nut designed for a 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. Using 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) mark the antenna by cutting a nick where it will be flush with the end of the connector. Remove the connector and cut the antenna on 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.

Use a female barrel N connector 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 will allow you see if there is any power reaching the wattmeter while the transmitter is keyed. If there is any power compare the power output at the transmitter to the power on the wattmeter at the antenna end of the coax.

If a high SWR is observed and the antenna can not be tuned the tool can be used to check the coax and mount by placing the watt meter on the transmitter side of the coax and running the output through the tool to a dummy load. If this check good and the antenna is the right length then probably the antenna mount is not making good contact with the metal on the vehicle.

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 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 gets run through the coax. I have seen 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 connector. So I usually just change the connector first before getting too involved in trouble shooting but when that does not cure the problem then I use my tool to try to understand what is going on.

You may wonder about the earlier statement, “…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 wattmeter 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 for 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.