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Cushcraft MA5B

antenna

After a few different antennas, I finally found one that performs decent, and fits in the small amount of space that I am working with. My antenna Mast is the top two stages of a salvaged tubular crank up tower, I have it guyed to help stabilize it, and it is clamped to the house at about the 10 foot mark. Needless to say, I wont be running any monster sized beams in the near future, and that is what caught my attention with the Cushcraft MA5b

The MA5B is a 2 element beam on 10/15/20, and a dipole on 12/17. The antenna is rated at 1200 watts, but a web search will reveal that the traps, and balun will often fail below that point. I purchased the antenna before I got into QRO operation, so I wasn't too concerned about the power limits. The antenna is pretty light and isn't bad to put together. I followed the dimensions in the manual and I had good SWR on all 5 bands. The antenna is a drastic improvement over my inverted G5RV. I can hear stations on the MA5B that I cant hear on the G5RV, and even better, they can hear me.


antenna

Everything seemed good until I added the AL80 to the station. The amplifier produces between 800-950 watts depending on the band. I only run SSB, and the occasional digital mode(without the amplifier), and after a few days, I noticed that My SWR on 20 meters was up to 8:1. I knew something had gone wrong, so I set out to find the issue. all the cables and meters in the shack checked out, so it was onto the roof to take a look at the antenna. I started with the balun box, I opened up the box, and everything checked out. All of the cables looked good so I started looking into the traps.

I took off the traps one at a time and took them apart to see if I could find any signs of failure. To the Left, you will see what the coil inside of the trap should look like, This is one of the good coils. Notice the Shiny aluminum wire, and you can see the dark spots behind the wire where the rivets hold the coil to the aluminum shaft.


antenna

Here is a picture of what was found on the 10 meter trap on the driven element. I purchased this antenna used, so I don't know if maybe someone pushed the antenna too hard in the past to cause this damage. The traps melting is a somewhat common issue, but this one looks like it could be salvaged. I cleaned all of the melted plastic out of the burnt spot and it looked like it wasn't too far gone so I figured I would try to salvage it.

The rivet that was in the hole was completely missing so I wondered if maybe the sharp edge of The tube was making the trap arc. I needed to at least fill in the section that melted, so I decided that some epoxy would be a good candidate to try to get the trap patched up. I added some epoxy in the hole, overlapped the threads of the coil a little to make sure that the tube was fully insulated from the coil. I let everything cure, rewound the coil on the form, and got everything put back up. I checked everything with my SWR analyzer, and it looked good. after a little bit of on the air testing with good results, I fired up the amplifier and after that, it lasted about 10 minutes.


antenna

Here is a view of the somewhat cleaned up trap. I am assuming that the dielectric properties of epoxy isn't quite what I was hoping for. I got a little creative at this point and decided that the reflector trap would be the same form, just another half a turn of wire. I pulled the reflector trap apart, drilled out the rivet and wound the driven trap wire onto the form from the reflector that looked new.

The thinking behind this idea was that the reflector is not a driven element, so there should be no arcing in the reflector. I use the good condition form in the driven element and hopefully everything works as it should. I have gotten everything together, and hopefully will have it up in the air in the next few days, and then I can see if my thinking was correct.