Monday, January 16, 2012

Finishing the R7000 Rebuild

At our last installment, we were going to re-assemble the R7000. But, I forget to mention one other rebuild step I performed. When I took the R7000 down, the center insulator was looking green. As in slimy mildew green. Yuck. That growth was going to slowly destroy the insulator.

Refinished R7000 center insulator.
Disassembly was the first step. I thought I might be able to separate the insulator from the aluminum tubes, and I removed the screws holding them together. With the original R7000, there are eight short screws inserted from the inside of the tubes. I'm sure they use some special jig for this, because once you remove them, you can't get them back in. That's OK, because later versions of the R7000 simply used four 2 1/2 inch 8-32 screws.

Even with the screws out, I couldn't separate the insulator from the tubing. It was just too tight a fit to try and separate and risk damaging the insulator.

With all the hardware out of the way, the next step was to clean the green slime off the insulator. Soap and water took care of that with a little gentle scrubbing with a sponge. The fiberglass was a little frayed on the surface, but the insulator looked practically new. 

To prevent a new growth of green goo, I painted the insulator with a thick layer of flat black spray paint. The photo shows the screws re-assembled. 
R7000 Assembled. The chair keeps the
capacity hat rods from bending.
Following the measurements in the manual, I re-assembled the antenna in the driveway. Although the manual directs you to install the radial rods first, it's much easier to work with the antenna if you leave them until later.

Once you put the capacity hat rods on, you'll need to elevate the antenna to keep them from bending. I used a patio chair for this purpose. 

After finishing the assembly, I double-checked the tubing lengths, just to be sure. This is an important step. When I first assembled my Cushcraft A3S, for example, I got the lengths on the director element about 6 inches short on each side.

Before doing all the work of putting the antenna back on the mast, I wanted to make sure it was working correctly. I figured if I could hold it a foot or so off the ground, I ought to be able to see the resonance dips near each amateur band. 

I used a wooden stepladder and bungee cords to  hold the antenna just off the ground. Using an antenna analyzer, I found resonance dips roughly where they should be. Having the antenna so close to the ground means it's going to be slightly out of tune. But it was a good enough test to give me some confidence the antenna would be working as it should. (or, so I hoped)

Wooden stepladder used as a test stand.
After this, I re-mounted the antenna to the mast and raised it back up in position. Initial tests seemed to show good results on 10 and 12m. 15m - 20m were a bit out of whack, and 30m and 40m were pretty much unusable.

Hmm. I contemplated this problem for about a week. I figured it might be a problem with the CT1 trap. In the meantime, however, my replacement capacitor came. I replaced my chain of 4 100 pF caps with a single 27 pF 1 kV capacitor.

After replacing the MN7000, I was shocked to see no SWR dips at all on the antenna analyzer. However, punching the antenna with a little bit of RF remedied that. In fact, the antenna appears to be resonant on each of the expected ham bands with a reasonable SWR.

I used this antenna in the recent NAQP CW. It performed decently. I even managed to hold a run for about a half hour on 40m. I'd say it's now working as  designed. I don't know if it will break any pileups, but having a decent radiator on seven ham bands with one eleven foot support is nice.

1 comment:

  1. Congratulations on rebuilding the antenna. I had owned one years ago and had great success with it.