Friday, December 30, 2011

Rebuilding the R7000

Given the number of times that Cushcraft has re-designed it's multi-band, no radials vertical, I'm not sure it has been their most successful product. The R5 / R7 designs had problems with trap stability and mechanical complexity, so Cushcraft introduced the R7000, using enclosed traps very similar to designs they used on their very successful trapped tribander series. Later this became the R6000 / R8, where some traps were exchanged for multiple vertical elements. 

The R7000 also went through a trap re-design in mid-1998. There are three different manual available, the original from October 1996, one from June 1998, and the last from May 1999. The latter two are hardly different -- only a slight difference in assembly hardware. However, the 1996 and 1998 versions don't even share the same dimensions.

For those of us using the original version, there's at least one change from the 1998 version that might be worth adopting. When adding the R80 80m add-on kit, there are two tubes BH and BI that are inserted into the ends of the CT1 trap. This strengthened the tubes on the trap, which can help avoid fold-overs like this. The 1998 changes incorporated these tubes as part of the standard product. Since this only requires two pieces of 5/8 inch tubing about 6 inches long, it should be recommended for any rebuild. (Provided you can find 5/8 inch tubing)

First step on any rebuild is disassembly. My unit had seen 15 years out in the weather, but was actually in pretty good shape considering. No parts missing, and only a very slight bend in the bottom of the CT3 trap tube.

When I opened the MN7000, I found several spiderwebs and debris. Getting the circuit board out of the box requires unsoldering the SO239 center contact -- this requires a large soldering gun. Once I lifted the board out, I was surprised to find a very live spider who was not happy I disturbed his home. Sorry, fellow, but the eviction has been posted.

MN7000 after re-assembly.
I found the MN7000 in pretty good shape. None of the components seemed charred, cracked or damaged. Some of the connections on the circuit board showed a bit of corrosion, and I re-soldered some of the connections. 

Unfortunately, in trying to shorten the leads on the 27 pF capacitor, I ended up destroying it. In the picture, you can see my substitute -- four 100 pF 50V capacitors in series. As soon as I get the correct part, I'll swap these out.

Pay attention when disassembling the MN7000. I found some of the hardware to be slightly corroded, along the brackets. Remove as much of this as practical.

The general advice is that high SWR on all bands means an MN7000 failure of some kind -- but I couldn't find anything wrong, other than a few questionable connections.

Not sure if the MN7000 was the problem, I turned my attention to the traps. After a short debate, I decided to disassemble all the traps. 

CT3 trap disassembled. The 20m trap is on the bottom
(right) of the assembly, and the 30m trap on the top (left).
The traps consist of two nylon bobbins that are held inside the aluminum tubes by four self-tapping screws and four dimples. You have to drill out the dimples with a 1/8 inch drill bit. The top rubberized weatherproofing was removed by slitting it part way up the non-connected side (the smaller lump) and pulling it off. Try to cut as little of the weather proofing as possible. Remove the screws and pull the bobbins out, or you can tap them through (but be sure to move the aluminum wire out of the way).

CT2 trap half-way assembled. 15m trap is inside the tube,
17m trap waiting to be installed. 
The traps were in pretty decent shape. The top trap bobbins were clean, with the bottom trap showing a bit of insect debris. The bottom traps showed a bit of yellowing as well. A few checks with an ohmmeter showed the connections between the wire and inner trap tube were good. 

Re-assembly was easier than disassembly. Since the dimples were drilled out, I substituted 3/8 inch long #6 self-tapping screws. The alternative would be to cover the dimple holes with tape, but using screws seemed to add to the structure of the trap.

CT1 trap, reassembled on the antenna with a protective
layer of electrical tape.
Weatherproofing is held in place with a layer of tape, wound from the bottom to the top. I also added a layer of tape over the bottom screws, in order to keep the weather off the aluminum wire connection.

With all the components cleaned and re-assembled, it is time to assemble the antenna. Which we will do in our next installment.

R7000 Moves to Micro-Shack

Putting up the R7000 at the Micro-Shack took a bit of doing. I didn't want to leave any concrete monuments in the yard, so I had to look for something a bit more temporary. Fortunately, there's an outbuilding behind the house that appears to be made of salvaged lumber. This little shed has seen better days, but looked sturdy enough to support wall brackets. A few 2x4 reinforcements inside the framing received lag bolts for a couple of wall brackets I had on hand.

The hard question was -- what to do for a mast? The 1 1/2 inch rigid EMT I used worked ok for 15 years, but EMT isn't meant to be a mast product. Plus, it wasn't the right outside diameter to fit the mounting U-bolts for the antenna. A 12 to 15 foot piece of 2 inch galvanized, chrome-moly steel would have been ideal, but not easy to come by.

Top end of mast, showing R7000 attached. Notice the R7000
radials are not installed. They get in the way so it is easier
just to put them on just before raising the antenna.
I ended up using an different combination. First, a 10 foot piece of 1 1/4 inch steel pipe, a 1 1/4 to 1 1/2 inch adapter, then a 12-inch long 1 1/2 inch steel pipe nipple, and finally a pipe cap. Steel pipe is quite a bit thicker than rigid EMT, and a lot more expensive. A 10 foot 1 1/2 inch pipe would have been almost $40! The 1 1/4 inch pipe was just over $25, and the 1 1/2 inch nipple made it 11 feet total height. 

The 1 1/2 inch nipple fit the mounting U-bolts much better than the old rigid EMT did. Close, but not as perfect as a 2 inch mast would be.

It just took a warm afternoon to put up the brackets, assemble the antenna and raise the mast. Unfortunately, the antenna did not show the characteristic low SWR on the ham bands. Some work with an antenna analyzer showed relatively high SWR on every frequency from 7 to 26 MHz. Only on 10m did it show a slightly lower SWR of about 3:1. 

Uh-oh. Looks like this 15-year-old antenna needs a rebuild.

Wednesday, December 28, 2011

R7000 History

After my move in November 1994, it took over a year before I put up any antennas -- I was much too busy with young kids, new job, finishing the basement, etc. In January 1996, I put up a 125 foot doublet up about 15 feet and operated the NAQP CW contest. That doublet got mounted higher by the NAQP phone, and was joined with some attic antennas.

By the fall of 1996, I was in the market for an antenna that could reasonably support several bands, including the WARC bands. The R7000 seemed like a pretty nice solution -- seven bands with an option to  add 80m as well. I bought one in November of 1996.

To mount it, I joined a 10 foot piece of 1 1/2 inch rigid EMT to a 12 inch steel pipe nipple and planted it 3 feet in the ground using two sacks of concrete. The pipe union was buried in the concrete. The resulting vertical pipe was about 1.9 inches in diameter -- almost large enough to fit the mounting U-bolts -- and nearly 8 feet tall.

While not as optimal as the specified 2 inch mast, this support worked very well for 15 years. I only stopped using the R7000 a couple of years ago when the coax to it was cut when the cable company re-buried the cable. (That one was cut by septic tank workers repairing the system)

The R7000 was not a great performer, in my opinion. I first called it the R7000 attenuator. I later discovered that it was designed to work at a height of 18 feet, not the 8 feet I had installed it. After a few years, I got the idea to add some radials at the base of the mast. Seven 20-foot radials made the antenna appear to work a lot more reasonably.

Since I wasn't using it at the old QTH, the R7000 seemed like the perfect antenna to move to the Micro-Shack. It required only one support, covered seven bands, and with a few radials seemed to work OK. A mediocre antenna seemed better than none at all.

Friday, December 23, 2011

The Micro-Shack

It eventually happens. You get a QTH built up with a reasonable set of antennas, then you have to move. My friend Mike used to think he lived under a curse -- two years after he put a tower up, he ended up moving. This happened to him three separate times. He solved that problem when he moved last -- he hasn't put up a tower since.

My wife was moved to a new church, and with that posting came a parsonage. The little problem with the parsonage was it was a bit smaller than the previous house. So, where could I set up some radios? No more basement shack, cause, there is no basement. And there's no spare or "bonus" room. What to do?

There was a little utility room next to the car port. This is an unheated / (cooled) storage room, that houses the water heater and the electrical box. It's about 5x7 feet, and not good for terribly much. Perfect. (Well, not perfect, but it could work....)

The storage room was pretty dismal. While it had a window, the walls were thin, unpainted plywood that had seen a fair amount of abuse in 40+ years. I removed a beat-up cabinet that apparently had been salvaged out of someone's kitchen a few decades ago. A couple of coats of white paint considerably brightened the place up.

On the wall opposite the water heater, I put some adjustable shelf brackets in place, and had enough brackets on hand for five shelves. Then there was the small matter of the operating desk. I mounted 2x4 blocks on opposite walls by screwing into the studs and placed a piece of 23/32" plywood on top of them. (Do you know that it is crazy you cannot buy 3/4" plywood any more?) A couple of 2x4 blocks along the wall hold up the back end of the desk, and two strips of 1x4 glued to the bottom give it enough reinforcement that it would probably hold up my weight. Should be good enough for any boat-anchor I choose to put on it.

The operating desk fitted the space available: 5 feet 3 3/4 inches by 29 inches deep and 30 inches above the floor. The shelves just above the desk would support some equipment to free up a little desk space. It would be small, but quite usable.

Now, it's just a small matter of getting power, ground and some antennas up.