Entries from October 2009 ↓

Moxon or KT34XA

I have been thinking about turning my KT-34 into a three element moxon for 20 meters. Why? Because I think it would reduce the weight by a bunch and work better with my old AR-22 antenna rotator.

Yesterday I was reading up on moxon performance. The two element moxon was consistently 2 S units below a 160 meter loop at 50 feet. Granted, the loop has a capture area the size of Texas compared to the moxon, but this moxon performance is just a little better than what you can expect from a normal two element beam, about 5db.

I am already getting close to 8db from the KT-34 on all three bands. Why would I settle for about 7db on only one band. That is what I would end up with using a three element moxon on 20 meters.

No, it would make more sense to reinforce the mast so that it can take the additional load and just hope the rotator will slip its moorings when the strong winds arrive. It will screw up the feed line but at least save the rotator.

I am pretty much convinced that the winds that did in my HD-73 would also have taken out a prop pitch motor. So going lighter on the rotator should not make that much difference as long as it is not pinned to the shaft of the mast like the HD-73 was.

I am not about to spend 1000 bucks on a heavy duty rotator when I am pretty sure it would get shelled out too.

So the KT-34 is being rebuilt as is the mast. The antenna just needs the feedline attached and the capacitor insulators replaced. The mast needs to be taken down and the bent portion needs to be cut out. Then I need to come up with a clever way to mary the 2.5 inch pipe to the 3 inch pipe without welding. My guess is slip the smaller into the larger for a couple of feet and use two bolts spaced 90 degrees appart. Will also find a solid steel shaft to insert into the hollow of the 2 inch pipe at the junction. The lower 15 feet of the 2.5 inch pipe will be reinforced by welding some heavy streached chain to its backside. This may result in a future bend closer to the top. However, the 2.5 inch pipe is only 20 feet long. That puts the reinforcement just five feet from the top. I am hoping that will be enough to support the additional weight of the extra boom and two elements to convert the KT-34 to a KT-34XA custom.

Custom because the wide spaced additional three band director is going to be an old Mosely tri-band driven element sporting two traps. The extra ten meter director will be made from tubing scaps of which I have plenty.

I have already used the two Mosley traps and tubing to build a rotatable dipole using some scrap tubing. It worked very well on all three bands. All that needs to be done is telescope the tubing further together to make the element shorter and turn it into a director.

The only thing I am short on is boom material. I need to double the length of the existing boom. Increase it by 16 feet to a total of 32 feet. I only have about 13 feet available and it is not all 3 inch. Got three 19 inch sections of boom from an old single bander beam that was destroyed in a storm. Got another six foot section of 2.5 inch steel mast I would rather not use since it is heavy. Then there is a five foot section of scaffold tubing which is extremely light weight and hopefully extremely strong.

There is another 16 feet of mast now being used on the second tiltover but I believe that mast is steel (heavy) and about 1.5 inches in diameter. Probably not at all suitable.

I might be able to use three feet of the 1.5 to make up the difference in boom length for the KT but I am tempted just to let is go short. A three foot reduction in boom length can’t effect the gain that much.

The first task is to securely mount the gin pole mast by drilling holes for bolts in the top plate of the tower. Then removing the tiltover portion and loweing it to the patio. Secure both ends and the middle. Then cut the bent portion of the pipe out of the middle. Move the 3 inch pipe into the garage to weld on the uppper cable stays and paint it.

Next comes the 2.5 inch pipe. Move it off the roof and into the garage to install cable stays and reinforcing chain. Also need to figure out a good way to bolt the two pipes together. Paint the smaller pipe.

Move the finished pipes to the patio and position to bolt them together. Install the cables before raising to the hinge point. Once it is at the hinge point you wont be able to get to the cable stays.

The cables will be installed with a little more foresight. Will be using four cables. One RG6, two RG 213, one rotator cable, and one openwire 450 ohm line.

The 450 ohm line will be terminated to PVC insulators support off the tops of the cable stays. Rotor cable and both RG 213 runs will come down through the cable stays as will the RG6.

One RG-213 to the KT-XA. One RG-213 to the vertically polarized two meter beam. RG-6 to the TV antenna. Maybe use RG six for the two meter beam too? Then there will be the UHF TV antenna wich will use the 450 ohm open wire line for feedline.

It may not be entirely obvious but the two TV antennas need to be fixed mounted. That is, I know where the TV stations are and I need those antennas to point in that direction and stay there. Dont need the TV antennas rotating around with the ham antennas. The UHF TV antenna does not present much of a problem. Just mount it to the mast with two u-bolts. (might need to get new U-bolts to fit the larger mast.) The VHF/UHF combo TV antenna will probably need to be mounted to an extension arm off the side of the main mast. Ten feet below the large HF beam would be a good place. Need to determine the length of the extension arm

There will be a junction box at the top of the mast. Perhaps one at the hinge point as well. All cables will terminate in connectors at the juntion box at the top. This being done so that the antennas can be removed or serviced without worry about feedlines. Just disconnect at the junction box. The juntion box will be located high enough on the mast so that it is accessible from the roof of the garage as the mast it tilted down.

Getting the beam up onto the mast is going to be trick. The last time we did that, we used the step ladder straddling the garage roof peak to gain enough height to attach the boom to the mast. The beam was fully assembled as we installed it this way.

This time we will fully assemble the entire beam on the patio so we can test it. Then dissassemble and take the main boom section and short elements up to install the boom to the mast. This should be fairly easy because the balance point of the beam will have shifted to one end of the boom. My guess is that three elements will be up and one down as the boom is mounted to the mast.

Further assembly will be done at standing height from the roof by rotating the beam to position it then raising or lowering the mast by tilting it to bring the elements into play one by one.

Once all four original elements are installed we can to the XA modification. Add boom sections to the first ten meter director. Install ten meter director. Raise beam higher, install more boom lenghts, install the tri-band director.

Raise the antenna to full height and hope the swr is acceptable on all bands.

The official weight of the factory KT-34XA is 69 lbs. I am hoping my version will be under 60 lbs. Still, that is very heavy. Some of the weight of the previous installation will be offset by lowering the mounting point of the rotator. I am guessing I can set it lower by about ten feet. The only problem there is going to be attaching the saddle shaft to the antenna. Ten feet lower may put off the edge of the garage roof. My extension ladder is not long enough to reach that point. Will just have to play this by ear. I might only get a five foot lower mounting point. I definately do not want to have to install the rotator before I raise the pipe to hinge point. The whole purpose of the rotator mount is so that is can be accessed for replacement since it seems to be most likely part to fail.

Need to make some cable clamps and stays to support the new longer boom. There is a sturdy pipe extending ten feet up from the point where the beam is connected to the mast. The upper reaches of that pipe need a clamp with two ears to enable attaching the ends of two cable stays, one on each side. The cable stays will connect to boom clamps located a few feet in from the boom ends. The stays will be preadjusted to length to keep the boom level and prevent any sag or droop. I have a length of aluminum bar stock that is heavy enough for making clamps. I just hope there is enough to make three clamps. The mast clamp is no sweat but the boom clamps will need about six to eight inches of material per side times two. Need at least three feet of stock. Clamps will be bolted together and have extensions to permit attaching the stays. They should be a tight fit to the mast and boom so that they will not slip.

This is longer than I intended. Sort of thinking out loud and sort of off topic at the end. Then this is the ‘Off Topic’ blog. Continue reading →