Yesterday I built a choke balun. The objective was a good resistance (<4 kΩ) with low reactance. I’ve been playing with RG316, FT140 43 and 52 cores. After hours and dozens of differents arrangements I came to the conclusion that a good one would be this one:
In blue on the graphic, my CMC brings 4 to 6 kΩ of resistance on the entire range with low enough reactance. I’m not entirely sure it was really necessary to be so picky but I from what I red about CMCs, this is pretty much what we are looking for.
Most of the well-known recipes from the internet did not work as well with the components I intended to use. Stacked cores, single cores, fewer turns, more turns… I tried a lot of different configurations, but only the cores in series provided satisfactory results. The other solutions led to various problems, such as Xs > Rs , narrow bandwidth, efficiency located in a different frequency range, and so on. I added two additional experiments in the graphic, for comparison (12 turns on a single ft140-43 and 9 turns on two stacked ft140-43 and 52 cores).
Ah, so you’re working on the same conundrum I am. What I’ve seen from KIO, they originally used (page 8) a segment of T-beam with 4 drilled holes to from a flange that the stub-mast was bolted to slightly off-center. They now offer a flange for 1" pipe that has the same bolt hole requirements. Where you procure these or smaller ones I haven’t yet figured out.
Another design I saw but don’t see the reference in my open tabs is drilling/tapping holes into the end of thick-walled aluminum tubing to use that as a pseudo-flange adapter. A through-drill of both the tube and the mast allows a key-bolt to be threaded through along with a couple set screws to help hold things steady.
If the diameter of your center post is similar to that of your upper mast, you might be able to find two flanges with the same drilling pattern and “match” the upper flange to the lower one.
Personally, I just wasn’t in the mood to buy too many pre-made parts if I could avoid it. I’m going for maximum DIY on this one. Also, the price rises quickly when you start spending €10 to €20 on shipping costs for every single component.
In a similar dilemma, I’d love to make my own mast, but I’m not sure how to go about it. So here are my 4 options:
A tilt-over mast is relatively simple to DIY, but it seems very impractical for a hexbeam.
A telescopic mast is much more convenient since you can lower the antenna to ground level in just a few minutes. Sadly, it’s unlikely I’ll find galvanized pipes that fit tightly into each other at a local store. Most of the time, they’re sold in 6-meter lengths, whereas I need 2-meter sections. It’s a bit frustrating to buy so much and only use a third of it.
Second-hand telescopic mast: Very difficult to find, and I’m not driving 500 km just for a pipe.
Buy new: Easy and convenient, but expensive and lacks the satisfaction of DIY.
Hm. Well, part of my search problem seems to be that neither Home Depot nor Ace Hardware (local options in home improvement/tools/parts stores) sell the former type of pipe flange! I guess I’ll have to see what the tradeoff is between potential welding at work or adding such a flange to one of my orders. Since I won’t be going for as large of an assembly or as tall of a mast as you, I might be able to get away with less. Mechanical engineering: remaining my weakness.
Thick-walled inch tubes can be pushed into one another. The 1 1/2 inch tube fits perfectly into the 2 inch tube. At the time I used the galvanized seamless tubes for my 4x17 Elt group for 2m.
I pushed them up with a car jack and 2 angle plates. I always worked with strong plates and U-clamps (like exhaust clamps)… and used them to secure the tubes against twisting or to mount antennas. (For the elevation I used a push rod from a satellite dish and used it to tilt the antenna group.)
You can also use an antenna rotor mount as the top flange. These parts made of cast aluminum are very stable.
If you leave the thinner mast 20 - 30 cm in the thicker mast, then that’s no problem. I simply secured the upper mast with a U-clamp to prevent it from slipping in.
I had an 8mm aluminum plate about 15 x 30 cm and used a U-clamp at the top and bottom to prevent the mast from twisting. Simply drill 4 holes and mount a U-clamp at the top and bottom.
I also attached 2 angle plates with U-clamps at the top and bottom and placed a car jack between them and pushed the upper tube with the antenna upwards.
Simply remove the U-clamp and secure the upper tube against slipping in… then secure the upper angle plate deeper again and push the upper tube further and further up.
I attached my tubes to the house wall with brackets and worked from the roof.
The advantage was that I could always lower the mast to work on the antenna.
The mast itself was held by the upper bearing… and I could also remove the rotor.
It took the hurricane of 1999 to bend the upper tube… but nothing was broken
Great. My system will be a bit different since the rotor will be on the ground, rotating the whole mast.
Let’s see if I can find some good pipes for half the price or less compared to commercial solutions…
Edit: quick search on the internet learned me what I was afraid of, the pipes have very standard dimensions everywhere I look, and none of them seem to be usable for telescopic purpose. Gaps bewteen sizes are way to big, even with the thickests.
Ok so we have the same. 53ID for 48.3OD works well ? Hmm… I imagined that a 5mm gap would be a no go but if you tried it and it was good then I will also give it a try
I had a tiltable 4 x 17 elt Tonna group for 2m on the mast… and a small Yagi for 2m and one for 70cm…
If you have less wind load on it, you can also choose thinner tubes. You need a few mm of space to be able to move the tubes. The tubes are fixed together and so you still have less than a cm offset at the top if the tubes are still a little bit stuck together.
For steel tubes without steel cable guying, 1/6 should be clamped in. The rotor to the upper bearing was 1.4 m for me and the top antenna was about 7 m above the upper bearing.
I need to go at the metal recycle / junkyard from time to time. Sometimes we find incredible things and it’s almost free (0.50€ per kg I think). I got a lot of stuff from this place.
The spindle / flange / bearings from the back of a washing machine drum might be useful.
I used such things to build a wood turning lathe a few decades ago!
