I've been working on creating a decent Unun that is good for full legal limit power, and provided good coverage from 1.6 - 30 MHz. Achieving that isn't all that hard but doing it in a way that will not present a hazard has been one of my main goals. Though I attribute some of the alleged problems to either pure fiction, or poor design. Nevertheless my intentions have been to build something that was:
I purchased a waterproof electrical box from Home Depot as my case, and it resembles pretty much every image I've seen of commercial Ununs. The link to the electrical box is here:
4 in. x 4 in. Junction Box-E987NR at The Home Depot
That provided a suitable outdoor housing (it's waterproof), with easy mounting options for mounting at the base of your antenna (vertical). It has plenty of room to fit my core choice, which is a pair of Amidon FT-240-61 ferrite cores. You can obtain these for about $13/ea from the fine people over at AllTronics:
Amidon FT-240-61 | Alltronics
Now since Home Depot annoyed me to no end, after getting the electrical box I went quite literally across the street to Lowes to obtain most of the rest of the parts. You'll need the following items (regardless of where you get them) ideally these will be stainless or alternatively zinc plated:
(2) 1/4-20 1" - 1 1/2" hex bolts
(8) Suitable 1/4" washers
(2) 1/4-20 hex nuts
(4) #4-40 screws
(4) #4-40 nuts
You will need an SO-239 connector I have been using American Radio Supply for my connectors and such, but you can get them from DXEngineering, or ebay or where ever you like. I personally love the prices over at ARS and he ships FAST!
SO-239 UHF-Female 4-Hole Panel Mount Coennector (ARS-0511A)
You'll also want a pack of ring terminals for 14-16ga wire with the ring small enough to be used with the #4 screws. You will also need 14-16ga ring terminals capable of be used with 1/4" bolt, I got mine from DXEngineering at the same time I purchased the radial ground plate. They have them 20 for $5:
http://www.dxengineering.com/parts/dxe-225rt-20
You'll also want 14ga stranded (You can use solid core enameled wire, but you'll want to put PTFE tubing around it, so just avoid the hassle) PTFE insulated wire. There's lots of sources for that, but since you don't need very much (like 4 feet worth or so), I bought a tiny quantity from ebay at:
10 Feet 14 Stranded PTFE Insulated Silver Plated Copper Wire | eBay
14ga will handle maximum power, and the PTFE insulation will handle extremely high temps without catching fire, it is after all teflon and they put that in frying pans. Lastly and for safety concerns you'll want to obtain some 3M 27 Glass cloth electrical tape, to wrap the FT-240-61 cores with. This will provide a little extra insurance against fire and the like in the case of high loads, shorts, and bad impedance matching. You can get that from Amazon for about $15.
What you'll want to do regardless of the wire source, is cut the wire into two (2) five foot sections. Here you face a choice, bifilar or parallel wire. To be honest I've done both and have personally not noticed much of a difference. In fact all the photos I've seen of commercial Ununs are all using parallel winding, so if that's easiest for you, you can do it. Otherwise you can create your own bifilar winding by taping the ends of your 5 foot sections, and then gently twisting the sections around each other. You don't need to make the twists tight, a nice loose job will do. I've personally used parallel winding in my core but you can do whatever you like. You will want to leave 4-5 inches on both ends of your winding because you'll need to make some connections when you're done, and if you leave them long enough you can avoid having to make superfluous soldiering that otherwise could of been avoided. I also recommend marking the two ends of the wire that will be your positive wire so you can identify it easily after you're done wrapping the core.
At this point, you'll want to wrap the cores with your 3M 27 glass cloth tape. I've wrapped both separately then wrapped them as a unit. But you may just want to wrap them as a single unit from the start and avoid the extra taping. To be honest I have no idea which is better. Theory would say wrapping them as a unit is better due to concerns over altering the thickness of the cores. But I'm not sure a thin layer of tape is going to make THAT MUCH difference overall, in fact I've read that the differences are pretty negligible. The fact of the matter is, it's most likely to effect the far ends of the range the unun will cover, and I'm pretty dubious of anyone operating much there. I know I don't operate much at 30MHz nor at 1.6MHz.
After wrapping your cores, you're ready to begin winding the wire around your core, I've done a lot of research on how many turns. I've read everything from 15-18 turns, but honestly that's an unruly mess I've done it. It does improve your low end response slightly but it's a real pain in the back side. I've been using 12-13 turns with excellent results, and suggest you try the same this will give you pretty good response from 1.5-1.6MHz thru 30-40MHz. You can read all about this here:
http://webzoom.freewebs.com/g8jnj/33ft Verticals and Ununs.pdf
He did a lot of research into these devices and I decided to take my 12-13 turns directly from that document.
Also one of the things you'll see on some of the Do-It-Yourself unun designs on the net if you google is that they use a superfluous piece of low loss coax (RG-8X etc). Well there's a much better way to design this thing than that, also avoiding the coax component you'll avoid additional soldier connections that can go bad, or break or whatever over time. My design ultimately uses only ONE (1) soldier joint, the rest are lugs, which also have their problems obviously but diagnosing the problem with 3 lugs is a relatively easy problem and can also be easily fixed and done relatively quickly.
Since it's late at the moment and we're in the middle of a tropical storm (or certainly getting gusty now), I'll post up a wiring diagram, and some photos in the next day or two so you can see how this all comes together. I'll be assembling the Unun from scratch (sans the extra core and the 3M 27 tape which unfortunately I don't have on hand yet .. they're both on order sadly). But you will get the basic idea of how it all goes together.
I purchased a waterproof electrical box from Home Depot as my case, and it resembles pretty much every image I've seen of commercial Ununs. The link to the electrical box is here:
4 in. x 4 in. Junction Box-E987NR at The Home Depot
That provided a suitable outdoor housing (it's waterproof), with easy mounting options for mounting at the base of your antenna (vertical). It has plenty of room to fit my core choice, which is a pair of Amidon FT-240-61 ferrite cores. You can obtain these for about $13/ea from the fine people over at AllTronics:
Amidon FT-240-61 | Alltronics
Now since Home Depot annoyed me to no end, after getting the electrical box I went quite literally across the street to Lowes to obtain most of the rest of the parts. You'll need the following items (regardless of where you get them) ideally these will be stainless or alternatively zinc plated:
(2) 1/4-20 1" - 1 1/2" hex bolts
(8) Suitable 1/4" washers
(2) 1/4-20 hex nuts
(4) #4-40 screws
(4) #4-40 nuts
You will need an SO-239 connector I have been using American Radio Supply for my connectors and such, but you can get them from DXEngineering, or ebay or where ever you like. I personally love the prices over at ARS and he ships FAST!
SO-239 UHF-Female 4-Hole Panel Mount Coennector (ARS-0511A)
You'll also want a pack of ring terminals for 14-16ga wire with the ring small enough to be used with the #4 screws. You will also need 14-16ga ring terminals capable of be used with 1/4" bolt, I got mine from DXEngineering at the same time I purchased the radial ground plate. They have them 20 for $5:
http://www.dxengineering.com/parts/dxe-225rt-20
You'll also want 14ga stranded (You can use solid core enameled wire, but you'll want to put PTFE tubing around it, so just avoid the hassle) PTFE insulated wire. There's lots of sources for that, but since you don't need very much (like 4 feet worth or so), I bought a tiny quantity from ebay at:
10 Feet 14 Stranded PTFE Insulated Silver Plated Copper Wire | eBay
14ga will handle maximum power, and the PTFE insulation will handle extremely high temps without catching fire, it is after all teflon and they put that in frying pans. Lastly and for safety concerns you'll want to obtain some 3M 27 Glass cloth electrical tape, to wrap the FT-240-61 cores with. This will provide a little extra insurance against fire and the like in the case of high loads, shorts, and bad impedance matching. You can get that from Amazon for about $15.
What you'll want to do regardless of the wire source, is cut the wire into two (2) five foot sections. Here you face a choice, bifilar or parallel wire. To be honest I've done both and have personally not noticed much of a difference. In fact all the photos I've seen of commercial Ununs are all using parallel winding, so if that's easiest for you, you can do it. Otherwise you can create your own bifilar winding by taping the ends of your 5 foot sections, and then gently twisting the sections around each other. You don't need to make the twists tight, a nice loose job will do. I've personally used parallel winding in my core but you can do whatever you like. You will want to leave 4-5 inches on both ends of your winding because you'll need to make some connections when you're done, and if you leave them long enough you can avoid having to make superfluous soldiering that otherwise could of been avoided. I also recommend marking the two ends of the wire that will be your positive wire so you can identify it easily after you're done wrapping the core.
At this point, you'll want to wrap the cores with your 3M 27 glass cloth tape. I've wrapped both separately then wrapped them as a unit. But you may just want to wrap them as a single unit from the start and avoid the extra taping. To be honest I have no idea which is better. Theory would say wrapping them as a unit is better due to concerns over altering the thickness of the cores. But I'm not sure a thin layer of tape is going to make THAT MUCH difference overall, in fact I've read that the differences are pretty negligible. The fact of the matter is, it's most likely to effect the far ends of the range the unun will cover, and I'm pretty dubious of anyone operating much there. I know I don't operate much at 30MHz nor at 1.6MHz.
After wrapping your cores, you're ready to begin winding the wire around your core, I've done a lot of research on how many turns. I've read everything from 15-18 turns, but honestly that's an unruly mess I've done it. It does improve your low end response slightly but it's a real pain in the back side. I've been using 12-13 turns with excellent results, and suggest you try the same this will give you pretty good response from 1.5-1.6MHz thru 30-40MHz. You can read all about this here:
http://webzoom.freewebs.com/g8jnj/33ft Verticals and Ununs.pdf
He did a lot of research into these devices and I decided to take my 12-13 turns directly from that document.
Also one of the things you'll see on some of the Do-It-Yourself unun designs on the net if you google is that they use a superfluous piece of low loss coax (RG-8X etc). Well there's a much better way to design this thing than that, also avoiding the coax component you'll avoid additional soldier connections that can go bad, or break or whatever over time. My design ultimately uses only ONE (1) soldier joint, the rest are lugs, which also have their problems obviously but diagnosing the problem with 3 lugs is a relatively easy problem and can also be easily fixed and done relatively quickly.
Since it's late at the moment and we're in the middle of a tropical storm (or certainly getting gusty now), I'll post up a wiring diagram, and some photos in the next day or two so you can see how this all comes together. I'll be assembling the Unun from scratch (sans the extra core and the 3M 27 tape which unfortunately I don't have on hand yet .. they're both on order sadly). But you will get the basic idea of how it all goes together.
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