Posted: 6/16/2015 5:47:36 PM EDT
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I've played with ferrite toroids a little bit, building chokes & current baluns, but the doughnut shapes are a pain sometimes. I ran across some 1/2" x 2 1/2" type 61 ferrite rods for about $1.95 apiece and thought I'd give them a try.
A plus for rods is they make winding stiff solid wire a whole lot easier, but the edges aren't relieved as much as on the toroid. (Unless your winding wire has tough coat, you'd be well advised to wrap the rod first) Being an empirical type, I tried a few different wire types to see how an SWR plot would look when the winding is terminated in a 50 ohm load. So far, my best combo for a 1:1 choke has been with 14 AWG solid wire stripped out of Romex, using one bare (or enamelled) wire & one insulated wire. (see below) 
The rod was wrapped with about 4 layers of teflon tape before starting. After soldering on an RG8x pigtail with PL259 on one end & a 50 ohm resistor on the other, I ran an SWR sweep with the Rigexpert AA-170. The result was a gently rising curve starting at 1.02:1 at 1.5 Mhz and not exceeding 1.29:1 through the end of the 12m band. The 10m band saw 1.30:1 at the lower bound and up to 1.32:1 at the upper end . Nick So far so good! The next step will be using this style of winding to build multi-core, impedance changing current baluns (guanella type) such as a 4:1 and a 9:1 to see how they look. Nick |
The 1:1 choke, built from a bifilar winding on a ferrite core, is a foundation component for a multi-core Guanella current balun. You see a lot of diagrams that show a 4:1 done with 2 bifilar windings on a single core, but I settled on the designs that use a separate core for each bifilar winding. The diagram below was lifted off a a web doc (I've seen the same one in a number of documents) but I don't know the original attributions.
Two cores make up a 4:1 Guanella current balun, extend the approach to three cores and you have a 9:1. None of this is my original work, I am just building my own implementations of the existing designs. Nick |
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Ok, put together both 4:1 (2 cores) and 9:1 (3 cores) current baluns using the materials described earlier.
I installed them in place of equivilant units made previously using 14 awg enamel wire and type 61 ferrite toriods (1.5" OD 1/2" thick). Both old and new units incorporated the same number of turns per core. I then compared SWR sweeps with the AA-170 for the connected antennae with archived sweeps from the prior baluns. No significant differences were noted. Band dips were in the same spots and of similar if not identical magnitude. For my uses, rod and toroid seem pretty much interchangable. On the plus side for rods, price is currently about 1/2 that of toroids and they are easier to wind. Nick |
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Here's the inside of a Hy-Gain BN-86 that was new in 1978 with my TH-3 MK3. Bad enough shape that I wouldn't think of using it so I tore it apart. I read that they were originally voltage baluns but redesigned at some point long ago to be proper current baluns.
ETA: Schematics on the web if you're interested. 
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Quoted:
Here's the inside of a Hy-Gain BN-86 that was new in 1978 with my TH-3 MK3. Bad enough shape that I wouldn't think of using it so I tore it apart. I read that they were originally voltage baluns but redesigned at some point long ago to be proper current baluns. ETA: Schematics on the web if you're interested. http://i.imgur.com/KlF5Y4L.jpg That's what I would expect the winding to look like on a balun using a ferrite rod. |
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