Calling Antenna Nerds - Choke Balun

ToddDubya

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Well, I just spent the entire morning until now working on a 1:1 choke balun with banana jack terminals to connect a dipole. I used an FT240-31 toroid and magnet wire, 12 turns of each pair on either side of the toroid, effectively two sets of inputs/outputs shorted together. I wired each side such that the left and right side are mirror images of each other.

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I probably should have checked the SWR along the way, but I at least made sure the correct wires were shorted and the incorrect wires weren't shorted. 0 ohms from inputs to respective outputs.

To test, I put a 50 ohm Dale power resistor across the terminals and got basically a 10:1 SWR from 3-50MHz with two narrow resonant points. Okay, that resistor is probably reactive as hell.

Next I tried two 100 ohm carbon resistors in parallel across the output terminals and ran a scan on that. It's better, but still terrible.

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I expected to see close to 1:1 across the board. Instead it starts out around 1KHz at 50 +j0, but as frequency climbs the resistance climbs and it becomes more reactive. Here's the Smith chart from 3.5MHz to 30MHz.

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If we ever have decent weather again I can try putting up one of my dipoles and see how that does.

In the meantime, what the hell? Anybody know what I'm doing wrong here? I'm hoping it's just my measurement technique that is causing it, but I didn't expect carbon resistors to be so reactive.
 
Update.

I just cut the leads to my dummy load down as short as possible and it's improved. Still not wonderful, but it makes me think my resistive load is not quite resistive. I still welcome any feedback anyone can give me.

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So, that’s what’s inside those grey boxes I buy from vendors? 😉

G’luck with that project!
 
I gotta wonder if one or more of the turns are shorted...what type of material is your core?
 
So, that’s what’s inside those grey boxes I buy from vendors? 😉

G’luck with that project!

Yeah, I was gonna buy one but I had all the pieces and it felt like a good weekend activity.

I gotta wonder if one or more of the turns are shorted...what type of material is your core?

The core is FT240-31. The DMM sees the 50 ohm resistance, and an open between the two conductors. It's not exactly a hi-pot test, but I think that's okay. The enamel on that magnet wire is pretty tough. It's actually a bitch to clean it off to solder.
 
The core is FT240-31. The DMM sees the 50 ohm resistance, and an open between the two conductors. It's not exactly a hi-pot test, but I think that's okay. The enamel on that magnet wire is pretty tough. It's actually a bitch to clean it off to solder.
OK...just wondering...whenever I've made baluns, I needed to make the wires different colors to keep everything straight. My eyes would go screwball o_O otherwise trying to keep everything correct.
 
OK...just wondering...whenever I've made baluns, I needed to make the wires different colors to keep everything straight. My eyes would go screwball o_O otherwise trying to keep everything correct.
Yeah, I tried marking them and then just checked continuity instead. Colors would have been easier but I thought magnet wire was better. What @Matt80 sent says not to use magnet wire. Back to the drawing board perhaps.
 
Pretty much the bible: http://k9yc.com/2018Cookbook.pdf

Running RG400 chokes on 2.4" cores for 80M, 20M, and 15-10M dipoles, as well as a 40M 1/4wave vertical. No SWR issues.
Damn, some good info in there. I didn't realize the windings would be band specific. I'd never heard that. I'll have to read that a few times to absorb it.

Do you wind for specific bands?
 
I don't have a way to measure what he measures in that article, I don't think. My analyzer will give me real/imaginary components (and R, L, C at a given frequency), but I don't know how he measures whatever that is in kOhms. There's gotta be some math going on.

Anyway, I shortened the leads to my resistors as must as possible and the SWR keeps getting closer and closer to 1:1, which tells me my biggest problems are measurement related. I don't know how effective it is, but it's at least looking like it won't be a detriment. I'm trying to work out in my head what impact having two sets of turns in parallel will have. Certainly it uses the whole toroid, so it should divide the current and spread the heat. But does it double capacitance while halving the resistance and inductance?

My setup seems to have the lowest SWR toward the 80m end, but decent up to 15m. According the handbook, my 13 turns should be effective up to 20m, so I might be in good shape. The proof will be when I can finally set something up outside and try it.

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I went with the assumption it was measurement error. Tonight I cut wires for a 40m dipole, trying to center on 7.150MHz. I can't be mad at the results considering it was all of 3' off the ground on electric fence posts.

I added bullet connectors to the ends, and the plan is to add elements for 60m, 75m, and 80m.

Now, I know the proof is in the pudding, and I didn't get it on the air. I brought it back in to solder the bullet connectors on, and it looks like rain (and dark). So maybe tomorrow I'll see what I can do with it. Under 1.8:1 SWR across 40m is nothing to shake a stick at. The intended use will be NVIS, so I'll raise it if I can but the step-in electric fence posts might just be good enough. SWR is one thing, but I need to see if it actually sends out the bananas, as they say.

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