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Posted: 5/14/2012 8:46:34 PM EDT
I built a 4:1 balun so I could start playing with an OCF inverted-V dipole.  I followed the ZS1AN hybrid balun
article which combines a voltage transformer and a choke. I used the
same cores he did, even though he admits they're not optimal.  So now
that it's built, I'm thinking about how to optimize the design for the
next build.

Where I'm having trouble is picking the right cores.  In this case, the
transformer is 8 turns of #16 (he used #14) enamel coated wire wound
over an FT240-61.  The choke is 8 turns of #16 over an FT114-61.  All
the windings are inside Teflon sleeves.

On the analyzer, it turned out to be between 3.4:1 and 3.6:1 across the
ham bands from 80m to 10m.  That's into a 200 ohm resistor.  I
guess that puts me in the right ballpark for an OCF.

A tiny bit of knowledge is a dangerous thing.  I was poking
around with some core calculator utilities and I'm thinking that
FT114-61 choke might saturate at 100 watts especially if I'm driving the antenna on bands that it doesn't handle naturally.  Also, I suppose it'd be better to align the toroids 90
degrees from each other so they don't share the same orientation.

So I'm wondering if I'd be better off with an FT240-43 core for the
transformer and and FT240-77 for the choke.  But really, I'm lost here.
What say you RF gurus?  

BTW, 100 watts max and I'm interested in whatever bands I can make work from 80m to 10m.
Link Posted: 5/15/2012 5:02:12 AM EDT
I can't answer your question, but I do want to ask what analyzer/software are you using?  Is this a high end spectrum analyzer with a VSWR bridge and tracking generator?
Link Posted: 5/15/2012 5:50:07 AM EDT
It's an AIM-4170C.  About a million times better than an MFJ for way less than twice the price.  
Link Posted: 5/15/2012 6:10:37 AM EDT
Originally Posted By BlammO:
It's an AIM-4170C.  About a million times better than an MFJ for way less than twice the price.    

Looks neat! 73, Rob
Link Posted: 5/17/2012 5:41:14 PM EDT
So, uh . . . anyone know anything about ferrite mixes?  
Link Posted: 5/17/2012 6:09:13 PM EDT
Originally Posted By BlammO:
So, uh . . . anyone know anything about ferrite mixes?    

I had a big reply typed up but lost it when I copied this from my search.  

The ultimate reference for baluns is the Jerry Sevick W2FMI book: "Transmission line transformers" ISBN 978-0872592964.

Your build looks great, and certainly more than I've ever tried to do.  Keep looking online, I think the info is out there, and email W8JI if you can't get anywhere.  He's an expert and loves a good technical RF discussion.  He's designed this stuff for Ameritron and DX Engineering.
Link Posted: 5/17/2012 6:33:41 PM EDT
Here is a really good write up on differing mix material and frequency by J9YC...  

This really is a good read...  

Chokes made easy in 66 pages of good reading
Link Posted: 5/17/2012 8:51:35 PM EDT
Thanks guys!  I really need to pick up Sevick's book.  I'll order that from ARRL, along with a couple others I've been eyeing.  I miss being able to hop in my truck and drive up to ARRL HQ to pick up goodies.  

I discovered that 66-page "Ham's Guide to RFI, Ferrites, Baluns & Audio Interfacing" day before yesterday and am trying to wrap my little jellyfish brain around it.  It appears that mix 31 is best for at least the common mode core.  Mix 31 is missing from mini Ring Core Calculator v1.2, but it appears to be a fairly new material.  Another resource I stumbled into is Common Mode Chokes by W1HIS.

Today, I started suspecting my wiring method to be the culprit behind the impedance deviation.  Sure enough, after testing it on the AIM-4170 in TDR mode, I can see a big spike up to 70 ohms at about 2.75 feet which corresponds to the connections between the choke and the transformer.  Splaying the wires apart from each other the way I did is what caused that.  Lesson:  When I'm winding parallel conductors, they need to stay parallel as much as possible, duhhhhh.

Back to the ol' workbench!  

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