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Author Topic: recognize coax on QRP Common Mode Choke?  (Read 203 times)

N3WTK

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recognize coax on QRP Common Mode Choke?
« on: June 04, 2021, 01:29:16 PM »

Does anyone recognize the coax on this QRP Common Mode Choke? It looks to me about the size of RG174, and I can even convince myself that I see "1 7 4" printed on it. But 174 isn't tan, and it isn't teflon. Maybe it's RG178?

I'd like to build a choke according to Jim Brown (K9YC)'s Cookbook, but smaller, for QRP. One of the reasons why Jim Brown used RG400 is because he found the center conductor didn't migrate so much through the PTFE dielectric when wound in a tight radius, while other coax cables would. I don't know if the folks at BalunDesigns.com put any thought into that or not, but it at least LOOKS like RG400, for what it's worth (?).

Now, for the inevitable posts that will come to say "don't do that, do this" -- I've been experimenting with my EFHW and find that its resonance is more stable when I insert a K9YC-designed common mode choke. Without the choke my resonance varies depending on how I hold or touch my radio, but with the choke it's solid and I have not been able to replicate the same performance out of coils of coax. But this choke is almost the same size and weight as my radio! So I'm trying to come up with something like it, only smaller. (And without having to pay $54 to have someone else make it.)

Thanks,
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Dave / N3WTK

N6MST

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Re: recognize coax on QRP Common Mode Choke?
« Reply #1 on: June 04, 2021, 03:02:40 PM »

RG400? Don't be so sure you can build it cheaper than LDG does, most places that sell RG400 do not let it go for cheap.

https://www.rfparts.com/rg400-mil17-therm.html

https://www.amazon.com/Eightwood-Double-Copper-Shielded-Coaxial/dp/B0116IRBHO/ref=sr_1_1?dchild=1&keywords=rg+400&qid=1622844121&sr=8-1

EDIT: LDG, Balun Designs, I get them all mixed up, my bad!
« Last Edit: June 04, 2021, 03:05:46 PM by N6MST »
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WB0LJK

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Re: recognize coax on QRP Common Mode Choke?
« Reply #2 on: June 04, 2021, 03:05:11 PM »

Might be RG-316 It's 50 ohm and about 0.1" in diameter.  I used it for a number of applications including a common mode choke on my active receive antenna.  Power rating at HF is more than enough for non QRO applications. 
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N3WTK

  • Posts: 84
Re: recognize coax on QRP Common Mode Choke?
« Reply #3 on: June 04, 2021, 04:22:02 PM »

yeah, maybe RG316, I can believe that.

It's not RG400. That's what I use for making my "QRO" baluns, as unto the K9YC cookbook referenced above. You need a 2.4 in dia ferrite to handle enough turns of that, and then they are big and heavy. Plus, you can see in the photo that the diameter of the coax is comparable in size to the center pin of that SO239, so it's smaller than RG400. Incidentally, using all new materials, RG400, connectors, and cores, I can build a choke for about $25-$30.

thanks,
« Last Edit: June 04, 2021, 04:24:52 PM by N3WTK »
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Dave / N3WTK

W6SDW

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Re: recognize coax on QRP Common Mode Choke?
« Reply #4 on: June 04, 2021, 04:47:57 PM »

Looking at your supplied picture - considering size of the unit, etc. - bet it's RG-316.
Most suppliers have minimum buys ( 50-100+ ft ) - Few years back (before retiring) I got a few feet of it as an "engineering sample" thru (I think) outfit called Fairview Microwave.  As I recall they would sell small quantities... Look 'em up and see if they still price-by-the-foot...  73,
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KB1NO

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Re: recognize coax on QRP Common Mode Choke?
« Reply #5 on: June 04, 2021, 09:22:41 PM »

It could be RG-303 (or RG-316 as W6SDW suggested.)    I believe either would work fine.   I was able to buy short lengths of both at reasonable prices on Amazon.  In fact Teflon coax may be overkill for QRP.  You could probably use RG-58A. Since the power is low, there won't be significant heating.    The length is short, so not much coax loss.

GL,
John

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W1VT

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Re: recognize coax on QRP Common Mode Choke?
« Reply #6 on: June 05, 2021, 06:33:01 AM »

The advantage of Teflon is that you can solder to Teflon coax without shorting the center conductor to the shield. 
This is a big issue with RG-174 pigtails--folks short out the coax when the soldering.

Zak W1VT
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