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Old 4th Sep 2009, 11:46 am   #1
YT2095UK
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Question RF Chokes (making of...)

has anyone here actually made one, ideally out of readily available bits.
the reason I ask is that I would like to make a version of the Regen radio in F.J Camm`s 1956 book "Beginners guide to radio", but all the chokes I`v seen for sale are crazy prices (over 20 quid a pop) and that`s a little excessive in my opinion.
I have some ideas but am uncertain as to how to proceed, one idea was to use a small 2 inch ferrite rod from am AM radio and wind as many turns of thin magnet wire around it as messilly as possible to keep the capacitance low, the other idea was to use a 3 teir plastic former and do the same on that to give me an Air-core version (that I could always add a ferrite grub scew to after if needed), and would it better to use Litz wire instead of thin magnet wire?

any advice that would point me in the right direction would be great
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Old 4th Sep 2009, 11:55 am   #2
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Default Re: RF Chokes (making of...)

I wouldn't use Litz wire, unless you've nothing else.

Litz wire is used to keep the Q high and losses low, in coils used as part of a tuned circuit. But an RF choke is almost always untuned and wideband, so there is no need for such refinements.
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Old 4th Sep 2009, 12:16 pm   #3
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Default Re: RF Chokes (making of...)

I`m so glad you said that, plain magnet wire is so much more plentifull and cheaper.
I have seen small inductors that look a bit like resistors, one chap on here (Variometer I think) mounts them in glass fuse cases, I was wondering if I bought a load of those and wired them in series if it would have the same effect?
and what sort of Total value I would need to be effective (it neglects to mention it in the book) although I expect it would be in the region of Mili henrys rather than Micro Henrys and would probably end up having to buy a shed load of them anyway
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Old 4th Sep 2009, 12:34 pm   #4
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Default Re: RF Chokes (making of...)

You shouldn't have to pay anything like 20 - RF chokes are frequently advertised, either NOS or used, you should only need to pay a or two.

Andy
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Old 4th Sep 2009, 1:02 pm   #5
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Default Re: RF Chokes (making of...)

Andy, if these chokes are suitable for the radio in question, then can you share your source or perhaps sell one to me?
I`de happily paypal you the P&P cost and a bit extra for a beer or 3
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Old 4th Sep 2009, 1:49 pm   #6
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Default Re: RF Chokes (making of...)

At the expense of a little loss in performance you could probably get away with using just a resistor instead of a RF choke unless the choke is required to couple magnetically with another circuit. So that's pence, not pounds.

To use a small resistor sized inductor, just choose the largest inductance that has a self resonant frequency above the range you're building the radio for- if you still need more inductance, put a few in series.

A 20 rf choke sounds more like a transmitter part than anything receiver related!
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Old 4th Sep 2009, 1:50 pm   #7
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Default Re: RF Chokes (making of...)

20 sounds more like the price you'd pay for a smoothing choke.
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Old 4th Sep 2009, 1:54 pm   #8
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Default Re: RF Chokes (making of...)

yeah, some of them were refered to in part with a current rating, others (about thumb size) with 3 to 4 cells of wave wound coil on it were min 12 quid and 26 quid for the 4 cell one, whereas the one pictured (illustrated) in the book looks to be a 4 cell choke with a laquer coating, again, sadly no ref as what value it was or even a part number
I imagine that back in the day, one could walk into any sort of radio dealership and ask broadcast band RF choke and be handed one straight away.
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Old 4th Sep 2009, 1:58 pm   #9
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Default Re: RF Chokes (making of...)

If a current is quoted and the inductance is in Henries then it's a smoothing choke. RF chokes will have a value in milli or microhenries.

You can wind your own on a high value resistor.
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Old 4th Sep 2009, 2:10 pm   #10
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Default Re: RF Chokes (making of...)

well the idea is to run the headphones as part of the circuit through this choke to present an anode voltage to work against, whilst simultaniously blocking any RF to the phones so that it can be used for the Regen coil ("Tickler" I beleive it`s called today), at least that`s how I understand it.
beyond that, I have no idea
but I will try a 1 meg with a messy winding of thinest wire I can find and give that a try too, the worst it can say is NO
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Old 4th Sep 2009, 2:17 pm   #11
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Default Re: RF Chokes (making of...)

Camm's book 'Wireless Coils, Chokes & Transformers', originally published in 1937, has a whole Chapter devoted to the subject. I've attached a couple of relevant pages. If you can't read it PM me with an address or two and I'll send better copies.
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Old 4th Sep 2009, 2:35 pm   #12
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Default Re: RF Chokes (making of...)

At medium wave frequencies you will need some sort of ferrite core, unless you have the patience to wind hundreds of turns. You need something like 1-5mH - should be available from Maplin, Rapid Electronics, JAB and many others. In a catalogue/website these might appear under RF, Inductors, Filters, EMC or Suppression.
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Old 4th Sep 2009, 3:35 pm   #13
M0FYA Andy
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Default Re: RF Chokes (making of...)

YT2095UK, I think I have the book you mention but it's gone into hiding somewhere! I must locate it.
If we are talking ordinary RF chokes, just search ebay. (Mods, tell me if am infringing the rules, please)
Only yesterday a lot of four used but very serviceable-looking RF chokes with three or four wave-wound coils sold for 5.50, There is a 'buy-it-now' for 6 at the moment. (OK, more than 2, but not 20!).
I hasten to add, I have no connection with any of the sellers.

Andy
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Old 4th Sep 2009, 5:53 pm   #14
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Default Re: RF Chokes (making of...)

Recently I bought these and they've worked fine even though I suspect they're not really intended for RF work.

http://uk.rs-online.com/web/search/s...uct&R=191-1226

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Old 4th Sep 2009, 7:40 pm   #15
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Default Re: RF Chokes (making of...)

I seem to recall that most of F J's circuits used something like a 2.5mH choke for the regen so as previously suggested, anything from 2 - 5mH should be OK. I might even have one somewhere but I'd need to find it and then check it on the bridge.

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Old 4th Sep 2009, 9:50 pm   #16
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Default Re: RF Chokes (making of...)

I have used 2W 10K resisors with as much 0.12mm wire I could fit on it with ok results in this kind of circuits, true a "real" 4 piece wavewound 2.5mH choke is better but...
edited a typo (decimal error)
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Old 5th Sep 2009, 3:44 pm   #17
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Default Re: RF Chokes (making of...)

There's some 2.5mH ones here on this site.

http://www.6v6.co.uk/cp/Scripts/default.asp
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Old 5th Sep 2009, 8:02 pm   #18
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Default Re: RF Chokes (making of...)

That's where I got mine from!


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Old 5th Sep 2009, 10:28 pm   #19
YT2095UK
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Default Re: RF Chokes (making of...)

ewww, I LIKE that! thanks a bunch
I`v seen a few other things on there too that I wouldn`t mind, I bought 20x 1000uH inductors today as well, I figured for 10p each it was worth it just to experiment with, and if they don`t do the job I can think of several apps where they would.
I`ll do the 10K resistor idea as well, it`ll be interesting just to see what works the best under certain conditions and get a little more data for future ref anyway.
Thanks for all your help each, it`s appreciated
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Old 5th Sep 2009, 10:33 pm   #20
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Default Re: RF Chokes (making of...)

That is the way to learn and a good way to gain understanding... experiment

Mike
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