What various RF Chokes look like

[Yorkie]

I've attached a pic of several vintage RF chokes, which might help anyone wishing to build old designs of homebrew radios that call for an RF choke and it might not be apparent just what one of a certain value actually looks like. I'm prompted to add the pic because a chum who isn't a member of this forum wanted a 2.5mH choke, which I happened to have. While I was scrabbling about I unearthed several others so took a pic of them. Chokes rarely have the value marked on them and if they're separated from their original packing, as they often are, without an inductance bridge or meter, it's difficult to 'guesstimate' let alone ascertain, the likely value.

Hope it comes in useful to someone!

[pwdrive]

Ah, Repanco, there's a name from the past, thanks David.

Lawrence.

[Yorkie]

Ah, Repanco, there's a name from the past, thanks David.

Lawrence.

Yes indeed Lawrence!

Their name was made up from Radio Experimental Products and Co - 'REP and Co' I notice that the address on the label was different from that which I recall - 'Much Park'. I don't know when they started or when the firm closed down, but somewhere I must have one of their crystal set coils which featured in the Boys' Book of Crystal Sets. Fond memories of happy far off days of long lost youth! As A.E. Housman put it so eloquently in 'A Shropshire Lad':

Into my heart an air that kills
From yon far country blows:
What are those blue remembered hills,
What spires, what farms are those?

That is the land of lost content,
I see it shining plain,
The happy highways where I went
And cannot come again.

Oh dear, that's a bit profound for a Saturday morning!

[pwdrive]

I remember saving up my pennies for one of those Repanco chokes for my first reflex job, 2.5 mh it was, from a components shop at the top end of Tibb St in Manchester. Happy days.

Lawrence.

[Skywave]

A useful & informative post, David: thanks.

However I feel that I must point out to some of our less-experienced readers that amongst all the passive components that one finds in electronics circuitry of any vintage, the 'R.F. choke' is the one item that is easy to mis-apply and whose behavior is generally not fully appreciated. The main reason (but not the only one) for this is simply that an 'R.F. choke' is far from being a 'pure inductance' - although a brief analysis of most designs where one is used seems to regard it as being such. Apart from its d.c. resistance, there is the all-important issue of its inherent capacitance: any R.F. choke, above a certain frequency, starts to behave more like a capacitor that an inductance! What can be even more disastrous to any given circuit is the frequency where that capacitance resonates with the choke's inductance - the last thing you want a 'choke' to do! Most of the chokes in the photograph show R.F. chokes wound in sections: this is an attempt to reduce that self-capacitance. So, in summary, when a cct. calls for a R.F. choke of so many mH, that specification is not really adequate: the manufacturer's reference ideally is also required so that the R.F. choke employed in your circuit will have the same R.F. characteristics as that used in the prototype design.
I have access to several inductance-measuring instruments, and many have been the occasion when, during circuit development, I have substituted on R.F. choke of a known inductance and d.c. resistance for another with those same parameters, only to find that the behavior of the cct. is then substantially different.

Al. / August 18th. //