[Boatanchors] constant impedance pigtail

Bruce Long coolbrucelong at yahoo.com
Sat Aug 4 18:03:31 EDT 2012

I would guess from the meaning of "constant impedance" and "pigtail"  that this means that one of the connections to the tuning capacitor is incorporated into a transmission line structure to reduce inductive coupling and reduce the effect of the inductance of that lead on the receiver tuning.   I remember reading an article by James Millen  published in the 1930-1935 time frame if I remember correctly where he discusses the instabilities- both electrical noise with mechanical tuning as well as inductive feedback to surrounding receiver circuitry that can be introduced into a regenerative receiver by the lead, wiper and bearing structure of the tuning capacitor.  He describes a method of reducing these problems by avoiding the use of a capacitor rotor wiper completely.  Instead he shows a coaxial hole drilled down the center of the tuning capacitor shaft with an insulated braided wire threaded through this hole and soldered at the hole termination
 to the capacitor shaft - by means of a second radial hole drilled in the shaft.  The other end of braided wire is then attached with a short lead to a solder lug mechanically secured to but insulated from the tuning capacitor frame. 

In this way the mechanically unstable wiping contact to the tuning capacitor rotor is avoided and a large portion of the self inductance of the lead is neutralized by the makeshift coaxial transmission line structure.

Sorry for the over wordy description   Just found my copy of the Millen reference

Please look at page 11 of the article   "The Evolution of the National SW-Series of Short Wave Receivers   complied by Bruce W. McCalley  and archived by antiqueradios.org  Should be easy to find with google

My apologies for attributing Bruce McCalleys work for that of James Millen but if you are going to write about early radio technology and be mis-attributed I can think of anyone better to be mis-attributed to than James Millen.  As an engineer who occasionally has to write I greatly admire Millen's design savvy as well as his excellent matter of fact writing style .

I also apologize for not providing a link or scanning my copy and providing that.  truth be told my wife and I sold the house and I am taking a break from packing  Have to be out in a few days and I have way too much radio stuff.

Looks like I got the attribution wrong twice   The original reference is from the June 1930 issue of Radio News
The article title is "An Analysis of A.C. Operated Short-Wave Receiver Design" by James Millen and Robert Kruse.

Thanks again to Bruce McCalley for making this very interesting and useful work more widely available.

bruce long

 From: Steve Mulder <smulder3 at cox.net>
To: boatanchor <boatanchors at puck.nether.net> 
Sent: Saturday, August 4, 2012 5:08 PM
Subject: [Boatanchors] constant impedance pigtail
I was reading an article in September 1931 QST about a three tube receiver, and the author mentioned that the tuning condenser had a "constant impedance pigtail". Can anyone tell me what a constant impedance pigtail is?



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