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Thursday, August 11, 2022

What Coil for the Polyakov Input Circuit? How to calculate a coil value for resonance.

So,  what is the value for L1 and L2?   What coil should I use?  
Michael AG5VG had that question.  And so did I when I built this receiver.  See below for the process I used in answering this question. 

On Tuesday, August 9, 2022 at 10:53:32 PM EDT, Michael S  wrote:

Good Evening Bill,

My name is Michael and I really enjoy your podcast with Pete. I have also spoken with him in regards to the design of a 20M bandpass filter I made for a homebrew rig.  I am currently in the process of making a 20 meter DSB - SC type. Thank you for all the information that you speak and teach about during your podcast. I also enjoy the humor. It's great.

The Polyakov is a simple DC receiver and it amazes me and how the sound quality is. My question is, what is the turns on the toroid for the antenna primary side and the radio secondary and how did you figure out the turns because looking at the schematic it doesn't give that information that I can see. Also how you resonated it with the variable capacitor that looks like a 365pf air variable. 

Thank you for your time and keep up the great work on the podcast and the content on YouTube.


My response: 

Good questions Michael.   When I saw the SPRAT article I too was struck by the fact that it didn't give a value for the coil.  But DK2RS did have a large value variable capacitor... And he was billing this as a dual-band (80-40) rig.  So I figured he wanted that LC circuit to resonate as low as 3.5 MHZ and as high as 7.3 MHZ.  So, with a variable cap that goes up to 350 pf, what value L should I use?   I started by calculating the resonant frequency of the frequency mid-way point: 5.1 MHz.   I figured the variable cap should be around 162 pf at the mid-way point.  At this point I went to the on-line resonant frequency calculator: https://www.1728.org/resfreq.htm (a REALLY useful site!).   This site revealed I needed a coil of about 6 uH.  This put me in the ballpark.   But then -- with the site -- I tested it with the values of the variable cap I had on hand.  Mine was 23pf to 372 pf.  (you really need an LC meter to do this kind of thing). 

Again at the resonant freq calculation site:  23 pf and 6uH = 13.5 MHz           372 pf and 6 uH = 3.3688 MHz

This would have been OK, but I wanted to move the frequency range down a bit, so I tried. 6.5 uH 

23 pf and 6.5 uH = 13 MHz                 372 and 6.5uH = 3.23 MHz

Now, how many turns?  First look at the overall coil -- don't worry about taps at this point.    I use the Toroid Turns Calculator: http://toroids.info/

 Start by asking yourself "What core do I have on-hand?    Let's say you have a T-50-2 (red/clear).   The calculator shows you need about 36 turns.  Do-able, but physically kind of tight.  

I found a big core in my junk box.  A T-106-2.  The calculator showed I'd need about 22 turns on this core. It was much easier to get these turns on the larger core.  

You have to measure the core after you wind it to make sure you are at the desired inductance.   One side of the main coil went to ground, the other side to the top of the variable cap. 

 Now for the taps and secondaries:  The schematic shows a tap.  This is usually about 1/4 of the number of turns up from ground.   I picked about 5 turns, and wound a little tap in there at that point -- that tap went to the antenna.  You also have a secondary coil --no value is given, but based on experience I guessed around 5 turns -- I wound these turns on top of the primary one lead went to ground, the other went to the diodes and the switch. 

 The last thing to do is to see if the circuit resonates on both bands that you want to receive.  You can do this with a signal generator, or with the band noise:  Hook up an 80 meter antenna.  Put the cap closer to its max value and tune the cap -- can you hear band noise?  Or can you hear (or see on a 'scope) a signal at 3.5 MHz?   You should be able to peak it with the main cap.  Try to do the same thing on 40 meters -- here the variable cap should be closer to minimum capacity. 

 That's it.  That's how I did it.   You can do it too!   Good luck with the Polyakov.  

 One hint:  Building the VFO is the hard part.  You can get started by using a signal generator in place of the VFO.  Just make sure you have the level right -- around 620 mV input.  

 Good luck -- Let us know if you have trouble. And please let us know how the project goes.   

 73    Bill    N2CQR  

20 meter rig built by AG5VG


  1. Dear Bill & Pete, with the current endeavours in temp stabilized VFOs in mind and the nouveau hardware defined radio spirit on the blog, I wonder whether there's a good suggestion for a 3.5 MHz VFO for the Polyakov DCR :) THX

    1. Andreas -- I'm reluctant to provide a schematic for this, because so much depends on what you have on hand for the coil and the variable capacitor. Just follow the guidance for stability: Solid construction, NP0 fixed caps, an air core coil... Good luck. 73 Bill

  2. https://www.n6qw.com/Resurrection_Radio.html

  3. The LC factor for a given frequency is 25330/(F*F) where L is inductance in uH, C is capacitance in pF and F is frequency in mhz.
    Thus the LC factor for 3.4mhz is 2191 and the LC factor for 7.5mhz is 450. The difference is 1741. If we have a variable capacitor with 350pf swing then 1741/350 = 4.97 (uH) and the minimum capacitance at 7.5mhz = 450/4.97 = 90.5.

  4. I may be missing something re the "LC factor" but, are you sure about this OM? My variable capacitor has a minimum capacitance of 23pf. With a 4.97uH coil it resonates at 14.885 MHz (too high!). The minimum capacitance is 23 pf. With a 4.97uH coil it resonates at 3.7014 Mhz (again, too high). With a 6.5 uH coil, the tuning range with this cap is 3.23 to 13.01 MHz. Are these calculations wrong? 73 Bill

    1. Bill - the problem is that I've used different frequencies. I used 3.4 and 7.5 as an example. If you plug in your 3.23 and 13.01 to my formula you'll find it works. 73 David

  5. Not our man Vladimir :)
    Vasily Polyakov, Soviet fighter pilot on film:
    @ 43: 14, https://youtu.be/0frbo09ocFU,
    The Secret Air Skirmishes Of The Cold War | Shot Down Over The Soviet Union | Timeline
    (1960s spy planes post-Gary Powers episode).


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