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Wednesday, February 7, 2024

Videos from Mike AG5VG -- His Homebrew BITX Rigs

Here are two great videos from Mike AG5VG showing his two homebrew receivers in action.  (The transmitter portions of Farhan's circuit will come later.) See yesterday's post for more details. 

On the video above (40 meters) 
-- I love that speaker.  
--  The enclosure and the reduction drive for the VFO is really great.  FB OM. 
-- Very cool that Mike captures a 40 meter QSO with "Wild Bill" ZS6CCY in South Africa, someone who we've spoken to many times, often on the long path, sometimes from Mozambique. 
-- I like how Mike demonstrates the effect of removing the antenna.  You can definitely see what we mean when we say you should be able to "hear the band noise."   

Above you see the 20 meter receiver in action.  You can see one of the physical benefits of using a wooden base:  You can easily mount connectors, switches and tuning controls using just pieces of copper-clad board screwed into the wood.   This is what I am doing with my latest BITX 15-10 rig. 

For the tuning of the VFO, it looks to me as if Mike has a large "main tuning" control in the center, with a smaller "fine tuning" or "bandspread" control off to the left.  Does that smaller control work with a varactor diode or with a smaller variable cap?  Also, to the right of the main tuning control we see a 3 pole switch.  Is that switch putting additional L or C into the VFO circuit to move the frequency around?  These techniques would all be quite valid;  Mike demonstrates that there are many ways to skin a cat (or tune a VFO!) 

Mike:  Please send more info. 


  1. I hope someone can give an opinion on this. I ask because I may not be able to identify a Schottky diode from the junkbox parts I have. Will Schottky diodes give better performance in a diode ring mixer or the Polyakov mixer than other diodes?

    1. Trevor: I just use ordinary 1N4148 diodes in my mixers. They work fine but YMMV. 73 Bill

    2. Thanks Bill. Before I read this I went and looked up some information on diodes. Then got carried away in writing mode, which I will post here I the blog.

  2. Bill,

    The switch on the side is for the band spread, with my first build at that time. I was not able to fully reach from 14 to 14.350 with the main 365pf cap, so I add in two caps for the other sides of the band so I can access it. I did research and I found that if I add more turns to L4, and I reduce the capacitance then I can get more band spread from what I have read. I applied that principle to the 40,meter but I still don’t have 300khz of band spread so I’m just focusing on the SSB portion 7.150 to 7.300

    Just learning more about these awesome LC VFOs. Such a great feeling to have them stable and working!

    The left smaller knob is using a 30v zener, I didn’t have a 36v so I used what I had. It works great for the fine tuning on the bitx20

    Mike AG5VG

  3. I may be able to identify any Schottky diodes that I may have by measuring the forward voltage. I found this on Wikipedia.

    'When sufficient forward voltage is applied, a current flows in the forward direction. A silicon p–n diode has a typical forward voltage of 600–700 mV, while the Schottky's forward voltage is 150–450 mV. This lower forward voltage requirement allows higher switching speeds and better system efficiency"

    Also of interest while I'm learning about Schottky diodes. Also from Wikipedia.

    "The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action. The cat's-whisker detectors used in the early days of wireless and metal rectifiers used in early power applications can be considered primitive Schottky diodes."

    I wouldn't mind doing some tests with different diodes in mixers to see the difference. I think I will need to move into something a bit more sophisticated than what I've done so far though. I played around with upconverting and down-converting in a very simplistic form. Using just my Sangean digital tuning radio, an old transistor radio and a single germanium diode. Plus some alligator clip leads. The transistor radio was actually made in the sixties sometime. I got it in a box of sundries at an auction years ago. National, made in Japan. Solid State written on the front. Just AM. Takes two AA cells. They came in red and black. Mine is black. It works fine. Anyway, the radio station I upconverted was on 783khz. I first put the Sangean on 1200khz. With the transistor radio on top I tuned it till the Sangean went silent. I connected one end of the diode to the antenna of the Sangean with a lead. A couple of leads together to the other end of the diode for the antenna. I placed the transistor radio on top of the diode. I changed to the shortwave band and tuned to 1983khz the sum of the two. It worked amazingly well. I now have a nanovna which I can use as an oscillator and have down converted one of the local taxi companies that transmit on 487.600 down to somewhere on the the shortwave band. I'm sure I down converted to the FM broadcast band with better performance, but the last time I tried I kept getting a local FM station coming through. It wasn't as good as listening with my Baofeng but quite listenable. This was still with the single germanium diode.

    My interest then went to direct conversion receivers. The idea that you could convert RF down to baseband as they put it seemed amazing. While I wouldn't be so bold to say I have built a proper DC receiver yet. I have experienced DC in a very crude form. With things cobbled together, I had very limited and poor results. But it did work. Using the nanovna for the LO, still the single diode and the output going into a small amplified speaker. I tried other powered speakers but this worked the best. I used the three local AM radio stations because of their strength of signal. Now I know DC receivers are not good with AM, but you can hear it and understand speech no problem. Music sounds warbly. Bill knows all about radio Marti. In fact, when I was doing all my reading from the transcripts of Arnie Coro on his radio program, I remember him talking about having a very stable LO and able to listen to AM quite well. Anyway I was able to change the oscillator frequency and hear those three AM radio stations but it was not very loud. What could I expect? No bandpass filter. Not a proper mixer. No diplexer. And I'm sure a lot more. But it was just to see what I could do with what at hand without melting solder. And I did enjoy it.


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