Taking the Knack a Bit Too Far

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BITX 17 Build Update: More Filter Maintenance

 

 

 

Having had great success in straightening out the 11 MHz filter in my BITX 2040 (scroll down for details),  today I decided to see what I could do with the filter in my 17 meter BITX (four 5 MHz crystals in Cohn MIN LOSS configuration, with 40 pf caps all around).  The key to my success in all this has been the filter programs that came with EMRFD.  After characterizing my crystals with the G3UUR method, I plugged the values into LADBUILD 8, then took a look at the expected results.  As you can see from the image above the predictions were not pretty.  Yuck.  Lots of ripple and lots of insertion loss.

I went into the rig and using my DDS sig generator and my RIGOL 'scope, measured actual performance.  It looked worse than the prediction (part of the worsening is a difference in vertical scale):

LADBUILD lets you play around with the values of the components in the filters.  I know that ripple is usually related to an impedance mismatch.  So in LADBUILD I experimented (virtually) with different impedance values at the end.   I noticed that at about 1000 ohms, the ripple and insertion loss got better:

So I went and built two broadband toroidal transformers.   4 turns primary with 12 turns secondary (1:9 Z).  I'm assuming that the BITX has around 150 ohms at either end of the filter.  That would put about 1350 ohms at the ends of the filter.

Here are the results:

Much better.



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ISEE-3 Spacecraft Returns after Being Forgotten

Hack-A-Day has issued a challenge to hams and hackers:

http://hackaday.com/2014/02/14/call-for-hams-and-hackers-welcome-iceisee-3-home/#more-114769

If a BITX 20/40 would help, I stand ready to assist!  


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Homebrew Junkbox Machine Tools

In the book "The Ugly American"  the hero (the ugly guy!) come up with a way to use old engine blocks to build water pumps to help farmers in South East Asia. Now, Mr. Delany is putting them to similar good use.  Homebrew junkbox machine tools!  Yea!

http://makezine.com/magazine/make-37/patdelany/

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

BITX 2040 Build Update #10 : Fixing my Filter

That dip on the high side of the passband was bothering me.  The GPLA crystal design program predicted it, and indeed, when I measured the performance of the actual filter, there it was.   The rig was working fine -- the receiver sounded fine and everyone tells me that it sounds great on transmit.  But still, it bothered me.

So I started working out with the various crystal filter software packages.  

This filter was -- sort of -- a Cohn Min-loss filter, but I had built it with four crystals and three shunt caps (80 pf each) and no series caps at the input/output.  This morning I decided to see what would happen if I put the series caps in there.  Here is what Wes's GPLA predicted:

Wow, that looked a lot nicer.  And the 150 ohm terminations seemed to be just about perfect for the BITX design -- no need for impedance transformation.  I heated up the soldering iron and searched the junkbox for suitable caps (I found 2 82 pf caps -- close enough).

Here is what the results looked like (I didn't measure insertion loss so the top of the curve is just the peak of the response curve).

Exactly as predicted!  Thanks Wes! And thanks to Farhan for encouraging me to characterize my crystals and to use the available software

Now I have to go back and de-ripple the 5 MHz filter in my BITX 17.

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Back to the W7ZOI/W7PUA Power Meter

Recent e-mails and Facebook postings from Jim (W8NSA) and Michael (AA1TJ) got me thinking about my old W7ZOI/W7PUA power meter.  The 15 inches (38 centimeters) of snow that fell last night gave me the day off -- and time to play with this very useful and interesting piece of gear. 

The last time I used it I remember thinking that a digital readout would be nice.  But I didn't feel like going back into the world of Arduinos and LCD screens.  So I came up with a real Kludge solution:  I had cheap little DVM that I wasn't using, so I just velcroed it to the side of the power meter.  That little connector above the BNC is the output for a DVM.  I might work on calibration later today.

Wes has some very interesting info follow-up info on the meter on his site: http://w7zoi.net/qststuff.html
I really like the part about how the meter is so sensitive that you can see the thermal noise in the input circuit and can actually measure the strength of signals from your antenna. 

I think I might need a low pass filter at the input of the meter.  There are strong FM broadcast transmitters in this area (some of you may have listened to them in the background of early episode of the SolderSmoke podcast!). I notice that just bringing my fingers close to the input causes the meter and the DVM readout to swing up.  That's not good.

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A Very Simple SSB Transmitter

 

 

I found this in the files section of the BITX20 Yahoo Group.  There is no information on the source.  I see two balanced modulators and two very simple phase shift networks. 

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Mars Star Party from La Palma (video)

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BITX 2040 Build Update #9: On-the-air Observations

The BITX 2040 has moved from the bench to the operating position and is producing a steady stream of contacts on 20 and 40 meters.  In the picture above it is the rig with the copper-clad front panel (the BITX 17 is below it).  It has already crossed the pond on both bands.

Some observations:

I get significantly more power out on 40 than on 20:  about 7.2 watts PEP on 40 and about 4.4 watts PEP on 20.  I saw a QST article that showed similar frequency/power out variations from IRF510 amps.   But I notice I get more power out from my 17 meter rig.  On that rig I am using trifilar (9:1 Z) transformer instead of the standard BITX bifilars. 

My 40 meter receiver is LOUD.   Too much AF out.  I am not used to having this problem!  On this rig I am using the same discrete component 2n3904 2n3906 transformer-less circuit that I used in the BITX 17.  But AF out on 40 was so loud that I had to go back and add 20 k ohms to the top of the volume control pot.   I didn't have this problem with the 17 meter rig, and I didn't have it on 20 with this rig.  Any ideas why this rig would be so loud on 40?

I still want to go in and fine tune the crystal filters in both rigs.  I am studying the various software packages out there (especially Wes's LADPAC).  I hope to get rid of the ripple.   

In most of my contacts with these rigs, I end up describing the circuit and its Indian origins.  Most people are really fascinated.  Yesterday W1IDL in Michigan suggested that I contact my Indian friends and get some assistance in making some Hindi or Urdu labels for the rig and the controls.   I think that is a very cool idea. 

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Coils, Magnets, and Special Relativity (video)

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Inductive Reactance and Special Relativity

Bill,

I'd been meaning to share these stories with you after I read your book a couple years ago but I never got to it.  I thought you might enjoy them, from an "engineering perspective", I guess.

One of the courses I had to take for my undergrad was an engineering physics type class.  I loved it.  I think a lot of hams seem to have more curiosity about the physics of electronics than regular non-ham engineers, at least that's how it's always seemed to me.  Anyway, I'm sending you a snapshot of the relativistic length contraction figure in the book "Concepts of Modern Physics", 4th Ed by Arthur Beiser.  I thought you'd enjoy it as it is almost identical to what you mentioned in Soldersmoke (from your "Atoms to Amperes" book I think).

Hopefully there's enough resolution there to make it out.  Basically, when you flow current in the same direction in both wires, they attract.  That's because the electrons see effectively many more positively charged nuclei from the other wire than they do other electrons due to the nuclei distances being compressed by the Lorentz-Fitzgerald contraction (later refined by Einstein). 

When I first saw this, in my early 20s, I was completely floored!  Nowhere had I ever learned anything like this from the ham license manuals or even my basic physics course.  The implications were also very profound -- magnetism was nothing more than electrostatic attraction, the attraction between charges.  The "electromagnetic" force was really just an electric force.  Relative motion between charges gives the illusion of "magnetism". 

Much later, I listened to some of the old Feynmann lectures.  In them at one point he adamantly proclaimed that there's only the electric force between charges, and there is no magnetic force!  I still find this confusing.  Recently I brought this up to a university RF engineering professor.  I wondered why we dealt with Maxwell's equations when in reality the magnetic field is an illusion.  The "real" formulas come from Feynmann's theory of quantum electrodynamics!  His reply was something along the lines of Maxwell's equations being a solution of quantum theory that worked well for our purposes.  To be honest, I didn't really understand his reply and I'm still skeptical!  I think his point was that the QED calculations are overly complicated and unnecessary for most problems we deal with, things like patterns from an antenna.  I don't think Maxwell's equations appropriately describe things like lasers though, which are more quantum in nature with the coherent beam.

FYI, most engineering students I ran across had only passing curiosity for these things.  Only in graduate school did I start to find people curious enough to really try to understand "what lies beneath" some of this stuff, mainly this physics.  Honestly not even everyone in grad school was all that captivated.  As you've said before, there's a lot of "turn the crank" mentality in engineering where you wade through mathematics to get answers, not always thinking about the physics.  It's even worse in the digital world, where everything gets boiled down to computer code! 

One more quick thing.  I talked to a physics prof once, asking him if there was any research happening in his department focused on electromagnetics and radio waves, etc.  His reply: "radio waves are nothing more than the result of accelerating electrons".  Period!  Discussion over.  In other words, that's ancient history.  Engineers are still very much involved with new technologies involving antennas and amplifiers, etc.  But as far as the physicists are concerned, I get the impression that our whole field is pretty ho-hum.  But he was right about accelerating electrons, I also found out later.  And it doesn't have to be electrons.  Anything carrying charge undergoing acceleration will emit photons.  That's another crazy situation that I only more recently learned.

Hope that was entertaining!

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BITX 2040 Build Update #8: All Boxed Up and Ready to Go!

Its kind of scary:  my BITX 20/40 is starting to look vaguely appliance-like!   Once again I find myself missing having the rig exposed -- sans-cabinet -- on the bench.  For me, the contacts made under those conditions, with solder smoke still in the air, are the most satisfying and memorable.  

But anyway, it is in the box now.  I think it looks pretty good.  I enjoyed working with the copper foil.  I did some trial and error testing and found that ordinary Krazy Glue is the best adhesive for joining the plastic material on the back of the foil to the wood of the box.  I like the look of the foil -- it reminds me of the copper (or gold?)  foil covering that they put on some spacecraft. 

There is always the danger that a rig that works well on the bench will go into rebellious oscillation when confined to a metallic box.   That didn't happen with mine. 

We discovered that the wood in the box is not actually walnut.  But I'm not complaining because whatever it is, it is very easy to work with.  BITX or Minima builders should consider these boxes (available via Amazon). 

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