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  • Author Author: kmikemoo
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Tap Tap Tap

kmikemoo
Tap Tap Tap

A few months back while looking for information on oscillators, I stumbled across a 40dB tap. Okay, the article was really about a RF power meter, but that looked complicated and, besides, I already have a NanoVNA. I would really like to NOT blow up the NanoVNA so the tap caught my eye. The article is from the June 2001 edition of QST magazine by the ARRL. The authors are Wes Hayward and Bob Larkin. The article is attached below.

So... I ordered the little aluminum enclosures. Of course they come in a three pack. Fine. I need 1.5 inches of a brass strip. It comes in a 12 inch strip. Fine. I need a SMA bulkhead connector. They come in 5-packs. Fine. I don't have resistors of the correct power rating.  I can get 100 for the same price as 10.  Fine.  With all this excess material, I'll make a SO-239 version, a BNC version and a SMA version. It's a bit more than I was thinking about, but they will save me from using a bunch of adapters. This is a good thing.

So I built the SO-239 version. It turned out nice. It turned out that it wasn't quite as easy to build as I estimated AND... I built the easy one - the one in the article.  So the project stalled.  Onto the next bright shiny thing.

And the other versions? The only movement they have seen was to get pushed out of the camera shot for the 2-year antenna project.

In the spirit of Spring Clean, let's get this project completed.
Fantastic. I have just started and I'm using up all of the M2.5 hardware from my standoff kit. I need to order some hardware. Maybe this was why I stalled out a few months ago. At least I was able to find all of the SMA bulkhead connectors. The brass strip is still at the end of the work bench where I do my sawing. It hasn't moved in months. Even the resistors are still here – albeit buried under other stuff.

On a bright note, my custom brass plate feedline installation jig is still here as well.
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The build isn't super complicated.  Here was the starting point.  You can see that I started drilling some of the mounting holes while I waited for the M2.5 hardware.  Both the SO-239 and the SMA connector mounting holes were tapped into the body of the enclosure.
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The purple wire is a gimmick capacitor and is supposed to improve performance at higher frequencies (>500MHz).  

Pre-soldering the brass plate worked better than just buffing the plate and trying to solder on it while in place.

For the BNC version, the brass plate needed to be notched.  I used a hand nibbler for that.

For the SMA version, connecting to the brass plate proved to be quite a chore.  I used a piece of solid #14 to bridge the gap between SMA center pins.  Once they were finally secure, I cut out the middle so the signal would flow through the brass plate.  Soldering to the SMA connectors was by far the most difficult aspect of the project.

For the end SMA connector, I used longer mounting screws so I could secure the two 100 ohm resistors to chassis ground.  Wrap the resistor tail around the protruding screw and secure with a nut.  This proved to be quite handy.

Performance

So how do they work?  I give them a "Good" grade.  According to my NanoVNA, they start at about 33dB of attenuation at 3MHZ.  Since I really don't plan to blast 100 watts of power through these, I should be fine.  The span depicted below is from 3 MHz to 1 GHz, although the article said that the tap was good only to about 500MHz.  I could always try lengthening the gimmick capacitor.

SO-239 Version
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BNC Version
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SMA Version
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Conclusion

Again, soldering to the SMA bulkhead connectors was by far the most challenging aspect of this project.  Truth be told, this was probably why I didn't complete the BNC and SMA versions until now.  If I had to do it over again, I would try to use 22-26 AWG butt splice connectors to bridge between the center pin of the SMA connector and the plate or resistors.  I would also not connect the two 100 ohm resistors until after the center run is soldered in place.  Last build tip:  Use way more wire than you need for the gimmick capacitor.  You'll need some to hold on to while soldering.  Once it's soldered in place, then trim to length

There are now less random pieces parts laying about on the workbench and a couple of more pieces of test gear on the shelf.   If nothing else, my workbench is looking better than it has in months.  Now if I could only figure out why I bought those 4 MHz TCXO's...

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  • in reply to kmikemoo

    That's more reasonable. The nanoVNA looks to be a useful piece of kit.

    If you did want to refine it a little, perhaps try reworking the two 100R resistors at the bottom of the attenuator to lose the excess lead length, and experiment with the gimmick capacitor, which in the SMA case looks like it's overcompensating a bit at the top end up towards 1GHz.

    Good luck with the competition.

  • in reply to jc2048

      I have not been into the radio hobby long enough to know the deep secrets.  I do believe that the NanoVNA is "good" but it is not laboratory grade.  I also believe that it is better at the lower frequencies and less accurate at the higher frequencies.  Still... for $100-ish... it is fantastic.

    I chose not to try to get too deep into the "decibel" discussion because although it is a ratio, we tend to use it as an absolute value (because we use a standard reference value).  The discussion becomes too academic.  At work, we only worry about sound pressure decibels - but only the A-weighted version, dBA.

    HOWEVER, your comment about "into a fixed load" (50 ohms) got me thinking.  The screen shots in the original post were with no load - because my dummy loads were all built for 200MHz and below and they definitely impacted the performance curves.  But after reading your post a couple of times, I remembered that my calibration load is supposed to be good for something crazy - like 6GHz.

    SMA Version
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    SO-239 Version
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    As you can see, these have much better performance into the 50 ohm load.  THANK YOU - because now I won't feel the need to try to improve these. LaughingJoy

    My opinion:  In RF, there are always parasitics.  I tried to build a 100dB attenuator.  It's good for 46dB at 146MHz.  Why?  I don't know.  It could be that the parallel ground strips on either side of the main signal path are not at the same true potential and the inner tracks are allowing some RF signal conduction.  The signal could also just be jumping across the resistor.  After all, it is radiating signal.  And THIS is what makes RF a bit of magic.

    How is it that 46mW of transmitter power can be heard in Antarctica from Northeastern Wisconsin?  Or the West coast of Africa?  And yet... it is so.  Why does 1 cm of wire make all the difference between a good antenna and a terrible antenna?
    Sorry, not sorry, my friend.  Welcome to the Dark Side.  RF.  RelaxedThumbsup  Glad to have you on the team. Laughing

  • in reply to kmikemoo

    Thanks for doing the project. I immediately gave you a 'like' because it's so nice seeing someone build something physical and actually measure it (though you might have given us the characteristics of the pass-through route, as well as the tap). And you're prodding me to think about an area of electronics that's still, after all these years, a bit unfamiliar to me - perhaps I ought to buy a SA or VNA and have a go myself.

    I've always been fascinated by things like this that are a mix of electrical and mechanical; wondering about things like how they came up with the plate dimensions (would they have modelled it - unlikely unless they had work access to high-end RF field-solving stuff - or did they use pure trial and error with the VNA, or maybe approximate paper calculations based on working out the transmission line properties roughly, from estimates of the capacitance and inductance, and then finishing it off with the VNA, or perhaps its even lore handed down through generations of RF engineers.

    I didn't give a unit because (strictly - not that I have to worry about passing exams any longer: that's all half a century in the past now) a ratio is just a number. It doesn't matter my not indicating whether it's voltage or power because in both cases we get 40dB as the answer. That's why it was set up like that, with 10 log for power and 20 log for voltage, to bake in the power (square) relationship between voltage and power when working into a fixed load (50R for RF measurements).

    Thinking in terms of voltage, that 7dB difference between measured and theory is more than a factor of two (6dB), and for someone like me, coming from a world of resistors with tolerances of 1% and measuring instruments like oscilloscopes that can manage better than 1% (I'm talking about 3MHz here - passive probes won't maintain that accuracy up at several hundred MHz), it seems wildly inaccurate. Even if you used 10% resistors.

    So is this down to component parasitics? I wouldn't have thought a small amount of end-to-end capacitance (sub pF) and a low amount of lead inductance from a few mm of lead would affect it this much at 3MHz (am I wrong to think this?), but if the construction of the resistors looks anything like a coil that would certainly have an effect.

    A further factor is that the VNA might not be too accurate down at 3MHz - though it seems a real stretch to implicate the VNA (wouldn't it have had software calibration against a better instrument?).

    A way to cross-check the accuracy of the VNA, if you were interested and had the time, would be to drive the tap with a 3MHz sinewave, from a waveform generator, and do the attenuation measurement with an oscilloscope (with 50R terminations in place, obviously).

  • in reply to jc2048

     Fair question.  For a homebrew DIY project with imprecise components and questionable assembly techniques... having it turn out somewhat close to the goal is acceptable in my book.  If this were a professionally made item that I purchased, I would not be so generous with my assessment.

    I cannot speak to all of RF.  In Amateur Radio, we really use dBm - where 0dBm = 1 milliwatt (mw).  A -30dB (or 30dB of attenuation) gives me 1,000 times less power into my instrument.  -40dB gives me 10,000 times less.  -30dB should be around 77uV.  -40dB should be around 67uV.
    Because decibels seem like magic math to me sometimes, I try to use online calculators.  https://www.everythingrf.com/rf-calculators
    Folks are welcome to check my work. Laughing

    I am not chasing precision power measurements.  I'm really most concerned with keeping the magic smoke inside components while still being able to see what is going on in the system.  I also plan to use them with my TinySA to view harmonics.  There may come a time where I want to improve the performance of these taps, but it will probably just be for the assembly techniques.  If I need more precise measurements, I have a friend with much better equipment.  I would just like to start in the ball park.