A 2 meter and 222 MHz low-pass filter for the SocoTran ST-7900D

In an earlier post (link) I described a "hilariously bad" radio - the Socotran ST-7900D (a.k.a. the QYT "KT-7900D").  This radio, right out of the box, could not legally be used by U.S. amateurs on three of its four "bands".


One of these bands - that which covered 350-390 MHz - was understandably off-limits as there is no U.S. amateur band in this frequency range but the other two, the 2 Meter and the 1-1/4 meter (a.k.a. 222 MHz band), also covered by this radio, had poorly-filtered harmonic content:  It was even possible to key up a fairly-distant UHF repeater when one transmitted on a 2 meter frequency at precisely one-third of its input frequency!

The article noted that as it was shipped, the only band that might be legally used was the 70cm band as the strongest harmonics of the other bands weren't properly suppressed - not at all, actually...  It was observed that this radio seemed to have a single low-pass filter in its transmit path that was designed to start cutting off energy in the 550-575 MHz range - but this sort of filter would have no effect at all on the 2nd and 3rd harmonics on 2 meters and the 2nd harmonic on 222 MHz - which was the problem.

Besides just being cheap, one reason why someone might have been attracted to this radio is its ability to operate in the 222 MHz band - and paying $75 or so for a  25 watt radio that could only do 222 MHz might be a reasonable thing to do - so what about making some sort of low-pass filter that would kill two birds with one stone:  A single filter that would allow legal operation on both 2 meters and 222 MHz without having to switch filters?

Designing the filter:

What I needed was a filter that would pass the 222-225 MHz band with little attenuation, but still knock out the 2 meter band's 2nd harmonics in the 288-296 MHz range.  This sort of filter would permit operation on both the 2 meter and 222 MHz band without needing to switch or change anything - but if would, of course, preclude operation on 70cm unless it were removed.

Curious to see if this could be done I fired up the Elsie program, a software tool that is free for "student", non-commercial use (aren't we all students in this world?).  Designing a filter that would both adequately attenuate 2 meter's 2nd harmonics and pass 222-225 MHz would require at least a slight amount of complexity, so I went to work.

Knowing that a simple Butterworth or Chebychev filter would never meet the need for "sharpness", I immediately picked a Cauer (a.k.a. "Elliptical") low-pass filter design and plugged in the numbers, coming up with this:

Figure 1:
 Low-pass filter, inductor-input topology shown.  This "same" filter could have been constructed using capacitors on the input/output, but this version uses fewer capacitors and more inductors - which are both extremely cheap to make and are very easily adjusted - unlike fixed capacitors.
Click on the image for a larger version.

When I plotted the predicted response of this filter, Elsie showed me this:

Figure 2:
The predicted attenuation of the filter.  Part of the design goal was to place the 2 meters' second harmonics in the first "notch" in the filter.  While only 40dB was theoretically needed, a filter with 50dB attenuation was implemented knowing full-well that the real-world implementation of the filter may not do quite as well.
Click on the image for a larger version.

While I needed "only" 40dB to make this radio "clean enough" it is often the case that real-world filters aren't quite as good as their simulated counterparts, so I inputted 50dB into the program as the minimum attenuation.  If you look closely, you'll see that at the top of the flat part - just before it "rolls off" - the attenuation at 232 MHz, comfortably above the 222 MHz band, is just under 1dB while there is a deep "notch" at around 290 MHz - which is right where the 2nd harmonics of the 2 meter band will lie.  If this filter was, in fact, "build-able", it would neatly solve the problem of the harmonics from the 2 meter and 222 MHz bands.

When it came to specific filter types I had two choices for the "same" filter:  A capacitor-input low-pass filter and an inductor-input low-pass filter.  Both are theoretically equal in performance, but because the capacitor input version had 7 capacitors and the inductor input version had just 3 capacitors, I chose the latter as seen in Figure 1:  Inductors - which are just a few turns of wire - cost practically nothing to make and are easily adjusted!

Being familiar with VHF/UHF construction techniques I knew, when I saw the inductor values, that they would be very small, but easy to make.  For example, when 20 AWG wire is wound on a 3/16" (4.76mm) diameter drill bit with very short leads, you can expect to start with something along the lines of:

• 20-30nH:  2 turns
• 30-40nH:  3 turns
• 40-50nH:  4 turns

The precise value would then be obtained by squeezing/stretching the turns - or, possibly cutting a fraction of a turn off as necessary.  If you have ever looked at some commercially-made VHF/UHF gear you might have noticed that some of the coils look as though they were smashed or stretched, but this is a time-honored way of making fine adjustments to these circuits after assembly.

If one is constructing this using only small, surface-mount components the self inductance and stray capacitance of these tiny components on a well-designed board can almost be ignored at these frequencies - but I was going to use plain, old through-hole leaded disk ceramic capacitors, which would require a bit more consideration.

A good example of this is the first series-resonant section of the above filter - in the section marked "531.581M".  As you can guess, this is a series-tuned circuit that must be resonated at around 532 MHz using components of the approximate values shown.  Practically speaking, in this application one can "fudge" a bit on the values, so rather than trying to find a precision capacitor of about 17.5pF, I simply pulled a 18pF unit out of my capacitor bin with the idea that I would select the inductance to make it resonate somewhere in the area of 532 MHz.

But, there's a twist:  Noticing that resonating inductance is ideally 5.1nH, one may realize that even a rather short length of wire has a similar amount of inductance - and that is exactly what was done:   The capacitor's own lead - about 4 millimeters of it - plus the series inductance of the capacitor itself was enough to create a resonant circuit at the desired frequency.

What it takes to build this filter:

As you may have gathered, it is simply not possible to build this filter without some sort of test equipment at hand - and I used a spectrum analyzer with a tracking generator as I was building it.  In short, here's what I had to do:

• Fuss with the series L/C circuits to get the stated series resonant frequencies as indicated by deep notches on the sweep.
• Stretch/compress/adjust the other inductors as necessary to minimize the loss below the cut-off frequency
• Repeat the above two steps until it makes no difference - usually taking about a half-dozen iterations.

During construction I didn't bother breaking out any capacitance or inductance measuring gear - but very small inductors (those lower than a few hundred nanoHenries) can be very difficult to measure, anyway.  Using only "known" values of capacitance, by adjusting the inductors in the manner mentioned above, I have found via experience that such filters often "take care of themselves" when one takes a bit of care during assembly and adjustment - particularly when setting the resonant frequencies of the "notch" elements.

Amazingly, the filter went together without too much trouble with the test equipment indicating less than 1dB of insertion loss at either 2 meters or 222 MHz.  The hastily-kludged prototype looks like this:

Figure 3:
Constructed prototype.  This was constructed on a scrap piece of copper-clad PC board material using small PC board islands for some component support.  This version was built for testing the concept:  A "real" implementation of this filter would be crammed into a small metal box with the input/output inductors and ground plane soldered directly to the in/out RF connectors.
A circuit board could be designed, but to be effective it would need to be built on (at least) a double-sided board with a large-as-possible ground plane on the top connected to the bottom plane using lots of vias.
Click on the image for a larger version.

Figure 3 shows the prototype, constructed on a small piece of copper-clad circuit board material.  The input/output connections were made via some N connectors that were pre-attached to UT-141 rigid PTFE coaxial cable, being were used because they were on-hand.  Because we would need to handle "only" 25 watts, 100 volt NP0/C0G disk ceramic capacitors are more than adequate.

As can be seen in Figure 3 the junctions where the series L/C portions are attached are held off the ground plane with small pieces of circuit board (the capacitive effects of these are negligible at these frequencies) while the attaching hardline's center conductors supported the in/out inductors.  Also apparent is what looks like haphazard stretching/compressing of the various inductors to achieve the resonant frequencies for the three elements - which I marked on the board.

Once I connected it to a transmitter I did a bit of final tweaking, "adjusting" the series coils for minimum loss (as indicated on an RF power meter) on both 2 meters and 222 MHz but leaving the "notch" adjustments alone:  Only slight adjustments were needed.

Can I make such a filter?

Yes, you can - if you are familiar with VHF/UHF circuit techniques and have access to a spectrum analyzer with a tracking generator or some sort of equivalent.

If you don't have access to this sort of gear - and you don't know anyone else who does - then it is (unfortunately) not possible to properly "tweak" this filter for both harmonic attenuation and also to make its insertion loss and added VSWR low enough to both allow transmit power to pass through it without damaging the radio.

(And no, I won't build one for you...  Remember:  It's a $75 radio!)

Does it work?

Amazingly enough, it works pretty much as predicted!

After final tweaking the measured insertion loss was under half a dB:  With 25 watts in, around 20 watts exited the filter on both 2 meters and 222 MHz - hardly enough loss to worry about on receive or transmit.  After about a minute of solid key-down at 25 watts input the hottest of the filter's components were barely warm - lower than body temperature in a "not hot/not cold" room.

The real test was to put the filter inline and check it again on the spectrum analyzer - and the plots below show the results for 2 meters:

Figure 4:
The harmonics on 2 meters, through the filter.  The analyzer has been adjusted to read actual power, so the 2 meter fundamental is at about +43dBm.  The second marker (#2) shows the location and amplitude of where the 2nd harmonic would be - and this trace shows that it is at least 79dB down well within the FCC part 97 rules and  probably "cleaner" than your average "good" radio!
Click on the image for a larger version.

And here is the result from the 222 MHz band:

Figure 5:
The operation of the radio on the 222 MHz band.  The "2" marker shows the second harmonic - and other spurious signals may be seen at a similar level on the plot.  Like the plot in Figure 4, this is scaled to show the actual transmitter power (+43dBm) and thus the harmonics and spurious signals are around 80dB below the carrier - well within FCC part 97 rules!
Click on the image for a larger version.

As can be seen, the harmonics and other spurious signals are all but undetectable!

"Sweeping" the filter:

Curious as to how the actual attenuation curve of the filter looks?  Figure 6, below, shows its response over the frequency range of 100 through 400 MHz.

Figure 6:
A "sweep" of the prototype filter from 100 through 400 MHz.  On this plot marker #1 has been configured so that the difference in amplitude being measured and this indicates that the depth of attenuation is a bit over 53dB - but the settings of the analyzer used to make this plot likely reduce the apparent depth.  Outside that null the depth of attenuation is at least 40dB - more than enough to suppress the harmonics to meet FCC part 97 regulations.  If I'd taken a bit more care in building the filter (e.g. more thoughtful layout, some shielding between sections) more attenuation might have been obtained - but as it was, it worked better than was necessary.  Because of the configuration of the test jig, the absolute level of the passband portion of the response is arbitrary.
Click on the image for a larger version.

As can be seen, the "depth" of the low-pass filter is at least 40dB, but where the first "null" is located (which happens to be around the frequencies of 2 meter 2nd harmonics) the depth is much greater.

As expected, the simulated filter's ">=50dB" attenuation above the designed cut-off frequency wasn't quite met over its intended range (likely due to the physical layout of the filter - not to mention the difference between simulated and real-world components) but it is more than capable of rendering this radio "legal" when it is operating on the 2 meter and 222 MHz bands.

Using the filter:

Some time in the near future it is likely that this page will show a version of this filter that is built into a small box with UHF connectors which will allow the radio to be used legally on 2 meters and 222 MHz by U.S. amateurs - but having this filter inline precludes its use on 70cm:  To do that, the filter would have to be manually removed.

If that is the case, one would consider this to be a "2 band" radio - and for around $75, it would to OK - aside from the possible tendency for its receiver to overload from nearby signals.

What about automatically switching the filter?

In theory, it should be possible to build into the filter a "bypass" circuit using some UHF-rated relays.

In poking about inside the radio I quickly found a circuit that was powered on only when the UHF (400 MHz) band was selected - and this could be used to "key" a relay to bypass such a filter.  In reality, one would want to design such switching so that when the relay was un-powered, the filter would be bypassed, but when the radio was powered up and not on UHF, use the absence of the aformentioned signal to pull in the relays in insert the filtering.

If we decide to do this, I'll post it here - but at some point, trying to make this radio do what it should have been capable of doing by design becomes an exercise of "turd polishing" when the time and money spent exceeds the gain.

Then again, the effort is sometimes worth the journey if the goal is to build and learn something!

(No, this filter won't help with the problem of this receiver being easily-overloaded by other signals in the same "band".)

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This page stolen from ka7oei.blogspot.com

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