A hiliariously bad multi-band radio: The SocoTran ST-7900D

A couple of days ago an "interesting" mobile transceiver crossed my path - the SocoTran ST-7900D.   
(This radio is also sold as the "QYT KT-7900D" and possibly a few other names.)

I say "interesting" - but what I really mean is "scary" - for anyone who ends up using this radio unawares!

Figure 1: Tiny "quad band" radio - but not really a quad band radio as only three of the ranges include valid amateur bands! Can you spot which of the frequencies above is not in an amateur band? (Answer at the bottom of the page.) Click on the image for a larger version.

This radio is diminutive - a little tiny thing that, when you see the "<=25 watt" power rating makes you wonder how much transmitting it would take to overheat it.  To be sure, it does have a thermostatically-controlled cooling fan, but it is (literally) only slightly bigger than a postage stamp.  Also a bit alarming is that just sitting there, receiving, the radio gets quite warm - probably about 98 degrees F (37C) in a 70 F (21C) room.

This radio is listed as being a "quad band" radio, with its frequency coverage being listed as follows:

• 136-174 MHz:  This includes 2 meters, plus lots of other things.
• 220-270 MHz:  This includes the 1-1/4 meter band (a.k.a. the "222 MHz" band)
• 350-390 MHz:  Used for military comms - There are no U.S. Amateur bands in this frequency range!
• 400-480 MHz:  This includes the 70cm amateur band.

 In retrospect, only three if its "bands" are available to a law-abiding U.S. citizen!

Using the radio:

As is typical for inexpensive Chinese radios the manual isn't very good - but it's "less bad" than many I've seen.

The menu system does not appear to be well though-out.  For example:

• Similar items are not necessarily grouped together.  For example, if you want to set transmit offset, subaudible tone, offset direction, power, etc. you must awkwardly jump around between 10s of menu items to do this.  In this radio, the front-panel knob (the one on the right) didn't seem to reliably change the menu item number up/down so one had to use the up-down button on the microphone or look up the menu item in the manual and enter its number on the microphone.
• The menu selection may not start at the current setting.  For example, if you were to set the subaudible tone to 100.0 Hz and then later change it to, say, 123.0 Hz, you would go into the menu and see it at 100.0 Hz.  However, when you pressed the menu button again to allow it to be changed, it will start at 77.0 Hz - the "first" tone in the list, rather than where it had previously been set.  For menu items with only a few selections this isn't too bad, but for the subaudible tone or for things like DCS, this can be a pain!
• Menu settings to not take effect until you enter them.  For example, if you go to the squelch setting in the menu, you hit "menu" again and can change the setting, but it is not until you press "menu" again to save it that the squelch setting actually changes.
• By strictly following the manual's instructions, it does not seem possible to save a current frequency/setting into a memory.  Perhaps there is some permutation of buttons that allows this, but the manual is not helpful on this point.

In short:  If you insist on using this radio (if you read on, you'll see why you might not want to!)you are best-off using a program like Chirp to set it and its memories up.

While many inexpensive Chinese radios seem to have low transmit audio, this radio has quite the opposite problem:  Even holding the microphone about 2 feet (50cm) away from one's mouth and talking in a normal speaking voice caused the modulation to smash into the clipper pretty hard.

To be sure, having a bit too much audio is usually better than having too little, but the mic gain is so "hot" that your voice will sound a bit harsh and compressed - and everyone listening to you on the air will be able to hear everything that is going on in the room that you are in - and likely the rooms next to you, and the bodily noises of any creature in your house.  In any but the quietest vehicle, road noise will be competing strongly with your voice.

Unfortunately, there is no menu item to adjust microphone gain, but it should be possible to make a change in the microphone itself to reset the gain to something more sane.

Figure 2: The top side of the board. The brass cover hides a large, surface-mount transistor that is the power amplifier.  The filtering may be seen in the lower-right corner of the board. Absent seem to be individual low-pass filter sections for 2 meters and the 222 MHz bands - or any means of switching these filters in/out. The cover plate for this radio is appears to be genuine unreinforced ABS plastic:  No annoying metal RF shielding here! Click on the image for a larger version.

Like many of these Chinese radios it will also receive FM broadcast.  For this radio, connecting it to a typical amateur antenna caused this receiver to be overloaded very badly, making it impossible to hear weaker "local" stations in the resulting muck:  It took about 30dB of RF attenuation to prevent the "FM broadcast" receiver from being clobbered in an area with relatively strong, local transmitters.

This radio appears to be based on the same (sort of)"everything-in-one" chips that the Baofengs are based on - quirks and all.  Unfortunately, the nature of these chips preclude their use in areas of very strong adjacent-frequency signals, for packet operation and for any sort of DF (Direction Finding) system that uses switched antennas.  Interestingly, the "S" meter on the front panel  seems to indicate a 7-bit binary number - possibly from one of the chip's registers - that is roughly proportional to the signal strength in dB, making it more useful than the typical radios' S-meter - but its useful range (a bit more than 60dB) makes it unusable for signals that are even moderately weak (e.g. weak enough to have any noise on them) or quite strong (e.g. within several blocks).

(I am surprised that this thing doesn't also have a flashlight!)

The real problem with this radio:

Having buried the lead, the real problem with this radio is when you use it on-air:

If you transmit with this radio on 2 meters or 222 MHz, you are breaking the law!

This is (literally) the worst commercially-made radio I have ever seen in terms of harmonic/spurious output!

With any inexpensive Chinese radio, my first inclination is to throw it on the workbench and see how it really performs - which includes checking things like its sensitivity, power output, microphone gain, and spectral purity.


The sensitivity of this radio on the amateur bands was very good:  At 0.15 microvolts, the received signal was at least 12dB SINAD - but this comes at a cost:  The receiver is easily overloaded by strong signals on the same "band".

The power output was also pretty close to what it should be.  The specifications oddly states "<=25 watts" - and this seems to be true:  At 15 volts, the output power was, in fact, 25 watts, dropping to 10-15 watts at 10 volts.  Aside from the obvious problem (mentioned below) that makes the legal use of this radio rather dubious, this wide voltage range (possibly) makes it a useful candidate for battery-powered portable operation.

But... don't use it!

The real problem was the actual transmitter specifications:  The literature states that spurious and harmonic energy is ">60dB" down - but it is not!

For 70cm, this radio seems to be "Okay" - but for 2 meters, the results were terrifying:  The 2^nd and 3^rd harmonics measured both as being -23dBc.

Figure 3: What appears to be the main CPU clock crystal:  "If it doesn't fit, just cram it in there!" - but hey, what do you expect for around $70? Click on the image for a larger version.

Putting this into other numbers:  For an output power of 25 watts at 2 meters, this means that the harmonics are approximately 125 milliwatts each - roughly as much power as many handie-talkies produce when set to low power!

As an experiment we did something that we probably should not have done:  Connected this radio to an antenna that is designed for both 2 meters and 70cm and transmitted - on a frequency that was 1/3^rd of a local UHF amateur repeater located about 20 miles away on a mountaintop.  The result was that the 3^rd harmonic was full quieting into that repeater!

If one peruses the FCC rules, one will spot FCC §97.307(e).  According to that rule, on a 2 meter transmitter if this power class we are allowed no more than 25 microwatts of spurious emission:  This radio exceeds that by a factor of approximately 5000 (about 47dB).

Remember:  This was the 3^rd harmonic of 2 meters which, if you are operating within the 2 meter amateur band, will always land somewhere in the 70cm band - but what about the 2^nd harmonic - which is just as strong as the 3^rd?  This would land somewhere in the 288-296 MHz range which is used for military communications - including aeronautical mobile.  What this means is that it is possible that your 2 meter transmissions made with this radio could be heard from, perhaps up to 100 miles away by an aircraft in line-of-sight!

What about 222 MHz?

The situation there isn't quite as bleak as the second harmonic was between 42 and 50dB down - the precise level varying wildly with power supply voltage.  Fortunately, any harmonic due to operation in the 222 MHz band (which covers 222-225 MHz) will land in the 70cm band, but its level will also be a bit high:  With 25 watts out on, say, 224.0 MHz the signal at twice this (448.0 MHz) will be around 1.5 milliwatts.

This may not sound like much, but this signal would be easily audible via line-of-sight at a distance of 10-20 miles - and it still can exeed the FCC rules by a factor of 63 (about 18 dB).

"But there's an FCC logo on the radio!"

If you look at the radio, you will find on it an FCC logo:  If you think that this means that the radio is "OK to use", but you would be wrong.

No matter what the radio's specs say, what the reviews say, or what others say, if you are using a radio that, for some reason, does not meet the legal requirements - YOU are responsible, even if you didn't know that it doesn't pass muster!

Why is this?  Because you agreed to this when you got your license.  In other words, the onus is ultimately on you to make sure your gear is working properly - not the manufacturer - and if you happen to buy something that doesn't meet specs and get into trouble, it's ultimately your fault - and now that you have read this, if you use one of these radios on 2 meters or 222 MHz, you have no excuse at all.

Practically speaking, the FCC certification does not mean that amateur gear is actually checked to see if its transmitter has spurious outputs or not:  If a piece of gear is checked at all it's usually just to see if it meets FCC Part 15 rules which cover non-amateur spurious radiation caused by the receiver, its computer, or other circuitry - but not the transmitter - which is covered by Part 97, and you are ultimately responsible for making sure that the gear that you are using is in compliance.

Having said that, anyone could make something and simply slap an FCC logo on it!


"I got one of these radios - can I modify it to make it legal"

The quick answer is NO, not if you use it anywhere other than the 70cm band.

The problem with this radio is it seems to have only one low-pass filter after its (single) power amplifier.  On other radios (e.g. Yaesu, Kenwood, Icom) there would be a separate filter for each amateur band after the power amplifier to remove the harmonics for that band - but this radio seems to have just one - and it doesn't seem to have too much of an effect below roughly 550 MHz.

What this means is that this low-pass filter does absolutely nothing for any harmonics or spurious signals below roughly 500 MHz - and this is why the 2^nd and 3^rd harmonics of the 2 meter band and the 2^nd harmonic of the 222 MHz band is way out of compliance!

Why did they do this?

It's cheaper, of course!

In "older" radios it was common to have a separate power amplifier for each band - each with its own filter - but with today's inexpensive power RF MOSFETs a single amplifier can work over a very wide range of frequencies - but this means that you must switch the appropriate filter inline for the band being used.

This switching is typically done with RF PIN diodes and/or relays - but either one of these options (particularly a PIN diode) gets to be pretty expensive (a couple of dollars) when you get into the 10s of watts at UHF frequencies.  Because this radio was "built to a price" it is almost inevitable that something was left out - and among those things that was omitted was proper low-pass filtering of the transmitter!




If you have one of these radios and wish to operate it on 2 meters or 222 MHz, DON'T do so unless:

• For 2 meters, you use an outboard low-pass filter that will attenuate the 2 meter 2^nd and 3^rd harmonics by at least 50dB.

• For 222 MHz, you use an outboard low-pass filter that will attenuate the 222 MHz 2^nd harmonic by least 30dB

What this means is that you would not be able to use this radio for transmitting for more than one band at a time without having to swap out low-pass filters:  You simply cannot use it "as is" on these 2 bands.


Answer to the question in Figure 1:  Two of the frequencies shown - 245.625 and 350.025 MHz are not amateur frequencies!


To be added:  Spectrum analyzer plots showing the levels of the transmitter's harmonics, picture of the tiny cooling fan.

This page stolen from ka7oei.blogspot.com


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