Nice full duplex build for under $200 (FCC cert components etc)

[NR9V]

Additional notes on some relatively minor details:

• Re. Squelch settings (applies to any nodes and repeater comms in general), Yaesu HTs (FT-530, FT5D) have very good squelch that drops very quickly (maybe ~20mS max) and quietly, and my Icom-9700 is even faster with basically no squelch tail at all, but my Kenwood TH-D72 and D74 are not so quick and have a much more noticeable squelch tail (seemingly more like 50-100mS and fairly loud). So I enabled Tone Encode on the node TX HT and Tone Squelch on my other radios to minimize squelch tails after the node unkeys. Both Kenwoods now drop squelch immediately ie. no squelch tail at all. (The FT-530 doesn’t drop squelch any faster with Tone Squelch on but that’s fine as the tail was short and quiet to begin with.)

• Cross-band full-duplex nodes do work fine with half-duplex radios if they are dual-band with dual-receive. For example my TH-D74 won’t do full-duplex but I have the node’s 70cm Rx frequency on one memory channel and the 2m Tx frequency on the next memory channel, with dual-watch enabled and when I transmit the D74 will not receive at the same time but once I unkey there’s then no difference vs. a half-duplex node. Thus you can use any radios you’d like with a full-duplex node as long as they are true dual-band radios, which pretty much all modern HTs are. (I rarely the use the D74 any more since it’s only half-duplex, really is a great HT otherwise but I’ll probably end up selling it.)

• Some of the Chinese HTs do partially support cross-band full-duplex (FDX), with the limitation that you can only Tx on 70cm and Rx on 2m – so that their 70cm Rx doesn’t get de-sensed/overloaded by the 3rd harmonic of their 2m Tx. I have seen several models such as the TYT 8000 that support this and that could work well for use with this node, and the 8000’s are available new for as little as $65. Haven’t tried one myself as I already have some good Japanese radios that fully support FDX, but it is nice to see the Chinese supporting it in new radios to at least some extent since there are few if any current production Japanese models that do. These would not be ideal for node radios though since the RT85’s are only $25 and have no limitation in transmitting on 2m while another one receives on 70cm.

BTW a size comparison of a TH-D72A and RT85:

• The Repeater-Builder RIM-Lite V2 Radio Interface Module looks like a good alternative to the Masters Communications DRA-30. DRA-30’s fully assembled with no case currently are $65 plus another $10.50 for shipping, whereas RIM-Lite V2’s are currently $60 and $5 for shipping. I just ordered 2 of the RIM-Lite’s, they were in stock and shipped today, and I’ll be testing those to see if there’s any significant difference vs. the DRA-30. Both use the same CM119A chip and have similar circuitry but the RIM-Lite is significantly more compact as it’s a fully surface-mount PCB with Mini-B USB jack. The DRA-30 has trim pots on the audio outs which is definitely helpful, whereas the RIM-Lite has a extra low-pass filter. A ~$2 trim potentiometer can easily be added to the wiring if needed to provide wider output audio level adjustment/dynamic range.

• It appears the ASL USB audio drivers use a fixed 8KHz sample rate (decimating the USB stream 1:6 from 48KHz) at 16-bits ie. a raw bitrate of 128Kbps. Once compressed by the codec it’s probably more like 32 or 64 Kbps sent over IP. (Which is about 10-20x more bandwidth than what’s used by AMBE/IMBE codecs ie. DMR, D-STAR, C4FM, P25, etc. - thus why AllStar sounds far better than all the digital radio modes.) There are a number of codecs that can be enabled in modules.conf, by default ADPCM, A/u-law, G.726, and GSM. It would be interesting to play with those on a test node and confirm if ASL nodes are going with the best-sounding codecs by default, or how much improvement could be made and what the differences would be in internet bandwidth. The 8KHz sample rate is probably the limiting factor though as that limits the frequency response to ~3.5KHz and my guess would be that the ASL-defaults are probably already pretty optimal.

• To have the node be self-contained and look somewhat “professional” all components could be mounted on a piece of ~8"x12" pegboard (eg. “Dura Board”) rather than in an enclosure, so that everything is easily accessible and well ventilated but is also well-secured and cleanly organized. The MicroPC can then be turned on and off with its power switch, and a separate power switch for the HTs can be used, and the node is then easy to move around, set on a shelf or hang on a wall. (Just add a picture frame and you’ll have an instant conversation piece.) Pegboard is very inexpensive and the holes every 1" are perfect for nylon zip ties to keep the components and wiring in place, along with velcro or twist ties for anything you might want to be able to take on and off more easily.

• Power to the radios will probably be best provided by a small switch-mode power supply. Most devices now use these small switching AC wall adapters and they’re very efficient and compact. AC adapters have come a long way in recent decades, they used to have just a transformer, rectifier and small filter cap providing unregulated output with a lot of ripple, which can easily cause 60Hz hum and harmonics if used to directly power an HT. But these supplies now have better regulation and go through many certifications. I had originally tried to avoid switching supplies as they can cause RFI (birdies) particularly in the HF bands on harmonics of the switching frequency (eg. 100KHz typically but as high as 1 MHz), but if it can be confirmed that’s not an issue for a specific adapter that should be the way to go.

My measurements of the RT85’s show that at 7.5V they have the following Current Draw & Power Output:
Off: 3mA
Standby: 100mA
Low Power: 475mA ~1.5W
Mid Power: 906mA ~2.8W
Hi Power: 1020mA ~3.7W

A 7.5VDC 1.5A power supply should thus be fine for 1 HT in receive and 1 transmitting low power. Note that a node Tx radio should only be used on Low transmit power because of the high Tx duty cycle. The RT85’s Low power setting is already significantly higher than most HTs (1500mW vs. more like 500mW on most other HTs or as little as 100mW on HTs with an Extremely Low power setting). Thus 1.5 Amps is over twice the current actually needed, ensuring the power supply should also stay cool. These power supplies are widely available on ebay/amazon for as little as $9 for what appear to be reputable brands that should have good performance. I ordered a couple of these and will be testing them to insure they work as they should without causing RFI to the HTs or on HF. Because switching-supplies switch at a high frequency there should be little or no 60Hz harmonics on the output and large filter capacitors should not be needed. Switching supplies are used with many devices such as internet routers and other electronics and they do not seem to cause any RFI issues at my QTH so I suspect this will work much better than linear supplies which are larger, less efficient and have more ripple.

A case for the audio interface might also be nice, but as these are fairly small simple PCBs I suspect they will work fine just zip tied or screwed onto the node backboard (though ideally not directly on top of the PC or power supplies).

• Re. wiring, Since the RT85s are very cheap and to keep the node as simple and inexpensive as possible the simplest thing to do is just solder 2 (~24 ga.) power wires onto the battery contacts on the back of the radio. These are very easy to solder to. Just pre-tin the wires and the contacts; the wires can then be tacked on in less than a second, and can be just as quickly and easily removed later if need be. These should then go to a small terminal block or a couple small wire nuts. If you use a DB9 with a terminal block (eg. this listing on ebay) all node wiring from the HTs, power supply, and radio interface can be connected with a minimum of wires and no soldering required.

The node ideally should have either one AC power cord (or one 12VDC power cord) that connects to the 2 power supplies for the PC and HTs. This type of full-featured high-quality node will probably make more sense for use on 120V than 12V since it is significantly larger and less portable than the small SHARI-type nodes. It’s pretty portable if properly secured in an enclosure or on a peg board type of mounting surface, but even then the node will take up approximately 10" x 15" x 1.5" of space which is ~5 times the size of the average portable node. If it were desired to run it on 12V the only thing that would change is that DC-DC converters would be used instead of AC-DC switching adapters, which are also widely available for less than $10 ea. online.

The above minor details will result in a nicely-integrated node, with a single power cord, separate power switches for the PC and HT power supply, that is easy to access, power on/off, change frequency settings, etc.

Updated drawing showing all components. Note that the power adapters can be placed behind the MicroPC and audio interface further reducing the overall size of the node. And the RIM-Lite is quite a bit smaller than the DRA-30 thus the overall dimensions can be significantly smaller than shown below:

With the PC’s Real-Time-Clock IC supporting powering on and off on a schedule, it would also be nice to have the PC be able to control power to the HTs. That can be done very easily with a USB cable and a small 5VDC relay. The 5V line on any of the 3040’s USB ports will only be on when the 3040 is on, thus with a $2 USB cable and a $2 relay the power to the HT power supply can be automatically controlled if desired.

Update 12/2/22: Received the RIM-Lite V2 and 7.5VDC 2A switching power supply today. After testing the power supply I found that for the best audio quality ferrite core filters should be placed on both of the switching power supplies, with the cord wrapped around the ferrite a few times. Without that there was significant buzz coming through from the 7.5V switching adapter. This is a known issue with nodes in general, relating to Tx RF being picked up by the power cord and then causing 60Hz harmonics when the diodes in the power supply are near their zero-crossing points - where they can be modulated by induced RF. This results in a broadbanded buzz on 60Hz harmonics that gets louder and quieter as you move antennas or cables around or move around with your HT that you’re talking into the node with. Sometimes it won’t be noticeable at all but then move 1 foot away and you start hearing buzzing. Ferrite cores knock this down by 99+%. With a linear power supply it’s not much of an issue because there’s a lot more capacitance between the diodes and the power supply output, since a linear supply’s filter time constant needs to at least 10x the 120Hz ripple period, whereas switching supplies are doing things up closer to 100KHz. Adding a couple good bypass capacitors to a switching-supply might be just as effective if added in or by the case, I may do some further experimentation there but a ferrite seems like a simple-enough solution. Thus as a general recommendation for any node, a few turns of power cables from switching-supplies should be wrapped around a ferrite core as close to the power supply case as possible. I found that adding a ferrite on the Tx audio cable helps a little also. (I was listening to the node on an SDRplay RSPdx with full audio bandwidth, whereas a normal radio bandpass filters everything to ~300 - 3K Hz. Thus an SDR can help with even further reducing quiet noise that wouldn’t be audible on a normal radio.) In total you’ll want 4 ferrite cores for a node with 2 switching supplies (one for the MicroPC and one for the HTs). The same would probably apply to a node that ran off 12V using switching DC-DC converters.

The RIM-Lite V2 looks nice, they are very compact, sealed in clear heat shrink and they come with a bonus DB9-M plug and shielded DB9 housing which is not mentioned on their site (which saves about another $5) as well as a USB A to MiniB cable. Once I get some more parts in the mail I’ll be building a new node with the idea of it being fully self-contained and “professionally” wired with all components mounted on on a ~8" x 12" piece of “Dura Board” or similar pegboard, plexiglass, etc. This should meet my goal of being the least-expensive possible way to build a self-contained full-duplex node using only high-quality off-the-shelf FCC-certified components. I plan to sell one or maybe a few of these on various sites, not really to make money or with any plan of making a lot of them, just to create and document the whole process and to know that the node is sufficiently well-built to be able to be sold as a turnkey product, for anyone who might prefer to pay a little more but have the node already fully assembled, configured and tested. And, so that anyone who builds one themselves will have all needed info and know that all the important details have been thought through and tested. Making a significant number of these is not something I’m interested in (I was a pro-audio OEM in the 90’s and then decided that wasn’t for me and went into software development instead) but because all the software is open-source and the node uses only commonly available off-the-shelf parts, anyone could make these and sell them on ebay/qrz/eham/etc., while adding their own unique touches, other features, options, or optimizations.