SDR: The Executive Summary

One of the RTL2832 dongles I ordered from Ebay finally arrived. The other I have chalked up as a no-show. I’ll try to get my money back, but even if it turns out to be a loss the total cost for both was under $20. That’s very cheap for a software defined radio.

The executive summary: using a general-purpose computer instead of a traditional radio interface generally sucks from a user-interface point of view, but it has its one very intriguing and useful aspect. That latter is how an SDR captures a whole swath of spectrum and typically has a waterfall display to visually plot what it receives.

It’s a most useful feature when searching for non-broadcast signals; one doesn’t have to rely on chance for the particular frequency one is listening to to become active. Instead, you can see activity as it appears on any nearby frequency in the range being displayed, and immediately point and click to tune to it.

I’d suspect the sucky parts of the user interface aren’t nearly so bad with some other SDR programs. Right now I’m using gqrx, because I have only Macs in my house and gqrx is basically the only SDR software that runs on a Mac. I’m going to be getting a dedicated Windows machine soon (there’s enough software I want to run that only runs on Windows that it’s become worthwhile to do so), and once I have that I’ll be giving some other software a try.

Hardware wise, an RTL2832 isn’t exactly the greatest receiver. It’s sensitivity isn’t that great and it’s full of birdies (I suspect a better antenna, further from my computer, would help with some of that). Even my 25-year-old Bearcat scanner runs circles around it. But what can one expect from a consumer-grade device that didn’t even cost $10? It is what I purchased it to be: a low-cost way to experiment with SDR.

Re-Thinking Software-Defined Radio

After blowing two evenings trying to get SDR working, I’m beginning to think I was correct in basically writing the technology off as not worth the trouble some years ago. I fight with computers in my day job. I don’t want to do it as a hobby.

First, I use Macs. If you use a Mac, you’re really left out. The vast majority of SDR software supports Windows and Windows only. The few exceptions tend to run on Linux and not Macs.

Sure, I could boot Linux on my Mac, but it’s Linux. That means it was written by hard-core geeks for hard-core geeks, so documentation is incomplete (if available at all). To prove my point, I tried to create a bootable Linux flash drive last night, following all the instructions meticulously. It didn’t work; it failed to even appear as a boot device when the system came up. That means there’s probably some missing step in the by-geeks, for-geeks instructions that was left out because it’s transparently obvious… obvious to a hard-core Linux geek that is. Figuring out the answer to that puzzle could easily involve me blowing my free time on it for the next several weeks. No thanks. I want to geek around with radio, not Linux systems administrivia.

The few exceptions, i.e. SDR programs that run on the Mac natively, tend to involve Mac Ports. Which is (link) currently broken. Sigh.

That leaves running Windows, which probably means buying and setting up a whole new computer. If it comes to that, there goes any cost advantage of SDR; even a sub-$10 dongle like the one working its way to me from Singapore will have a total cost about twice that of the Alinco receiver I just purchased. It actually might come to ruunning Windows… eventually. Right now, there’s higher priorities for spending that sort of cash.

Alinco DJ-X11 First Impressions

Why buy yet another radio? Two reasons:

  1. I’m currently trying to do noise mitigation in the HF bands, and that means walking around the neighborhood with a radio. While I can use one of my existing sets for this purpose, they tend to be very awkward, as they are all desktop models.
  2. I’m also interested in helping friends whose homes might be bugged (seriously; they’re known as activists, and the government has a nasty record of surveillance on such individuals) do some searching for bugs. It’s yet another something for which a small, battery-powered radio is a useful tool.

So I wanted a small, battery-powered wideband receiver that could tune as many frequencies as possible in as many modulation modes as possible. The latter is an important point; most of the wideband receivers out there (such as the Icom R6) can’t receive SSB or CW at all, which is a major limitation on the shortwave bands.

First impressions:

  1. It doesn’t feel super solid and professional, like I’d imagine the Icom R20 (discontinued) or the AOR 8200 (no raw I/Q output) to feel. It doesn’t feel super-fragile either; its plastic case does feel quite rugged and right. But there’s very little metal in the thing; it’s surprisingly lightweight.
  2. It’s very complex, and the manual isn’t the best in explaining the complexity. It can take some searching and experimentation to figure out how to do something.
  3. It won’t put my desktop HF receivers out of work; it’s significantly less sensitive than them, not so easy to use, and tuning SSB signals is somewhat painful.
  4. Notwithstanding the above, it does actually work acceptably on MF (aka AM broadcast) HF, VHF, and UHF signals. Given its small size and wide coverage range, its performance is quite remarkable.
  5. Forget about using the rubber duck antenna it was shipped with for HF; get an SMA-mount whip (thankfully I already have one).
  6. It comes with a rechargable lithium-ion battery and a drop-in charger. The latter was a pleasant surprise; I much prefer drop-in chargers to plug-in ones, and I was expecting the latter, given the price.

This is the USA, so mine is the crippled DX-X11T model with the stupid government-mandated gaps in the 800 MHz band (fuck you very much, Congress). I seriously entertained the idea of taking a trip to Canada and smuggling a non-crippled one across the border, but:

  1. That involves blowing most of a weekend.
  2. There’s always the (slight) risk of my purchase getting confiscated on the way back.
  3. I already have a desktop receiver capable of tuning such frequencies (completely legally; I bought it before the law became effective so it’s grandfathered).
  4. I also have an RTL2832 dongle on order which, together with a free software program, will be a software-defined radio that goes from about 24 to 1700 MHz with no such gaps.
  5. If I do find a bug, I’d rather do so with a 100% legal receiving device, to minimize the very real risk of governmental retribution should I be open to it by possessing contraband.

 

Icom R75 Tuner Repair

It’s apparently a common failure mode for the rotary encoder which processes the tuning knob input to fail in the Icom R75 (and many other Icom receivers and transceivers). Mind did recently.

A bit of background: most radios these days are completely computer-controlled. There is no variable capacitor connected to the tuner; the radio synthesizes its intermediate frequency under computer control, and if there is a tuning knob, it merely is a device for sending input to the computer. It may appear to be more traditional than a set of up/down buttons or a keypad, but it’s merely a show. A useful “show,” as tuning with a knob is often more convenient than using buttons.

The symptom of a dead rotary encoder is pretty much what logic would tell you it is: the radio still operates, and you can change frequencies by entering them with the keypad, but the knob simply does nothing.

There’s several fixes detailed in the initial link above. Initially, I was debating what to do and procrastinating the job, because none of them are cheap (replacing the chip in the controller requires de-soldering and re-soldering a surface-mount device, since I don’t have any equipment to do that, I’d have to buy it, since I couldn’t locate anyone who had it to lend, and there goes the clear price advantage of that option).

Then my (un)employment situation resolved itself, so I did the easy out and simply ordered a replacement from Icom, which costs just under $90 (not the $130 it is rumored). From the repair manual (which you should definitely download, as it shows how to take the set apart so you can get at the encoder), the part number is 6910012480. That part number may differ for products other than the R75, so please obtain a service manual and check it to be sure in that case.

OK, I suppose the real “easy out” would have been to send the thing in for repair. That would have doubtless cost at least twice as much as fixing it myself did, I was reasonably sure what the culprit was (the symptoms made that pretty obvious), I’ve taken apart reassembled electronic devices many times, I know the precautions to take to avoid damaging things, and this particular repair didn’t even require me to warm up a soldering iron.

So it was pretty much a no-brainer to do it myself. That said, the way the Icom parts person was taken aback at a mere customer troubleshooting his radio then having the audacity to order a part with which to repair it did make me have a little bit of doubt. Which was unfounded: when put back together and powered on, I was rewarded with a fully-working receiver.

Different than I Imagined, but Nice

P9291828wThe main difference between how I imagined the North Fork Skokomish Valley (a.k.a. the Staircase area of Olympic National Park) and how it actually was is that I had imagined it as being much more broad. In fact, there really wasn’t much of a flat valley bottom at all once one got above the campground.

It was, however, full of mile after mile of intact lowland forest, pretty much as I had imagined. It wasn’t all huge old trees (natural calamities do “reset the clock” in forests), but there still were an awful lot of them.

I also did manage to successfully make a couple of PSK31 contacts operating portable, so chalk up another goal for the summer season as accomplished.

The campground was surprisingly well-patronized, given that it was a weekday in late September and the water had been shut off. It’s probably just as well I didn’t even try to get a spot there last summer.

Where is the Null of my HF Loop Antenna?

I’ve been wondering that, since I’ve been using it to null out noise sources, yet I had long forgotten what the pattern of such antennas is.

I couldn’t just use the antenna itself plus noise to find out because I live in town and there’s potential noise sources all around. I couldn’t use the antenna plus signals because the ionosphere compromises the source-directionality of signals that pass through it, and it’s an HF-only antenna. (I suppose I could have spent most of the day delving into the physics of it, and coming up with the answer, but that would have taken most of the day and I just wanted a quick answer.)

Checking on-line wasn’t very helpful. I found articles claiming that both the null and the lobes were “in the plane of the loop”! Part of the issue, I think, is that there are different geometries of loop antennas. What one would tend to think of as the “plane” of a large, flat, air-core loop is perpendicular to what one would tend to think for a compact, multi-turn ferrite loop stick antenna.

A simple experiment with one of my old tube radios (which have a large, flat air-core loop antenna for the medium-wave broadcast band) and KVI’s signal on 570 KHz povided the answer: for “flat” loop antennas with a large diameter and low number of turns, the null is along the plane the loop exists in. It was pretty definitive: when I got the radio aligned so I had difficulty hearing KVI, its back (on which the loop is mounted) was aligned directly on a line running from KVI’s Vashon Island transmitter to me.

So there you have it.

Note that the while the peaks of a loop antenna are very broad, the nulls are by contrast very sharp. I’ve noticed this when nulling out interference; a slight bump on the HF loop antenna (changing its position by mere inches) often makes a significant difference.

Receiver Review: Bearcat 210 XLT

I bought one of these used for $10 at the Puyallup Hamfest last March, and it’s been a great addition to my lineup of receivers. I use it to monitor a couple local ham repeaters from the main living room so I won’t miss a call if I’m not in the radio room. For that purpose, it’s a steal of a deal at $10.

It is over 20 years old. Which means:

  • Don’t assume one you see for sale works. I looked at two at the Hamfest, plugging in and trying both. Good idea, as the one I passed on had some glitches.
  • No trunking; that technology was in its infancy a quarter-century ago.
  • No digital modes like P25. Same story as above.
  • “Only” 40 memory channels in 2 banks, not hundreds in dozens of banks.
  • No tone or digital squelch, carrier squelch only. Not a terribly big deal, as carrier squelch will still work in such situations.
  • “Wide band” FM only; it way predates the recent narrowbanding mandate. Not a big deal, just turn up the volume to compensate. Plus the ham repeaters I monitor still all use wide band.

It’s not all bad news, though. It has some nice features:

  • A simple user interface. It doesn’t have many features, and there’s basically a dedicated key or control for each one. I didn’t need to use a manual to figure out how to use it; it’s that self-explanatory.
  • A built-in power supply, a big plus if you leave it powered on for extended periods (as one tends to do with a scanner).
  • A nice bright vacuum-fluorescent display that’s easy to read in dim conditions.
  • Decent audio. I can easily hear it anywhere in the main part of the house.

It’s a scanner, not a general-purpose communications receiver. There’s no tuning knob or S-meter. It receives FM only in most of its range, and AM only in the air bands. Not a big deal for me, as I bought it to scan FM, and it works just fine for this purpose.

I even sometimes eavesdrop on the local police and fire services with it, because I’m in one of those areas that’s never adopted either trunking or digital technology for its public service communications.

All in all, if you can live with their limitations, those old scanners can, as mentioned earlier, be a steal of a deal.

Random Note: Comet SMA-503 on the Yaesu FT-60

I recently and without planning was thrust into the market for an aftermarket antenna for my Yaesu FT-60 HT. The original one became unscrewed fell off on a hike last week. Gotta love those losing SMA connectors that merely screw on instead of clicking securely into place with a bayonet mechanism like the far superior BNC ones.

Anyhow, I needed a new antenna ASAP because starting tomorrow I’m going to be in the woods doing botanical surveys for a few days, and I don’t like to be alone away from cell coverage without some other possible means of communication. And since it was easier to source an aftermarket antenna promptly rather than an exact factory replacement, that’s what I did.

While the SMA connector mates fine electrically, the factory antenna had a little skirt below the connector that made a tight seal. The new antenna has no skirt, and so leaves a gap that is both unsightly and a way for dirt and moisture to enter. Comet even included a small rubber washer with the antenna for this purpose, but it is too small to fill the gap in my case.

The solution is an O-ring, in this case a #83 (1/2″ OD X 5/16″ ID X 3/32″ thick) O-ring. Figured I’d mention that here in case anyone fires up Google to search for a solution, and mention both the size number and the dimensions of the O-ring.

My experience indicates that about 49% of hardware stores sell O-rings only by size number and look at you like you’re a visitor from Mars if you give them dimensions, and 49% of hardware stores sell O-rings only by dimensions and look at you like you’re a visitor from Mars if you give them a size number. (I’m one of the lucky 2% these days; my local hardware store sells them by both specifications.) So be sure and take both specifications with you.

If you’re radio is something other than an FT-60, and you have a gap to fill, don’t rely on my information above. Each radio is slightly different. If the included rubber washer doesn’t do the trick, there is no substitute for having the radio with you so you can find exactly the best size.

D-STAR Myths

Some time ago I wrote a post about D-STAR. It’s time to revisit the technology and do some quick summarizing.

D-STAR is an Open Standard

Maybe in an abstract theoretical sense, but in practice, it’s proprietary. Only one major ham radio manufacturer (Icom) supports D-STAR. None of the others have announced any intention to. In fact, one, (Yaesu) has come up with a digital protocol of their own. Plus, there’s plenty of hams utilizing used P25 digital equipment. So not only is it not “open,” it’s also not really a “standard.” The actual standard for local VHF/UHF voice communications remains FM, because that’s what all manufacturers support, and what the vast majority of repeaters continue to use.

D-STAR is the Wave of the Future

Highly unlikely. Digital modes in general are the wave of the future and will probably eventually displace FM and most other analog modes for most communications. But it’s hardly clear that D-STAR is the technology which will prevail. In fact, its lack of widespread adoption argues against it being the digital technology which will prevail. More than likely, that will end up being some yet-to-be-invented mode which offers far better weak-signal performance than either D-STAR of FM, and audio quality at least as good as current analog modes. That way, users will put up with the unavoidable “fall off a cliff” because it will happen significantly further out than where FM starts getting a bit hissy and choppy.

D-STAR, being Digital, Offers Better Audio Quality

No, it doesn’t. It offers worse audio quality. This isn’t CD-style digital audio we’re talking about; there’s significantly less bandwidth available than that on a ham radio channel. Circumstances dictate a slower sampling rate and very aggressive compression. Lossy compression. Quite audibly lossy compression, in fact.

It gets even more dramatic in weak-signal situations. Like all digital modes, a D-STAR signal quickly “drops off a cliff” when usable range is exceeded. Audio gets totally lost, first little bits, then big bits, then all of it. And it’s a much more disconcerting and intelligibility-compromising loss of information than one gets in analog FM, where rising background hiss provides an early warning and the drop-outs are not so sudden or so total.

D-STAR Is Needed Because We’re Out of Repeater Pairs

Maybe in Manhattan, Tokyo, or LA this is the case but for the vast majority of the Earth’s surface it is not. Most places have plenty of repeater pairs available. Others, such as most big cities that fall short of being megalopolises, might be short on empty repeater pairs but the repeaters themselves end up being mostly empty (you can listen all day and you’re lucky if you as much as hear another station ID). There’s plenty of room for more chit-chat.

D-STAR Therefore Is Useless

Not so fast. Nothing says you must use it for voice. There’s two parts to D-STAR, a CODEC for moving between the analog and digital worlds, and a way of sending streams of bits over the airwaves. The former is actually where most of the cost for the equipment comes in (since it’s a proprietary CODEC), while the latter is 100% open. So if you need a digital point-to-point link, the technology has some use.

In other words, it’s a specialized mode for special applications, not any sort of general-purpose replacement for FM.

It’s Not Just DMR, Either

Pretty much all communications-grade digital voice protocols sound awful, for the same reason that DMR sounds awful. Probably the least awful-sounding one, from the samples I’ve heard, is NXDN. Even that can’t hold a candle to plain old analog FM, however.

It all leaves me wondering if digital is just plain the wrong answer to cramming a voice signal into a smaller bandwidth. I’m inclined to thing using something like SSB plus an intelligent, software-defined receiver (with sophisticated noise filtering and precise, automatic carrier insertion) might be better.

Noise tends to be not a super-big issue at VHF and above, anyhow, so losing a degree of immunity to it might not be such a big deal, particularly if one uses digital signal processing techniques to remove as much of it as possible. SSB can sound as good as AM (which, unless noise gets in the way, blows digital out of the water when it comes to audio fidelity and overall intelligibility), provided you insert the carrier at just the right spot when receiving.

As a further plus, a voice-quality SSB signal uses half or less the bandwidth per channel of any of the digital protocols currently out there.

Sometimes the right answer to a question about employing a new technology is not to employ it at all (or, in this case, to employ it, but in a significantly different way than originally proposed).