Bels and Nepers - BSTJ 1929

Today's ARRL Contest Update mentions the on-line availability of all the issues of the Bell System Technical Journal. That's a wonderful thing, because so much of the cutting edge electronic and technical developments of the era 1922-1983 were written up there.  It is a spectacular testament to the value of AT&T's Bell Laboratories.  It is so sad that Bell Labs (and other private research labs like IBM's) no longer have the prominence in fundamental research they once did.

So anyway, enough sentimentality.  I thought I would open up a BSTJ article at random.  Sure enough, it was very interesting.  The first article in the Jan., 1929, issue was Decibel--The Name for the Transmission Unit, by W.H. Martin, wherein the "db" was introduced to the technical public!

Why do we always use "db" instead of bels? AT&T had been using the "transmission unit" for some time, which indicated a ratio of 10**0.1 -- which was identical to the new decibel.  It's a handy unit, too, because it is just about the minimum loudness ratio a person can detect in favorable conditions.

Quick Look at FUNcube Dongle SDR Radio

My candidate for the new radio with the most unlovely name is the FUNcube Dongle.  It is a part of the AMSAT-UK FUNcube satellite project, but it is attracting wide interest (not just for satellite work) as an example of what can be done by a volunteer group to build a very simple, but powerful Software Defined Radio to cover the 60 - 2000 MHz frequency range.  And it's pretty cheap.  The last lot went for £ 108.22 each to the US, about US $170.  They are only being made sporadically, but another batch is supposed to be coming at the end of February.

I have had a chance to borrow an FCD, and I've been trying to figure out what it does and does not do, and whether it's something I need to add to my stable.  Here are some of my lab notes:

My quick tests in my home shack without good signal generation & attenuation showed that the receiver (like any SDR) is sensitive to overload. The only signal source I could use easily was my 2M HT (¼ watt low power) transmitting into a dummy load a foot away from the receiver. I also noted that the dongle radiates at its LO frequency, breaking squelch on the HT. There seems to be little or no shielding on the dongle.  It has a plastic case, and there may or may not be shielding inside.

I used both the basic software combination (FCHid and SpectraVue) and the more user friendly (IMO) WRPlus with the G0MJW adapter DLL. The project's documentation for newbie users is fragmentary and frustrating, but that's understandable at this early stage.

At the ARRL Lab*, I found the FCD's crystal oscillator frequency in this unit is about 119 ppm high, but either software solution allows you to enter a frequency scale factor to correct for this. Warmup drift of nearly 1 ppm was observed from a room temperature start. (Lack of a high stability oscillator or an external sync capability might be a limit in some advanced applications.)

I was able to update the dongle firmware to version 18f, which is current. Surprisingly and unfortunately, I could find no indication in the PC software about what firmware version is actually installed.

I looked at the tweaks available for DC balancing (suppressing the zero frequency peak) and I/Q balance (maximizing image rejection). At 926 MHz, I was able to get 49 dB of image rejection, but the adjustment is critical. Switching to 2000 MHz, the image rejection is down to 30 dB. (Image rejection is typically about 24 dB if you don't do careful tweaking.)

Maximum input level before clipping occurs is about -55 dBm, varying somewhat with frequency.  You can easily reach this level depending on your RF environment.  A preselector filter may be needed.  (Strictly speaking, the -55 dBm limit applies to a simple sinewave input.  It's an instantaneous voltage limit, so a broadband input of greater power might be tolerated.)

A quick measurement of tangential sensitivity gave the following

60 MHz -122 dBm
144    -129
926    -130
1000   -129
2000   -133

Tangential sensitivity is a measure of the the minimum detectable signal. These values are by eye with a particular FFT and filtering setup. Don't hold me to them!

The frequency coverage was at least 60.1 – 2000 MHz.  I did not explore the outer limits of coverage.

The major limitations of the dongle are its limited sensitivity, lack of input filtering or attenuation or AGC, and limited instantaneous bandwidth (96 kHz). The USB dongle package is cheap and effective, but it's susceptible to mechanical damage if it's hanging off your laptop.

For about US $170, it's a steal. It is usable as a wide-coverage VHF/UHF receiver, especially with a low noise preamp, and it provides a good playground for learning about SDR.  I suppose it's even going to be a cheap and effective receiver for satellite work! I would want one as test equipment, but the bandwidth limitation becomes significant if you are searching for a signal with unknown frequency.

Update: The next sale of Dongles will begin Sunday 20 February 2011 22:00UTC.

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*I volunteer in the ARRL Lab in Newington, CT.  The work here is my own and ARRL is not responsible for my mistakes!

Very cool device

http://www.technologyreview.com/blog/arxiv/26194 or
http://arxiv.org/abs/1012.4415

This is a new nanoscale device that can detect RF.  You put in a bias current between D and S, it goes through the graphene (sheet of 1 atom thick carbon).  The graphene sheet resonates mechanically at an RF frequency (33 MHz in the reported device) when a signal is coupled in capacitively from the gate.  The vibration induces a signal on top of the D-S current.  The device works at 77 K (liquid nitrogen temperature), but hopefully can be developed for room temperature operation.

Ham Radio and Ngram

Google's new service "Ngram" is cool.  It lets you search for any word or phrase through all the books Google has indexed, displaying the count versus year.

Here is the graph for "amateur radio":

and here is the result for "ham radio":

The significance?  Ham radio (in books) didn't start until about 1950! The Ngram view page also gives links for the books in particular year ranges.  I looked up some of them prior to 1965 and found lots that were in the "popular" category: Popular Mechanics, Popular Science, Boy's Life, etc.  Amateur radio, however, has a book record back to 1900 if not before.

Locating amprnet

Remember ham radio's allocation of Internet address space?  (44.0.0.0/8) Thanks to xkcd, here it is in graphical form, ca. 2006 (red circle):

Right between Bell North and Japan Inet.  (Larger size at http://xkcd.com/195/.) You can buy a poster version of this at http://store.xkcd.com/.  (A good place to find gifts for the geek in your life.)

ARRL 10 Meters

I thought I would skip the 10 M contest -- too much happening on the family / holiday front, and surely the band is going to be dead. Well...

Image via Wikipedia

I checked in this afternoon just to see, and by golly there were openings to the south (FL - Caribbean - Brazil) and to the west (XE and W6).  Along with some local folks.  I ended up with 16 mighty Q's and 13 multipliers.  And I resisted the temptation to use the afterburner!  Ten meters is a low-power band, right?

Now, I'm a believer in gray-line propagation.

The End of IPv4 and the Amateur 44/8 amprnet

Image via WikipediaI wrote about amprnet (ampr.org) a while back - here.  It's a relatively unknown fact that ham radio has its very own Class A IP network -- 44.0.0.0/8.  This is quite a huge chunk of Internet IP v. 4 address space -- 16 million addresses, give or take.  As best I can determine, it was established in 1987 to support future TCP/IP networks that might be implemented either on radio links or as some combination of radio and alternative commercial or other links.  (The latter makes a lot of sense now, given the widespread availability of consumer Internet connections.)  The allocation is best documented in this ARIN WHOIS link.  I have not been able to find on-line documentation or other material relating to amprnet, beyond a number of blog and email postings.  (I'd be happy to get pointers!)

This whole subject came up once again for me, when I was reading how ARIN is trying to "capture" (contractually speaking) the legacy owners of IPv4 address space.  These are people, presumably including amprnet, who received their allocations well before the current Internet bureaucracy was established.  An interesting look at the issues is available in ComputerWorld.  (Thanks slashdot!)

The amprnet allocation usage must be tiny in percentage terms, and hard to justify in a world of address scarcity.   Standalone radio networks based on TCP/IP (if there really are any!) are likely based on VHF if not 2.4 GHz (HSMM), and they are fundamentally local or regional and are not likely to be routable from the Internet.   They could probably equally well use a network address like 10.0.0.0/8.

In recent years, various other network-based amateur systems have developed, including EchoLink, IRLP, D-Star, winLink, etc.  These make effective use of Internet links, but have no particular need for the amprnet addresses.

I don't want to argue the pros and cons so much as to point to the odd situation of our hobby holding such a valuable resource, with no visible institutional support. 

What are the chances to get a little more RF spectrum, if we hand back the IP addresses?

U-verse Downs & Ups

Image via WikipediaThe AT&T U-verse saga continues.  I was sad to find that, after my initial tests that showed I could run 80 M CW at 800 W, my latest round of tests showed really bad sensitivity.  I couldn't run over 20 or 30 W without causing the U-verse Residential Gateway (RG) to lose sync.  (That can mean a 1- or 2-minute TV and phone outage.)

After a bit of probing, it became clear that the RFI was entering on the internal home phone wiring.  Disconnecting it from the RG quieted the problem, but alas we are now very dependent on the U-verse VOIP service.  Disconnection is not an option, normally.  (Cell coverage is poor here - a story for another day.)

The AT&T installer had not fully appreciated my phone entrance system, which uses a special DSL surge suppressor that is bolted onto my Single-Point Ground system.  He ran his CAT5 cable from the wall plate straight into the RG. (That's OK for RFI, but not great for lightning surges.)  The house phones were fed via a circuitous route passing (backwards!) through the surge supressor, and generally making a nice RF pickup loop.

This afternoon, I pulled out half my station (the heavy part!) so I could get access, and rewired the phone system, adding ferrite chokes, etc.  Result: no trouble now with 800 W at 3600 kHz.  I'll do more tests later, when the contest goes away.

U-verse RFI tip:  You can use the ubiquitous DSL telephone line filters as an RFI choke.  These are transparent to telephone audio, but block the DSL frequencies.  (It's not clear whether they are effective at up to ~8 MHz that VDSL = U-verse uses, but it seems likely they are.)

More complete info will be at http://aa6e.net/wiki/RF_Compatibility.