Saturday, July 21, 2007

Amateur Radio: A funny thing happened on the way to the contest today...

Just a quick note on a funny story; at least one of those ones that radio amateurs find funny.

Most amateurs have stories about RF (especially in the HF bands) causing trouble around the house. Usually just simple TV interference, harmless stuff. One guy I knew said his digital bedside clock would reset itself when he transmitted on certain frequencies. Another said his stereo would turn on by itself. I've heard my external hard drive park its heads during transmit, and in general USB hardware doesn't seem to like RF.

I've been dealing with some "RF in the shack" lately; because until I can get my heavy coax run back up to the roof I'm operating on a somewhat hacked setup. Specifically; at anything below 20 meters I get feedback paths into my audio, and one night I got "bit" by one of the screws on my radio's casing. (Caused by signals inducing currents into the radio itself.) Typical symptom of a grounding problem, and expected considering my current setup. I decided to try out an artifical ground; which is essentially like an antenna tuner between the station and ground. It provides a tuned circuit that electrically lengthens or shortens the ground, and it can even be used to make a random wire stretched along the floor act like a ground.

So as I fiddled with the artificial ground settings on one band I keyed up and as I did I heard my sprinklers come on. Maybe my wife's working in the garden, or...? I key up again, same thing. Apparently I was kicking out enough power to activate the driver transistors in my sprinkler controller and this is what caused them to turn on.


Friday, July 20, 2007

How to not be evil

Today's buzz is Google's Ex Parte FCC filing in which Eric Schmidt tells Chairman Martin that they have committed $4.6 billion to purchase spectrum in a future Upper 700 MHz auction - provided that the FCC structures the new service in such a way that license holders will have to offer at least 1/3rd of their spectrum to other companies on a wholesale market. The interesting thing about this is that the FCC's "auction reserve" has been said to be...$4.6 billion.

I think it's great that Google is trying to leverage their financial strength in order to ensure that wireless broadband remains somewhat open, and help prevent yet another telco monopoly from developing. The 1982 court-mandated breakup of the Bell system (the Greene Decision and subsequent Modified Final Judgement) opened the doors to increased competition and (some would argue) allowed technologies such as DSL to develop which would not have done so under the oligarchy of Ma Bell. To a large extent, today's cellular providers are no different than the wired telcos; largely run by a bunch of visionless business school graduates who are unresponsive to the needs of a market hungry for cutting-edge technologies. In a very real sense, the driving force behind wide-area broadband technologies such as WiMax exists because of the cellular carriers have consistently failed to meet those needs.

The carriers are of course not stupid, just short-sighted. Their initial response to alternatives was to be indifferent; then arrogant, and finally now they've become patronizing and suggested that the FCC should not allow "distractions" from smaller players (i.e. anyone who isn't a cellular carrier) and let them do what they will with the Upper 700 MHz spectrum. "Don't you worry your pretty little head about all this wireless stuff, young lady. This is man's work. Run along and play with your dolls now."

Allowing any one company or group to monopolize all or even a majority of the Upper 700 MHz would be a mistake of biblical proportions, and would result in the exact same kind of anti-competitive market and visionless wireless technology offerings the cellular carriers have today. The Upper 700 MHz spectrum is fertile soil in which entirely new markets can be grown and nurtured; wireless broadband, additional spectrum to relieve the crowding in 900/2400/5200 MHz ISM bands, and with any luck a nationwide interoperable emergency communications system which we desperately need and do not have today.

This isn't to say that if Google bought the Upper 700 MHz whole band that they would do the right thing either. Their mantra "Don't Be Evil" is simply that; a mantra. That and $3 will get you a latte at Starbucks, and I've always wondered when (not if) Google would devolve into a typical big company mentality. A few quarters with shortfalls in earnings ought to do the trick. But in the meantime, I think what they're doing is great and applaud them for their willingness to put their money where their mouth is.

Wednesday, July 4, 2007

How not to build a transceiver

Primarily due to its flexibility, one of the most popular mobile rigs for amateur radio is the Kenwood TM-742A and associated models such as the 942, 741, etc. The TM-742A is a tri-band rig which can accept up to three band modules out of an available five; 10m, 6m, 2m, 1.25m, and 70cm. Interesting trivia; the MSRP in 1994 was $660. Today, clean TM-742A rigs can and do go for over $750 and rising as replacement parts and band modules become harder to find.

Every rig has its quirks, and a quirk of the TM-742A is that the 2m module is prone to failure. The 2m power amplifier is a Toshiba S-AV17 which is a set of power transistors and associated components soldered onto a beryllium ceramic substrate. Symptom of the failure is that the rig will transmit enough power to be heard on other close-by (within a few tens of feet) rigs but makes no power at the antenna. Most people just pony up the $65 and replace the S-AV17. Others have discovered that the failure lies in a microscopic crack in the ceramic that breaks one of the microstrip filter traces. The fix for this is to remove the S-AV17, pry off the plastic cover, and run a rapid thermal recovery soldering iron (like a Metcal or Hakko) over the crack area. A standard resistive heater iron will not work; because the ceramic module is designed to absorb lots of heat so the trace won't get hot enough to flow. It takes 15 minutes to disassemble the rig and 15 seconds to solder it. Thanks to Kevin W3KKC for his webpage discussing the problem and walking through the repair process; complete with photos.

(Disclaimer: beryllium is nasty stuff. You don't want to inhale it. If you're not comfortable doing this; don't have the right equipment; etc blah insert dire warnings here then pay the $65 and don't try to repair the amplifier!)

Interestingly enough, and relevant to the title of this post, is to examine why the module fails. The reason for the failure is excess heat. The stock configuration for the band modules is to have the 2m in the middle, which means that the 2m power amplifier is buried about as deep in the rig as it can be. At 50W the 2m module is also capable of the highest power output, so therefore it gets hotter than the other modules. The ceramic cracks and you get a dead S-AV17. I would accept this explanation readily enough except that every 2m module I've disassembled has had the same problem; the S-AV17 is mounted dry. Not one has used any form of thermal grease to promote conductivity into the heatsink and transceiver structural frame. This would be like a high-speed CPU being installed onto a motherboard without thermal grease; the CPU is essentially guaranteed to fail from thermal overload. This is (or more accurately was) a blatantly stupid move on Kenwood's part that has cost radio amateurs thousands (if not tens of thousands) of dollars in unnecessary repairs, replacements parts, shipping costs and downtime. Can you demand a recall of a 14 year old product?