February 25, 2010
Antenna "diversity" always amazes me
By Bill Schweber

No, I am not talking here about diversity antennas, where multiple-antenna setup is used to overcome local problems of fading, multipath, and related ills, nor politically correct "diversity".

I am talking about the amazing variety of antenna configurations, designs, shapes, topologies, and physical implementations. To invoke a cliché or two, antennas are both awesome and thought-inspiring: so much true cleverness and creativity, and all so tangible and visible, "Antennas give me a headache". And the antenna itself is a passive component, which adds to the challenge– but is also liberating, in a way.

I was reminded of this when I saw the March 2010 "antenna" issue of QST, the official publication of the ARRL, an organization devoted to amateur (ham) radio (and yes, ham radio is still alive and well). Since the contents are not available online except to subscribers, you'll have to go over to your better local library (remember those? They have new and old material that is not available online, yet still is vital and valuable) or your better-stocked bookstore (such as Barnes & Noble).

Although there are just a few antenna designs in the theoretical sense (dipole, long-wire, to name two) the reality is that the number of both simple and complex actual realizations of these is staggering. Sometimes this is due to performance priorities, or wavelength/frequency issues, or the need for integral antennas such as in handheld devices (I still can't believe that the ones built in cell phones can possibly work at all, but they do). There are also PCB-based antennas, as well as huge dishes doing deep-pace tracking. And of course, the antenna itself is only part of the story, since your receiver preamp or transmitter driver must be matched to the antenna to achieve efficient power transfer.

The irony is that functionally the antenna is a very simple component with a straightforward mission: to capture or radiate RF; and to act as a transducer between electromagnetic energy and wire-confined energy. While today's antenna design is disciplined by complex and sophisticated simulation, unlike the "hey, this might work" approach of the early days, there are still interesting physical designs that must first be "thought up" by designers before the software can analyze them.

There's lots of room for innovation in antennas, and it is one area where "lone-wolf" individuals can still make a difference.♦

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February 13, 2010
Would you like some wireless with that sandwich?
By Bill Schweber

I recently saw a basic home gadget that came with an RF link which served a very minor purpose in the product's function (what it is, isn't important here). Seeing it made me wonder if perhaps "RF" and "wireless" are now so hot as marketing labels that we risk having too much of a good thing in our environment.

No, I am not talking I the sense of biological harm, there is no evidence for that and there is solid research negating it. I mean harm in terms of RF pollution, interference, hard-to-diagnose problems, and reduced link reliability and integrity. Even if each RF link meets appropriate regulatory stands–and let's be real, here: many don't, despite their labels–the channel loading due to built-up noise and interfering signals can compromise performance. There are even well-documented cases of home appliances such as clothes washers operating erratically due to nearby low-power RF sources such as cell phones.

One of the problems with this proliferation and profusion of RF links is that is often difficult to isolate and debug them and isolate them in a real-world situation. With a wired link, you can physically disconnect the cable/connector, of course. But with a wireless link, you often can only disable the link by shutting off the associated device. Turning the unit "off" may not do this, so in some cases can only be done by removing its battery (and then there are devices which have no accessible battery. As we all know too well, the alleged "off" setting of many devices is really a sleep mode, not a hard disconnect of the power-supply subsystem.

Will all this RF, both useful and trivial, come back to haunt us in our homes, or public places, or as we sit down at the local coffee shop for a cup while surfing, talking on our cell phone with Bluetooth headset, using our wireless mouse, and more, plus similar from others around us? As they said in the old days: "stay tuned for more." ♦
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January 30, 2010
Are "RF" and "wireless" defined by their frequency or their media?
By Bill Schweber

Like most professions, we use the same words for different things, and different words for the same things. Think, for moment, of the many meanings of the word "buffer," or the many words and phrases we use for the analog/digital converter function.

Generally, this is not is not a problem, since the context of the discussion and expertise of the people we are talking to eliminates, or at least minimizes, ambiguity and misunderstanding. (I did, however, attend one conference where there was a heated argument about some aspect of "IP"–it turned out that one of the arguers meant intellectual property, while the other was referring to "Internet protocol"! And I'm sure they weren't talking about "intermodulation product.").

But what do we mean when we say RF? Do we mean radio frequency? OK, but what's a radio frequency? In the early days of radio–and I am talking Marconi-era here– radio operated down in the tens of kilohertz at its highest, which is almost DC from our present vantage point. But it was radio, in every sense of the word.

And when we have a signal on a wire or cable in the hundreds of MHz or above, we now routinely use the term "RF" to describe its attributes, even though the signal is confined to a physical conductor, rather than air or vacuum medium. Yes, it's RF, but is it radio?

What about infrared (IR)? Those ubiquitous remote controls are definitely wireless, but they operate well beyond the conventional RF bands, and they observe the same Maxwell's equations which conventional radio links must follow. Yet, when engineers speak of RF or wireless, they usually don't mean IR–but I suppose they could.

The lesson here is not so much to "watch your language" and try to be more precise. That's not going to happen, nor is it necessary in most cases. But notice that I said "in most cases": there will be times you'll want to make your definitions clear up front, to avoid misunderstandings and even embarrassment among participants! ♦
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January 03, 2010
Why I am sometimes not so enamored of short RF links
By Bill Schweber

Earlier in 2009, I wrote about how the wireless bike speedometer I got to replace an older wired one was a case where an RF link can beneficially take the placed of even a short wire link, see

Well, times change and thrills fade. It turns out that the wireless set-up is just not as reliable as a wired one was. Whether it is this particular unit, or this model, or wireless speedometers as a class, I can't say. The handlebar-mounted readout shows the right speed much of the time, but often drops down to about half or a third of the actual speed and then recovers, even when I am going at a fairly constant rate.

I know the sensor pickup is OK, since I can hear the clicks of the internal reed switch as the spoke magnet goes by. But after that, I don't know where the unreliability is. It could be that I am at the border of the claimed 70-cm wireless range, or that the frame geometry is somehow in the way, or maybe the fork-mounted sender and/or the handlebar-mounted receiver/display don't like colder weather. Whatever the cause, it's frustrating. (And yes, I have put fresh batteries in both units.)

But the underlying issue and subsequent lesson goes beyond this non-critical application. To save power, reduce size, and keep costs down, the wireless bike speedometer is a simplex (unidirectional) design. There is no reverse channel and acknowledgement from the handlebar unit to the sender. And that's the weak link, so to speak. There is no way for the fork-mounted unit to know that its clicks were received (it's a fairly slow repetition rate, 0.2 clicks/sec per mile/hour speed).

Unlike a wireless mouse or TV remote control, for example, where the user can see a problem, and thus (in effect) implement the ACK/NACK protocol, and repeat the action if the signal was not received (frustrating, for sure, but at least doable), I don't have that luxury on the bike. Either the switch pulses are received and totaled, or they are not.

While low-cost, unidirectional wireless links may seem attractive—and they are in many cases, since they eliminate connectors and cables—be sure to take time to understand what the impact of a lost message will be. Regardless of the cause, whether it is distance interference, weak batteries, or other, the consequences can span minor to major. And if they are more than minor, you may want to embed a most costly, complex half- or full-duplex link, with associated protocols. Or, you may decide a wired link, with its greater reliability and immunity to outside factors, is a better choice after all. ♦
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