Steven S. Holland

Back in the 1960’s antenna sites started getting a tad crowded, and one solution proposed was to connected many transmitters to a single antenna, using a parallel network of bandpass filters that would allow isolation between transmitters.  After looking for ways to accomplish this at VHF, it was found that resonant cavities could be formed using a readily found, well constructed, tight tolerance, cheap and widely available solution — beer kegs.

It was found that a coaxial resonantor could be formed by making a center conductor that is adjusted to tune the frequency. As the article states, many problems were overcome in order to make them as successful as they were, and the article estimates tens of thousands of radio transmitter sites around the world equipped with these bands of beer kegs transformed into resonators!

A lot of people in RF/Microwaves/Electromagnetics can probably trace back a path of teachers to Maxwell…and here is mine:

Maxwell taught emag to Jeans (at Cambridge),

Jeans taught emag to Smythe (at Princeton)

Smythe taught emag to Rusch (at CalTech)

Rusch taught emag to Strangeway (at University of Southern California)

Strangeway taught emag to me (at Milwaukee School of Engineering) – that’s 5th generation.

Hopefully someday I can add to the list:

Holland taught emag to _________ (at ___________) …

I got a Garmin FM/AM/Shortwave radio for Christmas, and here and there I scan around and see what I can pick up.  For a while I kept catching a channel out of the Netherlands, and a handful of other stations I can’t exactly place because they weren’t in English and I couldn’t listen to hear them say where they were broadcasting from .

The last week or so at night I have been picking up this station, broadcasting from New Zealand!  That is probably one of the furthest places from here I have picked up stations from.  It’s broadcasting on approximately 10MHz according to my little radio, which the site confirms is 9.8MHz.  

Shortwave is really fascinating, both from a sociological and technical standpoint.  On the human side, it allows people to broadcast all over the world, talk to people thousands of miles away, hold conversations, send data (albeit slowly – remember, you’re only at 10MHz, and bandwidth is a tiny fraction of this),  and in emergency situations potentially act as the only form of working, reliable means of communication that can be used to save lives.  I know what you’re thinking — the internet allows people from all over the world to connect already.  Yes and no.  First of all, shortwave has been going for much, much longer than the internet has been open to the public, so historically it was the first means of allowing individual people freedom to connect with people from all over the world.  Besides equipment, it is completely free – you don’t get charged to use the airwaves.  You have to get licensed, but there are no ongoing costs to use the frequency bands.  

From a technical standpoint, one of my favorite aspects is the fact that it is in a special range of frequencies that make “ducting” and groundwaves possible. 

Ducting

Shortwave is at a low enough frequency that it can reflect off the ionosphere, instead of traveling out into space, it bounces back towards the earth.  This is one of the major mechanisms that allows, for example, stations in New Zealand to reach the US — like using a mirror to see around a corner, the reflections off the ionosphere allows signals to reach places beyond line of sight.  This bouncing of the waves can happen multiple times, from the ground, to the sky, back to the ground, to the sky, …, and hence the name “ducting”, since it is like a waveguide. What’s really interesting is that this effect is modulated by the sun cycle and solar weather.  During the daytime, the solar wind pushes the ionosphere down towards the earth, and this can cause the waveguide setup to either be in cutoff or can make it so that the reflections are too shallow to propagate very far beyond line of sight.  But at night, the earth shields the sky from the solar wind, and the ionosphere is higher, allowing this phenomenon to take place.

The other phenomenon that can cause ducting is the change in refractive index in the air.  As altitude increases, the refractive index decreases, and so waves travelling through this medium tend to get bent back towards the earth.  Conversely there are also situations where the refractive index can increase with altitude, and the wave is bent further towards the sky into space.  But both of these allow propagation of radio signal well beyond line of sight, or the horizon.  

Ground Waves

It is possible to have, over a certain frequency range, propagating waves attach themselves to a ground system — in this case the surface of the earth — and propagate following the curvature of the earth instead of going in a straight line off into space as the horizon is reached.  These can also be referred to as surface waves, or Zenneck waves.  
Also, on top of the wave propagation aspect, it also allows people to learn hands on antenna and transmitter design and operation.  It has created a whole Ham radio, or amateur radio culture that build and operate these systems.

I have been using this book for the last 3 years as another source of some electromagnetic and antenna theory. It is an excellent book that is always being revised (similar to the electromagnetics book I posted about here) and is available in free pdf files. I highly recommend it to anyone looking for a good EM/Antenna reference.

I found online a downloadable, free book on electromagnetic field theory, more from an electrodynamics standpoint (physics).  So far I have only skimmed through but it gives treatment of spectral analysis of an arbitrary source, radiation problems, relativistic electrodynamics, etc.  Even more important, is that it is an ongoing project — people are welcome (infact, highly encouraged) to submit ideas/work to be included in the book or to find errors in it’s current form.  This provides the benefit of at least some sort of checks and balances on what is in the book, in terms of people reading it and submitting feedback, and also it is a collaborative effort to provide a completely free source for this knowledge, which they remind us should be free always.

Briefcase that “changed the world”

This BBC article tells the stort of the journey of one very important microwave component delivered to America during World War 2 –the magnetron.

During the war engineers in England developed a high power microwave source, but not having the money or industrial support to develop this new source, they brought it to the US. They gave it to the US — no catch, so long as they provided financial and industrial support. The source was developed and led to airborn radar as it was powerful and lightweight compared to the other sources available at the time.

Why should anyone care? Because it allowed radar to be exploited during the war. Oh, and it is also the high power source tucked in the back of every microwave oven. So next time you make popcorn or one of those breakfast bundle things with the sausage on the stick surrounded by a pancake….speaking in a British accent is encouraged.

A few months ago I had seen a thing for a clock with a smith chart as it’s clock face which I searched and can’t seem to find a link for that. Regardless, I have a cheap black wall clock that I picked up while cleaning up my room and thought that the clock face looked to be about the right size for a printed out smith chat, and I was right:
So I took a screw driver and pushed in two tabs on the back of the clock that hold in the clear plastic over the face of the clock, and removed the clear face:
Next I wanted to get the paper clock face out to use as a cutting guide on the smith chart, so I cut from the edge to the center hole so I could remove the face without trying to remove (and likely break) the hands of the clock:

Next, I took out the paper face, and aligned that on the smith chart so the smith chart would be more or less centered (with a perfect match at the axle of the clock arms!). I put tape on the edge of the clock face so it wouldn’t shift from the position I centered it on:
Then I cut out the circle on the smith chart around the edge of the clock face — the cutout needs to be the same size or smaller than the clock face or it won’t fit in the clock:

Then I cut out the circle in the center of the chart (shown started above) and cut a line from the outside of the smith chart to the center, again, so I can get it on past the arms of the clock. In retrospect, I could have cut a small line out from the center as long as the back end of the seconds hand, and then put the long end of the arms through the hold and then slide this slit down over the longer back of the seconds hand, thus avoiding cutting a line from edge to center, so if you are doing this, i recommend not cutting all the way from the edge.

I then put the smith chart in on top of the old clock face, and lined up the real impedance line (the horizontal line on the chart) with the 3 and the 9, so that a perfect short is 9 o’clock and a perfect open is 3 o’clock. I taped it down on the back side in a number of places to hold it snug to the clock face, to avoid it shifting around. I debated putting an extra circle in between the clock face and the smith chart to make sure the numbers don’t bleed through — they don’t unless under strong light, so I don’t think it’s necessary.

Here is the finished product!
So now, 12 is at Z = j, 6 is at Z = -j, and so forth. I am debating putting the clock numbers on this, but so far I am thinking I like it the way it is. I can now tell time in either standard, military, or impedance.

Some facts:
-the seconds hand traces out a circle at the tip that has a VSWR of about 12, the hour hand a VSWR of 2.8, and the minute hand VSWR = 9 or so.
- Between 3 and 9 o’clock each day, the hour is capacitive, while between 9 and 3 o’clock the hour is inductive. The minute hand, obviously, spends half of the hour between 15 after and 45 as capacitive, and the rest of the time the minutes are inductive.
- At any given hour (and impedance), moving 6 hours around the clock gives the admittance of the impedance. Same with the minute hand, only moving 30 minutes around the clock instead.
-9 o’clock is a voltage minimum, whereas 3 o’clock is a voltage maximum..

,,,I could go on, but I think I beat that one to death. Make your own smith chart clock!