What Is Radio:
A Lecture in the Making

Two friends discuss how to
explain radio to their children.

Date: Tue, 03 Dec 1996 20:56:36 -0800
From: James J. Besemer
To: Ward Cunningham

I volunteered to teach an electronics class at Michael's school. I was wondering if I could get YOU in turn to sign-up as a guest lecturer on the topic of Radio transmission and reception.

Class is 50 min and the target format is approx half lecture/discussion and half demonstration or hands-on project. My expectation is to survey the important devices, how they operate and how they are used. Then include some kind of demo or hands-on lab. K9OX mobile might be a successful demo. You could even put in a plug for amateur radio.

Date: Wed, 04 Dec 1996 11:04:58 -0800
From: Ward Cunningham
To: James J. Besemer

I'm going to decline but not without first making a suggestion and an offer. It has been my experience that ham radio demonstrations are a crushing bore for kids. The legal restrictions and accepted operating practice conspire to make it so. Not to mention that the only other hams on in the daytime are retired.

I've done a few demos for scouts. Although I did point to the ham rig, the scouts attention really picked up when I brought out our box of CB walkie-talkies. I have four of the Radio Shack model that will travel for a block or so. I gave them a simple communication task and sent them out the back door in teams to perform it. Here is what they learned:

Of course the same principles can be observed in the scout meeting itself. The den leader made that connection immediately. I don't recall them actually communicating effectively with the radios. We didn't make that a requirement for the Communicator badge.

Anyway, I suggest you design the lecture around an "operating exercise" and I will offer my set of CBs for your use. Here are some lecture points that you may want to include:

Gee, that is sounding like a fun lecture. I still don't want to do it because I can't reconcile the midweek commitment with my unpredictable travel schedule. Bummer.

Date: Wed, 04 Dec 1996 12:59:15 -0800
From: James J. Besemer
To: Ward Cunningham

One place I thought you could really help is where I'm unclear on the underlying physics involved. I know how to build a transmitter, but what is it about running current at high-frequency through an antenna that creates radio waves? Radio waves are simply low-frequency photons, right? So how do the electrons create the photons? Once you have the radio waves, I can sort of imagine how their striking an antenna might induce a current, though it wouldn't hurt to hear what's going on there also.

If you can explain it in terms I understand, then I can probably deliver a credible lecture.

This is important, as one of the things that is really cool about electricity is all the mysterious, magical effects. And I'd like to be able to give a credible explanation for what's happening.

I built one of those when I was in high school. I hooked up a Morse code key to a 12V relay and used that to switch the 110 to the transformer. Worked as expected. Many are unaware that Marconi's original "radio transmitter" was a large scale model of exactly the same thing. The sound of the sparks was so loud they could be heard miles away. The operator had to be put in a heavily sound-proofed booth to keep from injuring his hearing.

Date: Wed, 04 Dec 1996 16:33:17 -0800
From: Ward Cunningham
To: James J. Besemer

Well, let me tell you the story I was taught in EE411. It has the advantage of being slightly intuitive, unlike quantum electrodynamics. (See Feynman's QED: the Strange Theory of Light and Matter. or Carver Mead's Collective Electrodynamics)

Both electric and magnetic fields exist in empty space. Fluctuation in one will beget the other, which will itself be fluctuating. Energy transfers from E field to B field and back again, much like the transfer from kinetic to potential energy and back in a vibrating spring. Propagation of energy happens when the E fields and B fields are slightly displaced. The mechanical analogy is the slinky where each loop is a spring.

A key to understanding propagation is to realize that energy is transferred both forward and backwards, say from a collapsing E field to growing B fields both ahead and behind the wave front. However, the backward energy transfer is just enough to cancel out residual fields so that the net is a forward motion. Again, same with the slinky.

An antenna "sees" the B field, not the E field. (I suppose a pair of isolated terminals in space would see the E field instead of the B field, or I could have this point wrong.) The B field induces motion in the loosely bound outer shell electrons of the metal. They don't have to move very far since they influence neighboring electrons in a process similar (but not identical) to free space propagation.

The current induced into an antenna won't amount to much unless subsequent waves can reinforce the motion. This requires either very long wires or rapidly alternating fields. A half wavelength of wire works well because it will resonate, i.e., the current will bounce off of the ends of the wire exactly in synch with the alternating field. Less wire can be used if the current has somewhere to go while waiting for the next wave. We'll want to do this anyway since we want to tap off some of the energy from the wire so that we can amplify it and detect any signal it might carry.

A transmitter is just like a receiver except the direction of flow of energy is reversed. Now we use an amplifier to move a current in the antenna so that it will create the B field that starts the electromagnetic wave going. There is another practical difference. A receiving amplifier is designed for high gain so that it can take a weak signal and get it up to a watt or so. The transmitting amplifier is designed for high power, taking a watt and boosting it to hundreds, thousands or even millions of watts. Carefully tuning the resonance of an antenna is far more important when transmitting because you don't want to waste any of those watts.

Although all electromagnetic waves are the same stuff (E & B fields), high frequency radio waves are uniquely valuable because of the way they happen to interact with stuff that happens to be laying around, like electrons.

At the edge of space the air molecules are so widely spaced that when an electron gets knocked loose it can take a long time to find its way back home. At the highest levels there are permanent "clouds" of electrons and electron-short molecules. Closer to earth similar clouds form in daylight but turn back to regular air after sunset. The outer most cloud is called the F layer. The inner, daytime only cloud is known as the D layer.

A radio wave moves the electrons in these clouds too. If the wave is alternating at a very high frequency then they electrons don't move enough to change the direction of propagation. A radio signal from earth will head out into space. At a high frequency the cloud's electrons move enough to get their own fields headed back to earth (they are delayed enough by the mass of the electron to change the direction of propagation). At a medium frequency the induced electron movement is enough to cause molecular collisions in the D layer, the lower cloud. This ruins the effect since the collisions absorb energy that would otherwise be reflected. Medium frequencies are reflected by the F layer, the sparse permanent clouds at night, after the D layer has disappeared.

CB radio bounces off the F layer, especially when it is at it's thickest during a solar maximum. FM radio and TV are very high frequency and go right out into space. AM radio is transmitted at a medium frequency that will skip off of the F layer at night when it can avoid being absorbed by the D layer first. Hams use radios that can transmit on lots of different frequencies. They are masters at choosing the right frequency to skip their signals to distant cities.

In a way my hundred watt transmitter is just like a hundred watt light bulb except that the frequency of radiation has been selected to bounce off of the right cloud. Imagine putting a light bulb on your roof and expecting someone in a distant city to notice its reflection on a nighttime cloud.

My radio has two advantages over the light bulb. First the F cloud is a better reflector than the average cumulous, and second, my radio concentrates all of the energy into an extremely narrow portion of the spectrum overpowering what little noise falls in there. The receiver "knows" something about my signal that helps it distinguish it from noise. Likewise, the receiver of a spread spectrum signal "knows" something else (the pseudo-random code) that lets it separate the desired signal from uncorrelated noise. The trouble with a light bulb is that it both broadband and incoherent. There is nothing to know that would tell it from any other light in the sky.

My brother once fixed one of those optical doorbells for a friend by making it flash at a few kilohertz. With the appropriate filter in the receiver it would work over a hundred feet in broad daylight. Without the extra "knowledge" in the system it wouldn't work over the width of the door.

I see I've strayed a bit from my old EE fields class. I don't know what you want to use out of this in your lecture. But this is the stuff that makes me still like radio.

Date: Wed, 04 Dec 1996 21:38:16 -0800
From: James J. Besemer
To: Ward Cunningham

Great! Fascinating! And, yes, very helpful. Thanks.

Some follow-up questions...

So, if I'm getting this, what's really going on is that the transmitting antenna projects a magnetic field, same as any current through any wire. This magnetic field induces a current in the receiving antenna, again same as any wire by a magnet. SO in a sense, the antenna pairs work vaguely like the coils in a transformer (whereas I thought it was something completely different effect). The big difference is that the high frequency allows the field to effectively project over vast distances. Whereas at low frequencies we're more concerned with efficient coupling and power transfer.

Is this right?

This is good because I was going to cover basic electricity and magnetism, motors, generators and transformers early-on. That gives a natural spring-board for radio. Cool.

This is analogous to but unrelated to regular clouds, right? In the past when people talked about cloud skip, I thought they meant against real clouds. Or is the latter also a possibility?

And I can see why!

Date: Thu, 05 Dec 1996 14:04:29 -0800
From: Ward Cunningham
To: James J. Besemer

Yes, it's that we are dealing with a system where propagation effects are interesting, not that it is the only thing that propagates. A direct current will set up a B field that falls of by the inverse square too. But it's effects will be swamped by the earth's magnetic field, kinda like a light bulb in the sun.

There are a lot of wonderful ideas in play here, like forces, fields, atoms, amplifiers, resonance, reflection, propagation and detection. Radio needs all of them to work. Each will take a long time to settle into a kid's mind. That's why we must start early and be patient. (And we must remember that we are taking Maxwell's view, not Feynman's. The latter is yet to settle in my own mind.)

I've never heard of cloud skip. That doesn't mean it doesn't happen.

I sometimes wonder if I could image ionospheric "clouds" so that I could watch them come and go as I do regular clouds. I would need a narrow beam antenna to scan the sky in a raster pattern which I would record in time-lapse. I could scan several bands to get pseudocolor. The hundred megawatt international broadcasters would provide illumination.

There is an effect observed in VHF called tropospheric ducting. Variations in atmospheric density bend a signal along the curve of the earth much like light is bent in an optical fiber. Hams in Hawaii notice it several times a year when the powerful and busy 2 m repeaters in California start breaking squelch on that quiet island.

Ducts are harder to notice on the mainland because there are so many signals already. The only one I ever worked was from my car in the paring lot of my apartment back in college. It spanned a few hundred miles and lasted about ten minutes. I didn't budge until it was over.

Hams really are fanatics about unusual "openings". I'm most impressed with the guys that work skip off of meteor tails. It's a regular happening every summer during the showers. Contacts last a few seconds.

© ward@c2.com