GADGETEER

What I Bought at the Kempton Park Hamfest (Rally)

2006-11-25T09:54:00.000Z

The blue box is the power supply. The yellow box (tobacco tin) houses a broadband amp and seems to foreshadow the use of Altoid tins. The Black box is an old signal generator. I bought it because the model is "Windsor" (I like the town of that name). It was manufactured in Slough, Bucks, UK. You can also see the Hi-Z headphones, the PC boards, 73 magazine, and the RSGB's Handbook (a very good book!)

Dipole visible from space!

2006-11-23T08:39:00.000Z

I was surprised to be able to see my fishing pole dipole on Google Earth. I realize that this is probably a shot taken from an aircraft (not from space).

Should Class C Amps Saturate?

2006-11-10T11:06:00.000Z

I have been trying to figure this out and have been posting the following questions on rec.radio.amateur.homebrew. If anyone out there can help, please send me some comments or e-mail. Thanks, Bill

FIRST POST
I'm reading David Rutledge's excellent "The Electronics of Radio." In Chapter 10 -- Power Amplifiers, he discusses Class C amps and says, "In addition, if we drive the transistor clear to saturation, using the
transistor as a switch, the dissipated power can be greatly reduced, because the saturation voltage is low. This is Class C amplification..."
I'd always throught that in Class C, while you'd operate the device so that it was cutoff during most of the cycle, but not saturated. Is this just a different definition of Class C? I checked back with SSDRA and EMRFD, and didn't see anything about driving Class C amps into saturation? What says the group? Do we saturate in Class C or not? -------------------------------------------------------- SECOND POST
I've been thinking about this some more. The 1980 ARRL handbook points out that "Solid State power amplifiers should be operated just below their saturation points for best efficiency and stability." Also, the formula that we use to determine load resistance (Rl=Vcc^2/2Po) implies that we are looking for a combination of Vcc, Load resistance and power out that will prevent saturation. And wouldn't we end up with far lower harmonic content if we only clip one side of the wave form (at cutoff) instead of both sides (cutoff and saturation)?
I know there are more exotic modes beyond C, but for plain old ordinary ham radio applications, don't we normally avoid saturation in Class C amps?
Also, what about this business of having the efficiency improve through saturation "because the saturation voltage is low" Could that be right? If you put a voltage across a conductor and generate a large current, you can't sit back and say "Great! Power consumption across the conductor is low because the voltage drop across it is now minimal!"
THIRD POST
Thanks to all who responded. I think I'm starting to understand this. LTSpice helps a lot. I set up a class C amp and looked out power dissipated in the transistor vs. power dissipated in the load. The big efficiency gains that come with saturation were very apparent.
To better understand WHY this happens, I set up a spreadsheet that looked at power dissipated in a variable resistor as it swept from .1 ohms to 10 ohms. It had a fixed 10 ohm resistor in series and 10 volts DC across both of them. Yes indeed, the power dissipated in the variable resistor drops off dramatically when the resistance (and hence the voltage across it) gets very low. I guess is why this happens in the saturating Class C amp, Right?
But I still have some questions. When we design a Class A amp, the familiar formula Rload = (Vcc-Ve)^2/2Pout allows us to come up with a load value that will prevent the amplifier from saturating. A load of this value will cause the voltage across the transistor (collector to emitter) to vary from zero to twice Vcc. But it won't go into saturation.
Why then do so many of the books (EMRFD, SSDRA, the W1FB books) call for the use of essentially the same formula for the load when selecting a load for Class C amps? We're no longer worried about staying out of saturation, correct? In fact, we want to saturate. So why the same formula? In fact, it seems to me that if you have a Class C amplifier that is designed with this formula and is operating just below saturation, you can get it to saturate just by increasing the value of the load presented to the collector. Power out and efficiency immediately improves. Linearity, of course, does not.
Thanks, 73 Bill M0HBR http://www.gadgeteer.us

My amp design efforts

2006-10-29T06:59:00.000Z

24 OCTOBER 2006

HOW TO DESIGN AN AMPLIFIER
(CLASS A, COMMON EMITTER, NO FEEDBACK)

You have to keep in mind the differences between input (DC) power and output (AC) power. And throughout you have to keep in mind the differences between peak, average, and rms values. Remember that when we talk about power, we are talking about AVERAGE power: Vrms*Irms. Or (Vpeak*Ipeak)/2

I have an oscillator/amplifier circuit that produces .050 watts in a 50 ohm load. I want to build an amplifier stage that will increase power out to .5 Watts.

FOR DC BIAS:

Assume 20 percent efficiency. So I will need 2.5 Watts DC input.
Assume 11 volts on the collector and 3 volts bias on the emitter.
So there is 8 volts across the transistor.
For 2.5 watts DC input I’ll need 312 ma of idle current. (8*.3125=2.5 watts)
With 3 volts on the emitter, that corresponds to an emitter resistor of 9.6 ohms (bypassed)
To get the 3 volts on the emitter, you need approximately 3.6 volts on the base. (Because the silicon diode that forms the base-emitter junction drops .6 volts.) Use a simple resistor voltage divider network to get this.

INPUT (BASE) CIRCUIT

The 312 milliamps of idle current results in an emitter resistance of .0833 ohms (from the formula 26/312ma.)
To get the input impedance, you take this value and multiply it by (Beta+1) Beta= ft/f
250/14 = 17.8
So 18.8*.0833=1.566 ohms input impedance.

A transformer with a turns ratio of 5.6:1 will yield a 50 ohm input.

Using the 4X rule of thumb, I took the input impedance 1.6ohms * 4 and got 6.4 ohms. At 14Mhz that is .0727 uH for the secondary, .4072 uH for the primary.

OUTPUT (COLLECTOR) CIRCUIT

Remember that rms volts X rms amperes = AVERAGE power.
And Peak volts X Peak current = 2 X AVERAGE POWER

To determine necessary load: Rload= (Vcc-Ve)^2/2Pout

This formula derives from the standard formulas linking power calculations to ohms law: P= E^2/R. Vcc-Ve represents the highest possible peak voltage. If you have only 8 volts between the collector and the emitter, to avoid voltage flat-topping, you can’t have a load that will, at peak signal current, drop more than 8 volts. But the power equations need rms values. In the numerator, you are squaring the peak voltage value. Assume that the peak voltage was 1. 1^2=1 Now, take the peak voltage value down to rms: 1*.707 = .707 I you square .707 you get .5 volts. So, putting the 2 in the denominator of this equation is just simple way of converting the peak voltage value to an rms value needed by the P=E^2/R formula.

So: .5 = (11-3)^2/2(x) x= 64 ohms

Play with this a bit to see what it means.
8Vpeak * Ipeak = 2*(.5) (What peak current multiplied by 8 volts peak voltage will give you 1 watt?) Remember: Peak volts * Peak current = 2*Average Power out.
So Ipeak = .125 amps.

At peak, what resistance would drop 8V at .125 amps? Ohms law: 8/.125 = 64 ohms.

Now, drop the peak voltage and current values down to rms.
8*.707=5.56V rms
.125*.707= .088Amps rms

5.56V*.088A= .5 watts

.5 watts in 64 ohms implies .088 amps (rms) (P=I^2R)
.5 watts in 64 ohms yields 5.64 volts (rms) (P=E^2/R)

Now, convert these current and voltage figures from rms back up to peak to see how it all fits together:

5.64 V * 1.414 = 8 volts. That is the max maximum possible voltage drop (that would drop collector voltage from 11 down to 3).

.088 * 1.414 = .125 amps. .125 amps flowing through 64 ohms drops… 8 volts.

Another way of looking at this would be this. You have 8 volts peak to work with. You are saying you want .5 watts out. Since P=IE and I = E/R, this means you are calling for a certain impedance and a corresponding value for current. Take the volts down to rms.
.5 = 5.656I I=.088 amps rms. 125 ma peak

????Now, let’s take a look at the efficiency of this stage. Assume we put the idle current at 125ma. You have 8 volts across the transistor. That’s 1 watt DC input. 50 percent efficiency. This also works if you look at input power of one complete cycle of the signal. Voltage (collector to emitter) is varying from 8, up to 16, down to 0. Current is varying from 125 ma, up to 250, down to zero. So average current is .125 Average Voltage is 8 power input is 1 watt. ????/

So, to make my 50ohm antenna system look like 64 ohms. I use the formula
Np/Ns = square root Zp/Zs. That yields a turns ratio of 1.131:1.

Using the rule of thumb of making the Z = 4*Z of smaller coil, Z=200
At 14 Mhz this is 2.27 uH for the 50 ohm secondary, 2.567 uH on the primary.

Steve Weber's COOL 25 Watt Linear Amplifier

2006-08-19T07:00:00.000Z

I was looking at Steve "Melt Solder" Weber's (fairly) simple SSB rig. I really liked the 25 Watt linear he built for it. Take a look at his technique for mounting the MOSFETS on the big finned heat sink: he just cut holes in the PC board and, through these holes mounted the MOSFETS on the heatsink. Then the rest of the circuit is mounted on the board using our traditional ugly/Manhattan techniques. http://kd1jv.qrpradio.com/ssbrig2/SSB2.HTM

The Very First Homebrewers -- Hominid HB!

2006-08-11T07:00:00.000Z

I'm reading "A Short History of Nearly Everything" by Bill Bryson. In his description of early toolmaking, I came across some passages that I think will resonate with modern electronic homebrewers:

"Sometime about a million and a half years ago, some forgotten genius of the hominid world did an unexpected thing. He (or very possibly she) took one stone and carefully used it to shape another. The result was a simple teardrop-shaped hand axe, but it was the worlds first piece of advanced technology.

It was so superior to existing tools that soon others were following the inventor's lead and making hand axes of their own. Eventually whole societies existed that seemed to do nothing else. 'They made them in their thousands,' says Ian Tattersal... It's strange because they are quite intensive objects to make. It was as if they made them for the sheer pleasure of it.'

...The axes became known as Acheulean tools.... These early Homo sapiens loved their Acheulean tools... They carried them vast distances. Sometimes they even took unshaped rocks with them to make into tools later on. They were, in a word, devoted to the technology."

Sound familiar?

Watch Richard Feynman talk about radio waves!

2006-08-05T10:37:00.000Z

I'm a big fan of Richard Feynman. I've been reading his books for years, but until I came across this YouTube video clip, I'd never heard his voice or seen a film of him in action.

I was struck by his accent. Brooklyn comes through very clearly. It is a great thing that humble, working class origins didn't prevent Feynman from achieving what he did.

Check it out:

http://www.youtube.com/watch?v=83Dj9zAJsbs

Pegasus UK High Altitude Balloon Project

2006-06-21T04:45:00.000Z

I found this on the Make blog. An amateur high altitude balloon project. I always wanted to do this. http://www.srcf.ucam.org/~jac208/pegasus/

I was surprised that this kind of technical adventure was authorized. That's great! But sure enough, the rules (sadly) prevented them from using amateur radio for the balloon to ground link.

Three cheers for the Pegasus crew!

London Shack Photo

2006-06-18T05:56:00.000Z

Here is my London shack. This is also the UK studio of the world-famous SolderSmoke podcast (our other studio is in Juneau, Alaska).

The Drake 2-B has center stage. Above it is a homebrew 40 meter DSB transmitter, with an outboard MOSFET linear amp (based on a Ramsey kit) in the small box to the left.

On the bench to the left of the 2-BQ is my 17 meter SSB rig (TX and separate RX).

The SolderSmoke mailbag gong is visible on the floor under radios. The Estes Rocketcam is on the table, as is a small homebrew telescope (made from broken binoculars). My Heath HW-8 is also there.

In the window on the right you can see my two Volkswagen solar panels. They are connected to a 7 amp-hour gel cell battery, which powers the HW-8.

On the wall behind the radios is a poster from the UK's "Beagle 2" mission to Mars .

Cartoon about Podcasts

2006-05-23T04:34:00.000Z

One of our SolderSmoke listeners sent us this, and we really got a kick out of it.

India's Amateur Radio Satellite

2006-05-21T08:31:00.000Z

Steve Ford's column in the March issue of QST got me interested in a new ham radio satellite. Currently called Oscar 52 aka HAMSAT or VU-OSCAR 52, this satellite was built and launched in India. In low earth orbit, it is a CW/SSB repeater in the sky. The uplink is in the 70 cm. band, and the downlink is on 2 meters.

I already had a 2 meter loop antenna on my roof. Rummaging around the shack I soon put together a receive system: A Hamtronics 2 meter to 10 meter converter would feed into my trusty Sony SW7600GR travel radio.

The excellent "Heavens Above" website (http://www.heavens-above.com/) quickly gave me the times for the next pass. At 2239 BST on 20 May, right on cue the signals started pouring out of my little receiver. I quickly heard EA1FYM and EA7HG on USB, and DL1SMG on CW. Doppler shift seemed a bit more prounounced that what I had experienced with the RS10 and RS12 Russian sats.

It was great to hear this bird. Three cheers for the Indians who made this happen. Really impressive.

Linus Torvalds, Gadgeteer

2006-05-20T05:19:00.000Z

Slashdot led me today to an interesting CNN.com interview with Open Source leader Linus Torvalds. There were a couple of lines in the article that will resonate with Gadgeteers everwhere:

Linus Torvalds: A lot of the core people just feel excited about the technology. And that's why a lot of people just start; that's where I started from, it was just the excitement of doing something yourself. It's kind of like a hobby. You can tinker with cars, you can tinker with computers. There are a lot of technical issues that are just very exciting if you're that kind of person...

Kristie Lu Stout: Do you think there was a little bit of bragging involved?
LT: Absolutely. There was a bit of bragging, there was also a bit of, hey, I still, the way I do my work is I sit these days downstairs in my basement alone. And it's nice to just talk to people and a lot of it was probably just social, just saying, hey this is a way to interact with other geeks who are probably also socially inadequate in many ways.

Greetings Gadgeteers, welcome to The Shack

2006-05-16T05:02:00.000Z

Welcome to GADGETEER, a new blog for electronic homebrewers.

In an attic room in London, I maintain a small, secretive electronic workshop. I keep it quiet because it would definitely raise eyebrows among neighbors and office colleagues... "Oh, a workshop... what on earth FOR?" My kids occasionally blow my cover, and let slip that we have a room in the house known as "The Shack" ( a term derived from the traditional location for ham radio stations -- a small shack in the backyard). At first, the kids seemed surprised that their classmates didn't have shacks in their houses too.

The Shack is a moveable feast, and this is its fifth location. Originally established in Congers, N.Y. during the 1970s, my shack has also been set up in Santo Domingo, Dominican Republic (1992-1996); Falls Church, Virginia (1996-2000); and Sao Miguel island in the Azores (2000-2003). It has been in London since 2003. It is currently in a small room (maybe 10 feet x 12 feet) with windows that look southwest and that allow me to see the planes going into Heathrow Airport. The shack features a computer desk, a radio station table built from an old packing crate, a really excellent six foot long wooden workbench that my wife got me for Fathers Day, two book shelves holding technical manuals and boxes of spare parts, and LOTS of radio junk and memorabilia. It is getting a bit crowded in here!

I hope to use this blog to discuss the projects that are underway in the shack. I see this space as an adjunct to the SolderSmoke podcast that Mike, KL7R, and I have been producing for the last six months. See: http://www.gadgeteer.us

So again, welcome Gadgeteers. Welcome to the shack.