This Article has been taken from the Southgate Radio Club Web-Site,
http://www.southgatearc.org
with their permission.

Roger,  G3TDM

The Great ERG Race 2003

Another exciting Erg Race is over and, although what we had to make in the end - a MW or LW receiver - seemed easy, the box of goodies didn't quite contain what we all would have wished for.

There were 5 teams this year:

1) Verulam team 1: M3EDR, M0WRT, G8ATO
2) Verulam team 2: G8MFH, G0SNO, G8VRO
3) Southgate team 1: G8RPA, G0IDA, G0ESF
4) Southgate team 2: G4DFB, G0DZZ, G4YGH
5) ATC team: Dave Sharp, G8BCQ, G7VRT

As a reminder this was the scenario:

"Whilst flying off on a DXpedition, your plane crashes in a remote region. Miraculously you are unhurt, but the plane is wrecked and the cargo is completely destroyed - including all your precious amateur radio equipment. Whilst you are confident that a rescue effort has been launched, the fact that the plane was off course before the crash leads you to believe that the rescuers are searching for you in the wrong place. What you want to do is listen to broadcast stations, to discover from news bulletins where the plane is thought to have crashed and thus have an idea of which direction to go to have a better chance of being found."

The mission:

Using only the materials recovered from the plane, the crash site, plus the aircraft radio (all conveniently supplied for you), you need to construct an AM broadcast receiver.

I will write up on the team which I was in.

Keith G8RPA, John G0ESF and myself got our hands on the box like a bunch of kids on Christmas day and unravelled the present to find inside:

The ATC team get their coilwinding into top-gear

An empty toilet roll tube,
2 x 2m of wire
A defunked PMR radio
Potato (I guess for cooking later on)
A lump of coal (to keep us warm of course)
6 plastic pint cups
Strips of aluminium foil
A coil of thick copper wire
8 Ohm speaker
A very flat 9V battery
A sachet of vinegar
6 sachets of salt
2 bottles of water
Small plank of wood and some nails.

A circuit diagram of the PMR radio and a copy of the RSGB handbook was supplied and was located on a different table where all teams could have a look, but not take it away with them.

Our team got to work and we had to decide what sort of a radio we were going to build.
We first of all measured the voltage of the 9V battery only to find out that it was in fact a very good 1.5V battery. I was quite upset and felt done by that Steve gave us a duff battery so I went up to him to ask for a new one as this one was obviously flat only hear the reply that it is supposed to be flat L ….oh well!

The SARC team's working receiver. Note the power supply on the left

This meant making a battery from the cups, foil, copper wire and salt water. Before making the battery we went over to have a look at the circuit diagram to see if we could convert the PMR radio into something that would receive MW. After some time of debating what was going to be best we soon realised that realistically we are not going to be able to power any circuit of the PMR radio so we opted to make a crystal set.

Sounds easy doesn't it………..not without a germanium diode it isn't ! There wasn't one to be seen anywhere on the board, we searched and searched and then we had a look again but no diode.

We got our thoughts together and came up with a circuit diagram. In essence it was a crystal set but with one transistor as the audio amp and we used a silicon diode which was forward biased which meant that it was already on and needed no energy from the MW transmission to turn it on. We used a 47uH choke from the PMR radio and about 200pF of capacitance to resonate it which in the end gave us a station that we could hear very clearly. We used the microphone insert as the speaker as it was higher impedance than the 8R speaker and we seriesed up 4 cups of salt water, foil and copper as our battery to give us around 2.8 volts off load, which powered the transistor very nicely. We found it dropped to 1.5V on load.

Yes! It's a transistor

We worked well as a team and this was a key to getting the project done in under the 2 hrs which we were given. The work as divided up amongst us, John made the salt water batteries after discovering what was inside a 9V battery, Keith was swiftly whipping out needed components for our radio set from the PMR radio, unwinding strands of wire for the antenna and grounding system and I was soldering the components onto nails which I had banged in on the plank of wood.

After about an hour and 20 min we connected the battery to the bundle of components on the board and actually got a weak sound out of our speaker…what joy !!! I then changed a few values and got an even louder response in the speaker and we all decided that it was good enough. I soldered on a few more capacitors onto the choke which then tuned in a station very well and we left it at that. The grounding system was refined and connected to the radiator which made the volume unbearable in the speaker so we put the earth wire through a 47pF capacitor and that cured the problem and we were able to once again hear the one station clearly.

Although points would have been awarded if we could tune different stations, we decided that this would be a one station receiver. It just happened that we were the only ones to complete the task out of all the teams and make it work within the time given and hence we won the Erg Race.

There were various points awarded to everyone for selectivity, sensitivity, construction, etc…….. but the main issue was to receive at least something. In some respects it's a shame that all those hams with all that knowledge were not able to construct a crystal set and you don't get any simpler than that.

Steve White's (G3ZVW) working 1-transistor radio circuit

I would like to say thanks to the judges: Nick, G8DWF (Southgate) and Ralph, G1BSZ (Verulam) and especially Steve White G3ZVW for organising the boxes of goodies, scenario and event which ran very smoothly.

Steve White proves that it can be done

A sample of a working one transistor radio was produced by G3ZVW just to prove that it can be done with what was supplied in the box.

Steve's transistor demodulated the AM signal and amplified the audio and it was surprising to see just how few components Steve had used.

I would like to finish off by saying a big thanks to Keith and John for the splendid team effort which made it possible for us to win.

73, Berni G0IDA

Judges' notes:

Team 5 allowed a member to depart for a leak, wasting valuable electrolyte.
Team 1 used a transistor with ferrite beads on each leg.
Team 2 discovered variable capacitors and used a chassis.
Team 5 placed an aerial outside (the wire on the relay coil stretched as far as the next building!)
Team 4 made early provision of an earth and spotted the possibility of a ferrite ring to increase L.
All teams (except Team 1) knew what they were going to use as a detector.
By the half way stage, all teams (except Team 5) had documentation.