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Monday, 29 June 2015

Potato Polarity Test

Here's another little gem from "Radio, Television and Electrical Repairs (Illustrated)".
Does this actually work?!

Well yes it does! - I found a small piece of potato from the kitchen waste bin, and poked two stiff copper wires into it. The chapter in the book was talking about lead-acid battery chargers, so I applied 12 Volts from the bench power supply and very quickly heard a fizzing sound and saw a halo of green appear around the positive connection. Looking more closely at the the negative wire I could see small bubbles appearing. I assume the green colour is some kind of electro-chemical reaction taking place with the liquid in the potato to form a green copper compound.

Probably not a good idea to eat the potato after you have turned it green with copper!
As for a "quick and useful" method of testing polarity ... I think I will stick to the test meter for now.


Sunday, 28 June 2015

Fitting the SPF Amplifier in the 23cms BiQuad antenna

Today I installed the SPF amplifier in the dicast box on the back of the BiQuad antenna.

The new amp is MUCH smaller than the old one. Since I want to run the amp from a 12V supply I added the recommended 180 ohm series resistor, however I found that the Voltage across the amp was heading for over 5V once the supply exceeded 12V, so I  upped the value to 220 ohms to be on the safe side. The instructions contain warning about not letting the voltage on the drain of the chip exceed 5V. I'm not happy about the pig-tail on the coax going to the F-connector, but I can't think of a better way of terminating it yet.
At the other end, I kept the ends of the coax as short as possible at the feedpoint of the biquad.

Hopefully, with the nice weather, I might get a chance to try the aerial out this week.
73 Hugh

Friday, 26 June 2015

DXTV via Sporadic E on 50MHz

Nice strong signals from Europe on the 50MHz band this afternoon, I worked:
Jose, EA1HMP, in Spain,
Pinto CT1ANO, from Rio Tinto, Portugal
Uwe CT7AIU, in Portugal, and
Juha - using the OH5C Radio Club callsign in Finland.
But there was a lot of TV buzz. So I hastily connected up my old Plustron 5" portable and got some pictures on the channel marked "3" on the dial  ... but where are the pictures from? Is that Greek text? very few countries broadcast TV on VHF anymore.

All good fun - I haven't received any DXTV via sporadic E for ages ...

Sunday, 21 June 2015

GB1HA - Museums On The Air at Headcorn Airfield

Today we visited the Weald Radio Club at Headcorn Airfield as they operated their special event station GB1HA. Headcorn is the home of the Lashenden Air Warfare Museum, which is a good excuse to activate a station over the weekend. Tony, G4IMP has a 5 element beam for 6m, an ingenious multiband vertical for HF and a longwire which reaches over to the pole with the (ahem) wind-sock on it.

Here is Tony operating 6m (other operators are available). 6m was open to Europe with Sporadic E providing contacts to Spain, Portugal, Poland, and even Russia. There was a lot of contest traffic.

Plenty of light aircraft taking off and landing and the Sky-divers were making frequent flights in their Cessna Caravan.

There were even a couple of buzzards displaying although they weren't calling on the ATC frequency, and they didn't seem to like the parachutes.

73 Hugh

Wednesday, 17 June 2015

Some old radio and electronics books contain beautifully drawn illustrations. Often the artists are uncredited. The images are strangely evocative of a suburban era which has passed. I which I had more shelf space to collect these old books. Here's a picture from "Radio Television and Electrical Repairs (Illustrated)" which is edited by Roy C. Norris, Technical Editor of "Electrical and Radio Trading" magazine. The publishing date is not given, but a book plate indicates 1949. This is an illustration which might help those who are struggling to get rid of interfering "noises" on their radio reception!

73 Hugh

Tuesday, 16 June 2015

Four Meters open to Malta

Just worked 9H1BT on 70.200. Also heard 9H1xT. The skip was very short on 6m, - there was even a French station - that is a usually a sign that the MUF will support higher bands. While 6m was bedlam, 9H1BT was calling and calling on 4m and not getting many replies. Also heard 9H1XT working a UK station, but didn't manage to get him myself.

Sunday, 14 June 2015

Day out at Dungeness

So we've trundled out to Dungeness on the little railway ...
Oh dear looks like rain - better get back to the train.
No walk along the shingle today.
Still sunny out in the Channel.
Winston Churchill at Romney Sands Station.
New Romney Station

Thursday, 11 June 2015

Happy Hour on Six Meters

After a number of quiet days, the Six meter band was busy tonight. I worked:
CT1AVR Manuel, in Portugal,
S59A Drago in Slovenia
TK4LS Laurent on the Island of Corsica
OH1UM Pasi in Finland
OH1Z Juha in Finland
OH2FNR Jake in Finland
YT5TAS Nebojsa Serbia
IK0VWO Giovanni Italy
SM5KWU Hannu Sweden.

Back to the ATV

I got my Amateur TV folder down from the loft, last night. It contains lots of potentially useful magazine articles and datasheets that I salted away when I was trying to get active in Amateur TV ...erm ... 20 years ago!
The original design for the Twin-Quad, Bi-quad or "Bowtie" antenna that I built, is there, and I have scanned it and put it on Google Drive so that I can make it available again. Not sure about Copyright on this, I think the document came from the Kent Television group, but it looks like an extract from another publication. The original design seems to have come from DJ9HO. Anyway, if anyone objects to this being here, post a comment and I will take it down. On page 5 is a diagram showing how my version has the preamplifier mounted on the back, and it is this that I intend to replace with the SPF amplifier that I blogged about a few days ago. I found a very good article, on line, describing how to build a bi-quad antenna for WiFi use - i.e. 2.5GHz. There may be some useful design tips that could be incorporated into the 23cm version. The Author also offers a double biquad design, which offers 13dB of gain. 73 Hugh

Wednesday, 10 June 2015

Trimmer Capacitors

Today I have been looking at the trimmer capacitors that I bought at the Eastbourne Radio Rally. The bags of "10 for £2" are all marked with their values, which is good. I did check them with a capacitance meter, and they measured close to what was indicated.
Need to build some nice tuned circuits now!
I like the miniature air-spaced ones, they have a certain old-world charm, and presumably give quite high Q factor tuned circuits. The vanes are vulnerable to getting bent, but you can see clearly how much the vanes are meshed and estimate the capacitance from that. The ceramic ones are harder to "read".
I bought two of the "piston" style trimmers. They looks to be very low value pF, and can't really be measured on the LCR meter. I guess they are about 1 to 5pF, but I can't find anything quite like them on the internet. The threaded screw is wound down into the ceramic tube to adjust the value. I think I have seen similar components on UHF TV tuners in the days of analogue TV.

Saturday, 6 June 2015

SERFing at Eastbourne

Just got back from the Eastbourne Radio Rally, AKA Sussex Electronics and Radio Fair. Thanks to Peter G8WMZ for doing all the driving. Plenty of stalls, and a quite a lot of visitors, but I suspect some of the stall holders would have wished for more. I don't much like crowds to it was nice not to be jostled as one looked at the goodies. Nice bacon butties and even decaff coffee.
Peter G8WMZ and John G0GCQ looking at Peter's desk mike over a cup of coffee. Some of the stands at the show, which was in a large sports hall.
I came away with aluminium project boxes, trimmer capacitors, some very fine tweezers for surface mount assembly and a nice hand-magnifier. Also bought some books, and a Bench Power Supply for £10.
The bench power supply is a "Maplin Gold" one and seems to work ok. The meter scales are rather faded, and the mains lead is damaged and needs replacing. Also it was full of pet hair! The current limit is a bit strange, it makes a bit of a whistling noise, so i may need to check the output on the scope to make sure it is not oscillating. But a small supply like this was on my "shopping list" as it is a piece of test equipment that I didn't have. All in all a good day out. 73 Hugh

Thursday, 4 June 2015

Cycle to work and back, plus a bit extra

Hmmm... soldering is a bit blobby, but all the bits are on the board. I find 0603 components are about as small as I can do with a pair of tweezers and a soldering iron.
I'll clean off the flux after I've soldered on the coax cable. I'm going to look for some miniature PTFE 50 ohm coax at the radio fair on Saturday, but I have some 75 ohm UR111 which will probably do. It will, after all, be feeding a length of RG6 75 ohm coax to the satellite receiver. 73 Hugh

Wednesday, 3 June 2015

Here's a photo of the old Minicircuits MAR-6 amplifier which I built ages ago.
The actual amplifier is the little black dot in the middle. The p.c.b. is etched with a hacksaw. I used a junior hacksaw to cut gaps in the copper. The width of the strips do approximate to 50 ohm stripline. It may be crude but it worked well enough to receive TV pictures from G6EXU about 7 miles away, using a modified Amstrad SRX100 Satellite receiver. 73 Hugh
Today the SPF Amplifier kit arrived from Sam Jewel G4DDK. It is all very small and will easily fit in the space currently occupied by the Minicircuits MAR-6 amplifier on the back of the BiQuad antenna.
Sam has cleverly colour coded the surface mount parts so they are easily identifiable on the parts list.... that is until you take them out of the cardboard strip. Better put them on the board one at a time. The SPF amp has a noise figure of 0.8dB which compares very favourably with the Minicircuits MAR-6 which is about 2dB. Sam has sensibly advised a filter between the antenna and the amp as the amplifier is very wideband. However, since the MAR-6 worked ok, I think I shall try it without, and see how we get on. I now need to find some miniature PTFE coax to connect to this. 73 Hugh