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October 2015 - Simple QRP VSWR + Tuner.
Cheap kit of parts that seems to do the job.
Now that I am getting more into portable operation on the HF bands I found the in built VSWR meter of my FT 817 was not showing the VSWR as I expected. Not having an external meter that worked at less than 10 watts I decided a simple LED tuning indicator would be a better option. Checking through my stock of resistors and junk box I had every possible part except the ones needed so it was down to buying them. A quick check on my favourite auction site and I had found not just the parts for the indicator but a kit that included a tuner as well and at a lower price to boot. The kit cost £5.99 inc free p & p but didn't include any instructions but these are readily available on the net. It proved to be a relatively simple task to put it all together but there were a couple of minor problems. The pcb was screen printed with the component positions but were incorrect. The number of turns given for the FT37 - 43 toroid was 2 : 5 turns and on firing up the 817 the LED didn't even faintly glow. I looked up Dan Tayloe's design on which this kit is based and that showed the windings as being 5 : 25 so a little re-winding and soldering resulted in a nicely glowing LED at 0.5 to 5w.
Having finished the build I gave it a quick test on 20m using an inverted L and made a 59 contact from Bulgaria. Some build notes and circuit diagram, QRP_ATU.pdf.
I would suggest the following improvements:-
Rather than mount the caps and switch through the box fix these to an aluminium plate inside (there's room). This will act as a shield when trying to tune as without its a little sensitive to body parts, it will also mean the knobs will fit better and look neater. I've also fitted an extra toggle switch so I can switch the tuner section in and out to use just the LED bridge.
July 2015 - Light weight 6 Element 2m Yagi.
Following my first attempt at the RSGB Backpacker contest in July I decided to have a go at building a lighter weight portable Yagi. I was very pleased with the outcome and will be using it on the next contest. The theoretical gain is 11.2dBi and is allowing me to work repeaters over 50 miles distant with 2.5w all at a cost of around £8. Click the [button] below for full details.
Protecting a voltage sensitive rig in the car.
You may have picked up from my site that I have a Chinese TH 9800 quad bander in the car. There are some issues with these rigs in that some of the transistors used are very voltage sensitive ie 16v maximum. Assuming the rig is wired directly to the battery terminals suitable fused in both leads it is possible for the alternator voltage to rise to levels for several seconds when the engine starts and this could blow these sensitive transistors. High voltages can also appear when turning the engine off due to the collapsing magnetic field. You may think that turning the rig on only after starting the engine and off before the engine will protect the circuits but that may not be the case. Many modern rigs do not have actual power switches but rely on software to do the switching so whether the rig is switched on or off it can be subjected to voltage spikes. In reality how often would you switch the rig to safe guard it from the above, most times I hear you say so it may just be that time when Mr Fate catches you and blows your rig.
Well there's a simple answer to this, I fitted a low cost time delay board for around £2 which can be set to switch a power relay automatically on powering up the board. With that wired into the supply feed now when I start the engine the rig does not come on until around 4 seconds later so allowing time for the alternator voltage to stabilise so eliminating the chance of high voltages damaging my rig. There is still the risk of collapsing EMF on turning the engine off so I have also added several super caps across the battery and those have reduced that issue. Total cost for board and super caps was around £18.00. The caps are also supposed to improve fuel economy by keeping the battery at a more stable voltage which it does the ECU is also supposed to be more stable so control the engine to a more efficient state so save fuel, I'm not yet convinced about that but time will tell.
SDR Build.
I've been working on a Softrock Ensemble SDR reciever to cover the HF bands that are below my RTL dongle's limit of 22Mhz. So far I have the 12v power supply wired up and working and the next job is fitting the 3.3v regulator for the USB interface. Big problem the regulator is more like a speck of dust with 4 solder pads and even with a magnifying lamp its so small and the fine tipped soldering iron seem massive. It's my first attempt at working with surface mount and its certainly a challenge.
Flower pot 2m / 70cms antenna.
My first ham radio was a Boafeng UV-5R and the stock aerials are not the most efficient so I decided to build a flower pot antenna for 2m / 70cms based on John Bishop's (VK2ZOI) design. I have used a 2m length of plastic conduit to give a small amount of elevation for ground mounting. I have used the dimensions exactly as below which has resulted in a VSWR of 1.3:1 or better across the entire 2m band all for a total cost of under £3.00.
The flower pot mounted on a 25mm stainless steel pole.
My back garden is not overly large so stringing up a wire to cover 10 - 80m has been a challenge. I looked at several options, loops, delta but I have settled on an inverted L design as detailed here.
The house end is attached to a nylon line to allow it to be hoisted up and down and the far end is supported by an 8m fibre glass fishing poll. The antenna is fed through RG58 with the screen connected to a 1.2m earth spike.
The first tests produced a large number of UK stations and big thanks to all those who gave me signal reports. It was not the best day to test it out as the static level was very high but it does seem to be bringing in the signals. It is working well through an ATU across 10 through 80m.
As of July 5th 2014 I have managed contacts in the USA, Canada, Brazil, Lebanon & Bahrain so it does seem to be working.
The full details of this antenna can be found at http://www.clive.wankling.dsl.pipex.com/l.html
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