Monday, 26 January 2015

MAG Lyke Wake Walk

Myself and Bob M1BBV are planning on doing the Lyke Wake Walk, 40 miles across the North York Moors, with 5000ft of climb, in under 24h, around mid-summer, in aid of the charity MAG (Mines Awareness Group)

Many people do this walk, but we're planning something a bit different - we will maintain HF radio contact throughout. What this means in real terms, is we will operate HF portable, under special event station conditions, making as many QSOs as we can during the crossing.

It is our intention to solicit sponsorship for both the walk itself, and per QSO. We also intend that QSL cards will have to be 'bought', the cost covering printing, postage and a donation to MAG.

There are three main difficulties with this venture, which are Physical, Electrical, and Logistical.

Physical - well, its 40 miles over mountainous terrain. This difficulty speaks for itself! Mucho training walking needed and good foot care!

Logistical - The route is linear, so, we have to not only get to the start, but also get back from the finish!

The biggie really is Electrical - neither myself nor Bob have suitable radio equipment! We have of course VHF handies, but they wont get us enough contacts. We need HF, and we need man-portable HF. Essentially, this boils down to just one system, the Clansman PRC-320 HF Manpack.

Now, we could just go out and buy one, but neither of us can afford it, even if we invested together. So, the challenge is to find some kind hearted and charitable soul who will be willing to provide us the equipment on loan for an extended period.

An extended, long term loan is needed as not only will we need to train with the radio on the hills to acclimatise to its weight, but also as nether of us were ever signalers, we will need to train with it on-air to become competent at setting up and operating the system.

Training with it will comprise a good number of SOTA activations!

Another difficulty not mentioned will be persuading Ofcom to allow us a special event station callsign. We could operate under our own calls, but it wouldnt have the impact of a SES callsign, and to be honest we'll get far more contacts working as 'GB0MAG'

The plan is to do the walk the week after summer solstice, so we have the maximum number of daylight hours, although we will probably start the walk at midnight, and get the most of the climb out the way whilst its cool.

I'll update this blog with details until such time as I have enough to start a dedicated blog. A 'justgiving' page will be arranged in due course.

Sunday, 25 January 2015

Cellflex/Heliax DF Loop - Hmmm, Probably Not

Whilst sound in theory, the physical construction of a VHF RDF loop antenna using entirely 'hardline' coax has a number of drawbacks, which mean I probably won't pursue the design any further,

Firstly, these types of coax use a solid, continuous outer conductor, which is corrugated.  This makes it very difficult to remove just a little, theres no trimming it off with a pair of side cutters like with braid!

 The next problem, is that the inner conductor is just copper plated. This means it has a huge thermal mass, but very little of the heat you put into it goes towards warming up the copper so you can solder to it!

As one side of this antenna requires a 1/4wave of outer to remain, one end of the 1/2wave length of Heliax was bared back and the outer copper cut and soldered to the inner

Heres the cut in the middle. At VHF, this gap may be too small, and increasing it isnt easy. Likewise, adjusting the 1/4wave section to make a good match but cutting away a bit of the outer at a time, is going to be very tedious. 

But the big problem came here, with the Cellflex handle! Just look at the mass of copper! Even cutting it was difficult, and required the employment of a full size hacksaw

Soldering all this together was a nightmare. None of my soldering irons is capable of providing anywhere near enough heat for this job. Ultimately, the task fell to a butane torch. The whole thing was much closer to brazing than soldering.

And, with the loop now attached, a major structural problem came to view. The inner is taking all the bend strain at the 1/4wave cut.

 Although the Heliax is rigid, it will deform easily if bashed about. With the loop being some foot or so wide, I dont think its got the necessary strength to survive real life use. I will test it, and see if it performs as a loop antenna, and if it does indeed exhibit a single deep null, as to the design. But, I really think this is a dead end project. Indeed, im not yet even sure how the heck i'll connect a feedline to it!

 I think perhaps the 3-element 'Tape Measure' beam will be more successful. If, that is, I can find anywhere that stocks the T and cross pieces!

Friday, 23 January 2015

VHF DF Loop - with Integral Sensing

For some time now ive been planning building a 2m DF loop, something that can stay in the car as a ready to go device for tracking down interference and/or abusive/pirate transmissions on 2m. But as with a lot of stuff ive never quite got around to doing it!

Recently though I came across a simple design on the 'net, of a 1/2 wave loop made from coax. A 1/2 wave DF loop has a figure of eight radiation pattern, meaning the target can bearing can be found using one or other of the sharp nulls in the pattern, but leaving you with no clear idea which direction along that bearing is correct! A 1/4 whip would then be added to 'fill-in' one of the nulls, and allow the bearing to be established, this being known as a 'sense' antenna,

Where this design differs, and which lends itself to easy and simple construction, is that the 1/2wave loop is formed from the coax inner conductor, and the 1/4wave sense element is formed from the outer braid. The braid itself then also acts as a matching element, allowing the antenna to be tuned for best match to the 50 ohm feedline.

And it really is that simple. Add a bit of feedline to the radio, or a suitable connector, plus some hardware to make the whole lot rigid and provide a handle, and your away.

Im considering making one using some offcuts of Heliax, which will allow me to make the whole shebang - loop and handle, out of coax, and still be suitably rigid. A piece of 1" dia Heliax for the handle, and some half inch Cellflex for the loop, plus a pigtail length of RG-58 with a BNC connector on it. Perhaps, a single hole chassis mount BNC socket can be 'engineered' into the end of the Heliax handle...

The joints can then be coated in liquid rubber to seal them.

Thursday, 22 January 2015

Planning a new mic interface for FT-857D

My Yaesu FT-857D is equiped with a homebrew interface box that allows me to use either the stock fist mic, or a modified ex-telco Plantronics headset, along with a remote manual PTT button. This is a good system, but has three drawbacks -

1. The mic level out of the headset (which gives by far the best audio) is much greater than that from the stock mic, and hence the radios mic gain setting for ideal SSB modulation with the headset means the fist mic is almost unhearable!

2. I have no way to inject DTMF tones into the mic line to use IRLP and Echolink, and I dont have a DTMF mic!

3. The current arrangement is also not open to use with my proposed Bluetooth mic system.

So, I have redesigned the interface. The new version will include a 1:1 audio transformer in the mic line, and a level control pot, to allow injection of DTMF tones from an external dialler, or, audio from the Bluetooth receiver. There will also be a level control pot in the mic line from the Plantronics headset, to allow the level to be turned down to match the fist mic, and so let the radios mic gain be set to the best for both. A switch will be provided to select between the two main mic inputs.

Ive seen cheap copy Chinese DTMF mics on ebay, these look like the Yaesu mics, but I dont know if they are exact copies, or whether I could make one work properly with the FT-857D. If they can, then one of these would eliminate the need for an external dialler.

I dont know when i'll be able to put this together, it all depends on finding a small but suitable box for the interface.

Saturday, 17 January 2015

10" coil too low

I had hoped that my next coil would come out around the 250uH mark, but it seems this was not to be. The same technique was used as the huge 50cm beast to connect the turns, care being taken this time to select wires that were on the same 'face' of the coil as each other

The completed coil, including the screen connection, looks very good

But sadly, when tested, comes out a bit too low! It seems the ideal target is somewhere between 250 - 300uH. The higher inductance better for the receive circuit, the lower better for the transmit circuit!

Although perhaps not ideal, this coil is at least convenient for testing. I'll use this to try out the Surf PI, at least until I have some suitable cable for a closer inductance coil! Knowing that this one at 10" is a bit low, and the 50cm one is way over, I should be able to estimate a coil diameter that will give the holy grail 300uH!

More Chinese Modules

The sheer number and variety of electronic modules now available at small cost direct from Chinese factories lends itself to a great amount of experimentation. A couple of examples here, of modules or devices ive just placed orders for.

First up, is a Direct Digital Synthesis module. Based on the AD9850 chip, these little beasts can give a solid steady sine wave output in very small steps up to 40MHz, covering the whole HF spectrum, and with the addition of a suitable control circuit make a nice VFO for projects. My intention is to use one to make a multi band WSPR transceiver

These modules are under a fiver! The rest of the parts needed to control it, such as the PIC chip and a rotary encoder, and a 2x16 LCD display, I most likely have in the junk box. Flashing the PIC might be a problem, so i'll probably enlist help with that from a more experienced programmer.

The second item, is a bluetooth audio receiver. At a smidge over £2 shipped, this is ripe for a hack!

 Powered by 5v over USB, the idea here is to connect to it from another bluetooth device, and stream audio over the link to your speakers. Now, I have a very expensive and currently unused bluetooth 'behind the ear' type telephony headset. What I plan on doing is hacking this little gizmo to take supply from the mic line of my FT-857D, and feed the received headset audio into the mic input of the rig! The PTT will remain the gear lever mounted button, but if this works, then for a couple of quid ive removed the cables between me, the driver, and the radio. Much safer than using the fist mic, and more convenient than the wired headset.

Wednesday, 14 January 2015

Surf PI coils

The enormous 50cm diameter coil mentioned previously, is of course just a 'proof of concept', used to test the Inductance bridge and prove that that sort of multicore cable can be used to form such a coil. There are of course practical difficulties in actually testing it on the Surf PI circuit.

The 50cm coil was built to a spec of 350uH, but, ive since had information that 250uH should be the ideal target inductance. This apparently is what the original Whites Surfmaster used.

Plugging a few numbers into the online Coil32 multilayer calculator here shows that 250uH should be a reasonably close result if a coil is formed using this cable of 10 to 12 inch diameter

The wire dimensions are a rough guess, I need to take measurements to get a more accurate prediction. However, the winding length is also a guess, based on the internal diameter of the cable sheath. The above is for the 12 inch diameter coil. N, the number of turns, shows as 17, but this cable has only 16 cores. I dont think this will be a problem, as the test coils so far have been higher in inductance than predicted, and I fully expect a 12 inch coil to also be high in a real coil than in the calculations.

I will measure and cut a length of cable to try out a coil of between 10 -12 inch diameter, perhaps i'll have time to fabricate it over the weekend.

One thing that makes this project hard is the seemingly dispersed nature of the project build instructions! I have yet to find any comprehensive guide saying what the actual target inductance is, how to match the damping resistor to the coil, or how to actually set up the pulse parameters, either by test meter or on a 'scope.

Saturday, 10 January 2015

Versatile component tester for under a tenner!

Not too long ago, if you wanted to accurately measure your resistors and test your transistors, you bought a multimeter for about £20, but if you wanted to have something that automatically identified the transistor type, told you its pin out, and its beta, you needed a tester that cost near a hundred pounds. Add to that if you wanted to measure the equivalent series resistance of your capacitors, that was another £50 or so for a ESR meter, and if you wanted to measure inductance without messing about with noise bridges and tuned circuits, probably another few hundred!

This week, after only 8 days in a container from China, and for a fraction under £8 all in, I took delivery of a ATmega microcontroller based component tester. These are available in various forms, various connections, and as ready built modules or kits. I bought mine ready built, off ebay.

Most of the electronics are under the LCD, which is very well backlit. There is a contrast control preset at the top of the board. The module can be supplied either by a PP3 battery or an external supply. Ive been using a PP3 as I happened to have one handy. This version has screw terminals for the test connections which are a bit awkward, so i'll be adding test clips later. Initially it did some odd readings on inductance. I later found out that there is a calibration method using a three way shorting link. Once this is done, it reads quite accurately, certainly enough for most jobs, although there is a 35pF offset on the capacitance readings which is a bit annoying for dealing with small values, and its lower limit on inductance is 10uH, a bit too high for a lot of radio work.

Heres it measuring an unknown vintage Germanium transistor, just look at that atrocious Beta!

 It might not be the worlds most precise, its capacitance and inductance ranges might not be as wide as would be liked, but for under a tenner, all in, it does all this -

Capacitance & ESR
Inductance & series resistance
Diode polarity, capacitance and forward voltage
Transistor type, pinout, beta and base/gate forward bias

Fancy a slice of PI

Deciding I wanted to built another kit project, and with my interest in metal detecting rekindled, I purchased a Surf PI 1.2 kit from Silverdog,

The Surf PI is a Pulse Induction type metal detector. This makes it ideal for beach detecting. The documentation on the project is somewhat sparse, but the PCB is very well made, if a little tight. The sequence of photos below shows the board being populated

I wont go into the electronics of this thing. Suffice to say that the addition of a couple of potentiometers, a power supply and a search coil are all that remains of the electronics.

The search coil of course is the hard bit! A pulse induction machine works by pulsing a current into the coil, then rapidly switching back to its receive circuits to look for disturbances in the decay of the magnetic field. This requires a coil of a specific inductance, and low resistance to give a good strong current. Low capacitance helps make the recovery time faster.

I have now built a test coil. Its somewhat bigger than normal, at 50cm diameter (20cm being more usual), but thats the diameter I needed to make the right inductance from the multicore cable I decided to use. 16 core shielded computer cable, hence 16 turns, the calculations saying that would give the specified 350uH.

The following photo sequence shows the steps needed to make the coil

The shield of the cable has a messenger wire, which conveniently provides a way to connect to it. No attempt has been made to keep the pairing, but then again none has been made to obscure the pairing either, I have no idea how this will affect the capacitance. The shield only connects to one side, in this case the green coil wire. The final binding of insulation tape serves to keep the connections bundled and stop the sleaving from moving.

The final thing to do was to test the coil and measure its resistance and inductance. This was done on a very cheap but surprisingly versatile little tester from China

So, the resistance is nice and low. The inductance is a little higher than the theory said it would be, at 410uH, but I think this is within the tolerances allowable.

The reason I chose to build such a wide test coil, is that I wish to see how this sort of cable performs for search coils. This cable is available in a range of numbers of pairs, this piece being 8 pairs (16 wires). A smaller coil, such as a standard 10 inch coil, would need around 22 wires, or 11 pairs. This cable can be had in that form.

The next step will be to rig the coil and Surf PI module up in a metal free test environment, and see if it actually works.

Friday, 22 August 2014

Ramp Up The Amps

After much deliberation, reading and checking, a couple of things occurred to me. First was that I should try varying the emitter resistor in the Wispy LO, to try and increase the drive to the receive mixer. Germanium diodes typically turn on around 0.3V, so with a drive of barely 240mV pk-pk, I doubt they were on enough. The resistor in circuit is 2k2. So I tried tack soldering another 1k in parallel. This brought the level up to closer to 300mV pk-pk. A bit more playing here and I should be able to obtain the right level of drive. It will probably mean though that the Tx chain will need completely realigning!

The other thing that suddenly popped into my head was why my LM386 audio PA seemed to overload so badly when fed from the Wispy audio chain. I suddenly thought, perhaps I had the gain control pot wired wrong? Indeed I had! So after changing the wiper of the pot to go to the input capacitor and not the input pin of the chip, all should now be well.

My little self contained LM386 audio power amplifier
Oooh! Look, Ive just realised how to add captions to my pictures! Expect more of that!