Saturday, 26 September 2015

Chinese Airband Receiver Kit - Now with Digital readout!

Ok, so, its NOT a synthesiser, its not even a PLL, its still a free running LC VHF oscillator under varicap DC control. But, it is now much easier to put on frequency, or at least, close enough that the wide IF bandwidth means i'll hear the signal im listening for!

Chinese Airband Receiver with LCD Frequency Counter module

A couple of points here. First, the connections are only tag soldered onto the underside of the PCB at present, and the LCD is powered from the 8v regulator. I need to check that the additional current load isnt too much, although I doubt that the whole shebang takes the 100mA limit of the regulator. Second, the signal input to the counter is at present a single wire, eventually that will be replaced with a length of thin coax. Incidentally, the plugs and wires as supplied with the counter are such that the signal input is a black wire and its ground is a red one! Something to be wary of! I might modify the LCD module to allow the backlight brightness and the contrast to be adjusted.

On the back of the counter module are two push buttons. These allow the display to be changed between 1kHz or 100Hz resolution. Ive set it to 1kHz here, as the oscillators stability isnt up to 100Hz! The buttons also allow setting of an IF offset frequency and direction. By setting it to -10.7MHz, I have a readout of actual receive frequency, or as close as makes no odds.

If its of any interest, here its tuned to Leeds East Airport.

More musings on the Chinese Airband Receiver kit

After a fair bit of testing, several negatives have come to the fore with this kit. None is a really major problem, and for most builders just wanting to have a pootle about the air band they wont be of any consequence. These are as follows -

1. Sharp crackles from the loudspeaker if the antenna touches anything, or nearby switches are operated
2. IF bandwidth is about 800kHz!
3. A bit low on sensitivity
4. Squelch isnt very good
5. Tricky to tune - whole 118 - 136MHz in just a 270degree turn!

Addressing these in turn, no. 1 is of no real worry, and such noise susceptibility is to be expected working on AM. I expect a bit of extra front end protection is needed, maybe a high value resistor to ground or an RF choke. No. 2, well, ok, it means I receive several channels at once, but lets face it where I live the band isnt exactly crowded, I might have the ceramic filter in the wrong way around, although I thought they were bidirectional! No. 3, only really an issue picking up the ground station, and I can hear both my local airfields towers on it. No. 4 is a bit annoying, but really, see no.3!

So, no. 5 is the issue that is most problematic. With the supplied 10k pot the whole coverage is in a single turn, which means its very easy to miss a signal by tuning right over it. I considered replacing the oscillator with a DDS unit, but mine is only good to about 40MHz, far too low. A simple fix, which ive done, is to replace the 10k pot  with a 10k 10-turn precision potentiometer.

Tuning is now much cleaner, but more time consuming! And, since the knob goes around ten times, impossible to know what channel your on except from ID'ing the traffic! I could add a turns counter. But, instead, my thought is to add the el-cheapo LCD frequency meter module I have! I have yet to see if connecting it to the oscillator will cause unacceptable pulling, but will try it out. It might need a little FET buffer circuit.

With the addition of a multipole switch, and a bank of 10-turn presets, I can tune in several local frequencies to switch between.  And from there, its not much more effort to add a simple timer and stepping system to allow scanning.

Friday, 25 September 2015

Shack Teardown

Starting this evening, everything on the shelves in my 'shack' will be packed into boxes. The rigs will be disconnected and boxed, the cables coiled, and the desk itself dismantled.

No, im not closing down! But im finally getting a carpet (rather than random offcuts) in the shack, and the desk, which is attached to the wall at one side, has to come out. When its rebuilt, it will have two sides, and be movable, so I can access the cables at the back.

The only thing that will be left connected will be the MVT-7100 scanner and the coax to the discone, so I still have airband monitoring for tomorrow. Other than that, its handhelds, manpacks, or mobile for a while!

Thursday, 24 September 2015

Airband receiver kit complete - first thoughts

So, on arriving home I got out to the workshop and cracked on with the Chinese Airband radio. I had only intended doing the electrolytics and the connectors, but it took no time at all to solder those in, so I got on with the diodes, then the transistors and the 78L08 regulator. This done I tested it with 12v but without installing the ICs, and checked I had an 8v DC rail.

All was well, so I depowered and installed the ICs. The kit as supplied has a3.5mm jack socket audio output, so I tag soldered a loudspeaker across it for testing. Power applied, I was relieved to find that I had noise from the speaker, and that it stopped when I adjusted the squelch control.

With it connected to my Marconi 2955B and being fed a -100dBm 125MHz AM signal (1kHz at 70%), a bit of tweaking and I located the signal. This was quite poor, but much improved after adjusting the IF transformer. Ive pretty much left the band pass filter alone for now, as my to hand trimming tools dont fit.

It isnt quite as sensitive as i'd hoped, 12dB SINAD being around -105dBm, but its not too bad. After a lot of fiddling I managed to adjust the VFO coil to put 118MHz at the extreme of the tuning control, although I couldnt go any lower as the coils slug was all the way out. The tuning is varicap controlled by a 10k pot, and the range is so wide (top end at least 136MHz) that in its current form finding a signal is very tricky. Several ways might solve this, I could modify the circuit for a smaller bandspread, or swap the control for a 10 turn precision pot and a turns counter dial. Or, I could replace the VFO with a DDS synthesiser!

Even as is, mooching about with the control and with a 1m telescopic antenna, it pulled in several aircraft comms signals, ranging from late evening long haul ATC traffic, to my local field with a request that 'as were the last landing you can turn off the lights'!

Next step is to get the frequency counter on the VFO and see exactly what the bandspread is. If need be i'll modify the circuit to reduce this. Ideally, I want to modify the VFO anyway to get coverage down to 108MHz.

But, all in all, for about £12 its a fun little project.

A bit more done on the Airband kit

I had a few moments spare last night to progress building the Chinese airband receiver. The ceramic capacitors, ceramic filter, and inductors are now fitted.

(just realised that photo is before I fitted the IF can!)

I'll add the electrolytics, connectors and controls tonight, and if ive time might get onto the semiconductors. At this rate, I might have it operational for saturday morning, which is the day of the inaugural East Leeds Airshow. Despite being only a few miles up the road at RAF Church Fenton, I shant be attending this time, the number of ground exhibits, aerial displays and stalls doesnt to me justify the £50 family ticket price tag!

The only aircraft I really want to see that will be displaying is Vulcan XH558 - and I should be able to easily see her from my own garden!

Wednesday, 23 September 2015

4m FM - where's the activity?

After trying each lunchtime from work to make a contact on 70.450MHz FM using the PRC-351, I decided today to try the same using my FM-1100. This uses the same magmount antenna as i'd been using with the '351, but at 15W rather than 4W, and is capable of 12.5kHz steps, giving me the whole of the 4m FM allocation.

But not a sausage! Where has all the 4m FM activity midday gone? There used to be at least a couple of stations monitoring that i could reach from Emley!

Tuesday, 22 September 2015

Chinese Airband Receiver

Well, this finally arrived today! And it rather distracted me from my principle task of checking and adjusting the PRC-351's deviation, which is what I was meant to be doing since ive yet to manage a QSO with it, however I do think that might just be down to low power and low activity.

First impressions are good. It was all shipped in resealable plastic bags, all the parts are present (and as usual, a few extra!), and the PCB is high quality, through hole plated, with, as seems normal for Chinese kits, the groundplane on the solder side. The band pass filter components were already installed. Im not entirely sure why, but as the coils are the most tricky part for a less seasoned constructor, perhaps its actually for the best.

Ive installed the resistors, a couple of ferrite chokes, the molded coil, and the IC sockets. I'll do the capacitors and maybe a few more bits over the next few days.

Ive also had the PRC-320 open for its first inspection since coming into my ownership. Partly this was to allow inspection of the PSU module and the dreaded tantalum capacitors. I found that the case bolts were rather easy to unfasten, so thats something I need to ensure when im finished - that the case is up to correct torque. Some of the internal modules could also do with their screws nipping up a bit.

I was quite impressed by the build quality, especially the antenna tuner, which uses a permeability tuning technique. The module to the top left in the above picture is the synthesiser. This is where my attentions will be, along with the decade switches, when planning the VFO control system.
The picture below shows the underside, and the main board. The turret tuner is the big cylindrical block in the middle.

Taking the lid off the PSU module revealed that im too late! 

Instead of the MoD issue module with its dodgy tantalums, I found instead three modern DC-DC converter modules and a nixie type high voltage PSU module. So the PSU has already been thoroughly converted.
I have only one slight worry, and that is that this bit fell out -

and ive no idea where from! The radio works perfectly without it! I suspect its a locating stud from a module thats come loose.

Incidentally, if anyone who reads this blog is located either close to Selby or close to Emley Moor, and has 70MHz FM equipment, I really would appreciate a sked to get a check on my deviation level!

Thursday, 17 September 2015

A quick note about Clansman radios

The chances are that if your reading this blog on a regular basis, what im going to mention here is irrelevant. But, you might be considering getting an ex military radio, and might be wondering which of the Clansman range to go for.

I am not going to mention the vehicle sets here, as I have little experience of them. I will only talk briefly of the suitability for amateur use of the 'manpack' radios.

As far as the manpack Clansman radio go, there are six choices. Not all are any use to us!

The PRC-349 and PRC-344 are of no amateur use. In fact, unless you are a cadet instructor, using them on-air is illegal. Entirely. Dont even consider it!

PRC-349 is a 'handheld', yeah right. About the size of a house brick sliced in half lengthways, it covers 37 to 47MHz. These are still MoD frequencies, although the radio is a mere 250mW, so you probably wouldnt get caught, its illegal to use in the UK, even by amateurs. Yet strangely, working 349s sell for rather a lot of money, I do wonder who's using them...

PRC-344 is a backpack UHF AM transceiver for forward air control and operates on the military airband. Dont even think about it! You'd probably get away with a 349 if you were no where near an exercise, but this will get you DF'd and an Ofcom van and a scuffer jam sandwich outside your house in pretty short order!

This leaves us with the PRC-320, PRC-350, PRC-351 and -352.

Of these, perhaps the most useful and most popular, but also most expensive, is the 320. This is a HF manpack covering 2 - 30MHz in 100Hz steps, AM, CW and USB at 30W. It can be easily modified for LSB. Its only drawback is its decade switch tuning. These sets were not designed for trawling the bands! Its built in manual ATU and standard antenna wire spools and masts however make this a potent set.

Next up is the 350. Covering 36 to 56MHz at 2W FM, these are usable on 6m. They sell for low prices as they are low powered, of limited use, and use a 15V battery which is non standard. They are a bit smaller than the 351

The 351, as seen in my earlier post, covers 30 to 76.995MHz FM in 25kHz steps, 4W out. It can be used on 6m and 4m. Although heavier, its more practical than the 350 as it shares its battery options with the 320. The audio ancillaries are also all interchangeable. You will see these with a 'SURF' unit attached - dont bother unless you really want to, or your set came with one. The SURF, or Selector Unit Radio Frequency, is essentially a pre-selector. Its intended to allow two or more radios to operate in close proximity. Nice to have but not necessary for normal amateur use.

Last on the list is the 352. Giving out 20W, this is simply the 351 with an add-on amplifier block! It is not for use with the whip antennas used with the 351 though, and is for use with the various 'Ground Spike' antennas.

A word about antennas -
The 351 can use a 50cm 'battle' whip flexible antenna, or a 1.2m sectional antenna. It can also use the Ground Spike, Elevated Ground Spike, and vehicle whips. The 352 should always be used with the Ground Spike or Elevated Ground Spike.

The 320 can use a 2.4m sectional whip, or, pretty much any normal (or even abnormal) form of wire antenna - dipoles, slopers, random wires.

So there you go. As an amateur, the 320, 350, and 351/2 are useful sets. If your not an amateur, then all of them will make you a nice display of military comms kit, but theres nowhere you can legally transmit on them.

But dont let me put you off!

Wednesday, 16 September 2015

Hello Zero, this is Sunray

Ok, so maybe not quite. But I have now obtained a Clansman PRC-351 VHF FM company level manpack radio, which is the very type of radio that 'Sunray', that is, the company commander, would be using. Luckily, I never had to carry one of these too far or often. They might not be as heavy as the PRC-320, but they're not exactly light by modern standards!

As a basic FM radio, its capably of roughly 4W out, on any 25kHz step channel from 30 - 76.975MHz. Frequency is set by thumb switches on the side by the antenna. It therefore covers 6m and 4m, but because those bands use 10/20kHz steps, it doesnt match exactly. This is more awkward on 6m where the FM calling channel is 50.510MHz.

It does have many features that are not immediately useful for amateur radio though. Its two volume settings, Loud and Whisper, also vary the mic sensitivity. On Whisper, the mic is more sensitive and the volume lower, helping avoid being heard by the enemy, perhaps useful for late night ragchewing when the XYL is in bed? Likewise, a pair of terminal posts, and a handy wire stripper built in, mean that the radio can be attached to a remote handset and operated by line from up to 3km away! It can also work as an intercom between the local handset and the remote handset. Perhaps there are some amateur uses for those features!

But these radios are also designed for use as a 'Rebro', that is, a re-broadcast, or repeater, system. Two radios linked by field telephone cable (DON-10) and on different frequencies can be set to act as a repeater automatically, but, to do so require a 150Hz tone. Much like most modern amateur repeaters!

When I got it, it was pretty mucky. Now, contrary to usual thinking, a well bashed and mucky field radio usually means its been reliable in the field. A clean radio always begs the question - why was it left in the comms stores? So, ive given it a bit of a clean.

Because of the 150Hz rebro tone, these radios can get bad audio reports on amateur bands, where the tone isnt expected. In this case, the tone deviation was over 1.7kHz!!! Luckily, its not hard to turn this tone down, which I did by adjusting R9 on module 13, and made it deaf in the process!

Now, the reason for this is, that these radios dont use a normal noise gate squelch circuit, they use a tone squelch, but in order for them to be inter-operable with non-NATO/Clansman users, an internal 160Hz tone is generated that defeats the squelch on the users behalf. By turning the 150Hz Tx tone right down, I'd also turned down the 160Hz internal tone.

This can be corrected by adjusting various tone signal path presets, but to to so would require having the set powered whilst open, something I cant do yet as I dont have a battery extension cable. So, with a bit of intuition from 5yrs servicing public safety radios, I blind set the Tx tone to a low value that I hoped would work and not be annoying on air. With the radio back together, I tested it again, finding now that the squelch worked properly again, and that the 150Hz tone on Tx was now at a comfortable 500Hz deviation.

A check of receive at 70.450MHz shows a sensitivity for minimum discernible signal of about -123dBm, and the squelch break at about -118dBm, pretty much how I used to set Kenwood TK-349s for the prison service! Tx power at this frequency is about 3.3W, reaching a tad over 5W at 30MHz, and a bit over 4W at 76MHz. The only thing I really want to adjust now is the Tx deviation, which is a bit high for my liking and may cause distortion in modern narrow band amateur receivers.

Ive yet to have a QSO with it, and I fully appreciate that its low power means I need to get a good take off location to do so. As for claiming the callsign Sunray - I only manage 'Sunray Minor' in this house...

Monday, 14 September 2015

Call me, Zero

I am now, thanks to David G3RYP, the proud owner of a PRC-320 HF manpack radio. This is the same one we borrowed from David, who has allowed me to acquire it from him. So, I now claim the impromptu callsign ZERO! For anyone not familiar with British Army communications procedure - Zero is the standard callsign of the net control station. I hope in the next few days also to be able to claim the callsign SUNRAY...

Being now the owner of the PRC-320, the next step is to give it a thorough overhaul. To start with, I wish to see if it has had its PSU high voltage capacitors replaced, and if not, to do so. This will help extend the radios life by pre-empting the dreaded 110v fault.

It also means that I can begin working in earnest on the PIC controlled synthesiser remote control system ive been developing.

At the same time, I also obtained various Clansman spares, one item of which is a 'display only' battery.

The reason for acquiring this is simply to empty it out and refill it with a much more modern battery technology. Having drilled out the rivets holding the top on, and levering it off, I was confronted, not surprisingly, by a mass of expanded polyurethane foam. First job then was to remove as much of this as possible, until I could get at the cells

Using various tools, including a set of pipe wrenches, I managed to get a cell out. It was then a job of carefully chiseling out foam, and pushing, pulling and rocking the cells free, until the top layer was all out

The bottom layer needed the same techniques, but in the now cramped confines much more effort.

But, eventually, all the old cells were out, and most of the foam scraped away. Ive now to get the lid properly straightened out.

Eventually, I plan on fitting this case with a modern, 7S 5,000mAhr Lithium Polymer battery. That will drastically reduce the weight of the set-up, but also means investing in a suitable charger. Luckily, a charger suitable for 7S LiPo batteries is also capable of handling a 24v 4Ahr NiCd!

I also took 'delivery' of the new LCD modules from China, and have unfortunately found myself disappointed by my usual supplier. Firstly, whichever dork actually delivered the package, left it inside a recycling bin! I luckily noticed it. But then, on opening, I found that the LCDs had been shipped with the encoder I had also ordered, and not properly packaged. Two of the three LCDs, including the expensive 4x20, have impact damage to the screens as a result.

Im not at all happy about this, and have sent pictures off to the supplier and requested replacements. These displays all had projects waiting for them, which are now delayed.