Airband Kit

[veedub565]

Hi,

This isn't really vintage, but I wanted to have a go at building an airband receiver. I know I can buy one readymade fairly cheap, but I'd quite enjoy the process of building one and learning at the same time.

I picked up one of these

http://www.ebay.co.uk/itm/3219133450...%3AMEBIDX%3AIT

Anyone tried one before? it's supposedly the better version without the PCB trace inductors. I don't expect it is going to be brilliant, will most likely drift but will be interesting to see how it goes.

I have planned to add an LCD frequency readout and see if it's possible to extend it's range a bit into the 160mhz region.

[Martin G7MRV]

Find my build on my blog - g7mrv.blogspot.co.uk

also includes my addition of a cheap frequency counter for a digital display

[Wendymott]

Hi Veedub. It all depends upon what you want the receiver to do..... I have built two fully sythesized receivers... one with lots of whistles and bells.....a cut down version.... and a non synth version (hand held).. My full unit was for monitoring the airband away from the airfields.... the cut down one fits on my car dashboard for "on field" monitoring, and obviously the handheld is a portable unit...The portable unit has no real need for sensitivity, as long as it works "Near field"... i.e Radar control and Ground control.
For £12 its a bargain..........my handheld does not have any frequency readout... just two preset pots for the Leeds Bradford Radar and Ground control. If I go on a "Sortie" I check the alignment prior to setting off. If I need to go further I take the Version "B" receiver.

[veedub565]

Looks like an interesting build Martin, Which LCD module did you use for the display? Looks like the external pots not only improve things but also make it easier to mount in a project box. Would it be worthwhile installing it into a metal box to improve body proximity effect?

I'm not expecting amazing things Wendy, it's amazingly cheap if it works at all that would be great. It's more about the fun of building it, and the learning experience.

Ben

[Radio Wrangler]

Com band radio traffic is very terse and transmissions are rather sparse unless things are getting busy. A drifty receiver is going to quickly become annoying because once it drifts far enough off to miss transmissions in a gap between transmissions you don't know that you've started missing things. For airband listening, synthesisers have a lot of benefits.

The pot and varactor tuning is OK for getting to build something and showing that it works, but the resulting receiver isn't something you'd want to keep using.

David

[Martin G7MRV]

Yes I think a metal box would certainly be preferable to the plastic case I used. As for its stability, well you cant expect too much! But, after a warm up period mine is good enough for use monitoring the local airfields. The LCD frequency counter I used (cant remember the exact type but it has a programmable offset and only cost a few quid) means I can see if its drifted too far and correct it. The front end is rather broad anyway as is the IF, so a little drift is of not much consequence.

Its a fun functional project. Its not worth improving it beyond the frequency counter and multiturn pot for tuning, as the cost of further improvement gets close enough that a cheap commercial scanner is a better option. You could add a DDS synth, but then you need to add a controller, and then youd get miffed by the wide IF and start adding better filters, etc, etc....

Myself, I have this in the workshop to listen to the local fields. For any serious listening (and especially as I listen to UHF!) I use my MVT-7100!

[veedub565]

Yeah A fun little project is all I intend it to be really. One of the guys at work suggested adding a DDS Synth too, may do if I can find the bits cheap/free.

I've got a bit spoiled really when we had a £9,000 Icom receiver in for repair at work. It wasn't one of my jobs but I watched them test it after it was fixed and it was a really nice bit of kit (dare I say it on a vintage forum)

[veedub565]

Well it's arrived, looks all neatly packaged but no documentation of any kind regards component placement. All I have to go on is the jpg picture on the ebay listing which is barely legible (especially if printed off) makes things rather slow going and tedious.

[Steve_Bell]

I built one of these recently and it worked first time. Like you, I received no documentation but managed to find various bits and pieces on the web and compiled this document. The circuit diagram helped with the component numbers and values, although some are a bit blurred when zoomed or printed - fit those that are easily identifiable first and then you may need to apply some detective work.

Hope this helps,
Steve.

[Steve_Bell]

I also found this manual, although it has obviously been translated. Perhaps the resolution of the circuit diagram is better though.

[veedub565]

Thanks Steve, my kit is slightly different (I think it's the mk2 version) without the track print inductors. Apart from that though it looks the same, that document will really help. The diagram and board layout is much clearer than the one I have. And some setup up guide is always useful

[orbanp1]

Hi veedub,

Looks like a fun receiver to play with it!

Just a quick note about the IF-bandwidth of the receiver, it looks awfully wide for the Airband!
The Airband is AM modulation, channel spacing of 25kHz typically:
https://en.wikipedia.org/wiki/Airband
The E10.7A ceramic filter (from the picture in the manual), the main IF-filter, has a typical 3dB bandwidth of 450kHz, and a 20dB bandwidth of 750kHz:
http://www.datasheetarchive.com/dl/2...8a5f6/P/e10.7a
The T1 If-can probably does not help much either to narrow the bandwidth.

For experimenting, I would get some of the narrower band ceramic filters too, like the SFE10.7MFP, from the Murata datasheet.

As mentioned by others, if the LO is not stable enough, a narrow IF, while it helps channel separation, might make the use of the receiver too inconvenient.
Hence the emphasis on experimentation!

Good luck, Peter

[Martin G7MRV]

Yes the IF is as wide as a hanger door. Perversely though, this helps overcome the drift ;-)

The kit is a copy of an original Ramsey kit, the manual for which can be downloaded

[G6Tanuki]

Originally Posted by orbanp1

The Airband is AM modulation, channel spacing of 25kHz typically

That was the case a few decades back: these days to be compliant VHF civair AM needs to support 8.33KHz channel-spacing - http://www.javiation.co.uk/833.html

[AC/HL]

For commercial use yes, but most of us will find this adequate for casually roaming the airband.

[Radio Wrangler]

As Martin said earlier, the width lets it cover several channels at once, and full carrier AM becomes very uncritical on tuning, so this receiver will tolerate drift to an extent. You will hear more than one channel if nearby ones are in use at the same time.

This is actually used by ground stations to achieve wider area coverage. The same audio from a traffic controller is applied to up to five transmitters in different locations. One on the channel centre frequency, and the other four offset differently around it. A plane can fly around the area of these transmitters and hear one or the other. Its receiver is wide enough and the transmitter frequencies are close enough together that it will happily receive any of them. It will often be in range of more than one. BUT the beat between the offset carriers will create a very strong audio tone. This is fixed with a very sharp cutoff lowpass filter in the audio stage of the receiver.

A pilot can fly across a wide area talking with the same ATC guy and not noticing as one transmitter fades down as another fades up. The plane always transmits on the channel centre, and all ground stations receivers listen there. So you can get several kHz difference between aircraft and ground transmissions.

They even fit three frequencies into the bandwidth of an '8.33kHz' channel radio. By the way that's just a name. The channel spacing is really 25/3 kHz, and if you come across an aircraft radio to the current spec, the frequency display shows 5kHz increments and is now a channel name rther than an accurate statement of frequency. "118.000" gives you a wide channel for the old 25kHz spacing, actually centred on 118.000MHz. "118.005" stays on the same frequency! but the receiver bandwidth is narrowed for the 8.33 channel spacing. "118.005" stays narrow and shifts the channel centre up to 118.00833.. MHz.

It confuses the living daylights out of radio people the first time they come across it. Aviators mostly don't know that those aren't the actual frequencies in use, but it doesn't make any difference to them. The old 25kHz channels remain with their original names, so it's backwards compatible.

[m0cemdave]

David, thank you for that interesting info.

On the few occasions when I have tuned around the airband, I wondered why some light aircraft were so badly off net with the ground stations!

[Jon_G4MDC]

This radio person is confused all right David!
Have been trying to understand this for years.

118.0 is 11.8.0TX 118.0 wide RX
118.005 is 118.00833TX and 118.00833 narrow RX
what then is 118.010 please?
118.015 is 118.01666 TX and narrow RX I suppose?
118.020 is?
118.025 is 118.025 TX /RX and wide again I suppose.
Seems messy.

The staggered carriers look to be +/-10k from centre on an SDR.
It could be 8.33kHz I suppose - but it looks wider than that.

[veedub565]

Well I have made the kit up, got a few random components left over and a couple missing. No probs as I was able to find replacements. The mixer ic was defective, it didnt work well to begin with and then stopped working altogether. Again no problem as I was able to rummage around and find another.

Confirmed it's all working now, not bad for £13

Next job is to replace the oscillator with something a bit more stable, and the ceramic filter for something with a tighter spec. And then build it into an aluminium chassis. I may also add the display.

[veedub565]

I'm looking at adding the LCD display, looks like I need 16x2 LCD PIC counter module with programmable offset. Most of the ones I've seen only run to 50Mhz or so and the frequencies I'm looking at are 118Mhz+. So presumably some sort of prescaler is required and then the display will display whatever you tell it too.

Any suggestions? there's plenty of LCD display/counter modules out there, just a bit unsure of the prescaler.

I think I'm going to forgo the DDS for now.