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Old 16th Jun 2024, 10:10 am   #1
David G4EBT
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Default MATE Transmitter.

Split from this thread:-

https://www.vintage-radio.net/forum/...d.php?t=211769


It looks like Gabriel might have been sorted with the variable capacitor.

For anyone who doesn't know what the 'MATE' is but might want to, it's a valve-powered 'pantry transmitter', designed by the late Peter Lankshear from Invercargill, New Zealand, who passed away in March this year aged 95.

Peter will be known to BVWS members from his bulletin articles over the years, and to Eddystone User Group members, as the author of many articles on Eddystone receivers. He wrote close on 100 vintage radio articles for 'Electronics Australia' from 1988 to 1996. He kindly gave permission to New Zealand Vintage Radio Society, of which he was a Foundation and Life member, for the articles to be made freely available. They can be found at this link:

https://nzvrs.com/peter-lankshear-articles/

Full constructional details of the 'MATE' are in the 1989 section.

Peter's obituary on the Eddystone User Group is here:

https://eddystoneusergroup.org.uk/ob...n-new-zealand/

I hope that's of interest and relevance.
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Old 16th Jun 2024, 10:57 am   #2
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Default Re: MATE Transmitter.

I have the information on this and intend to build one at some point. In fact only the other day I was considering which valves to use.
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Old 16th Jun 2024, 11:08 am   #3
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Default Re: MATE Transmitter.

Thank you, David.

That's a rather good website with some useful resources.

The 250pF outout capacitor is interesting. It's the only thing standing between the antenna and the +250v HT supply, so it needs to be one whose insulation is considered a safety item. It would be wise to have belt and braces.... be sure the antenna wire is insulated or/and have an RF choke from the antenna connector to ground. The 250pF value can be reduced to ensure there is no significant signal off your property.

A variation might be to use the same type of valve in both positions. This leaves a spare triode doing nothing, but with the ECC81, 82, 83 series the heater power of the unused section can be saved. It seems that the dual triodes are easier to find than the singles (6C4 for example) and this might be a convenience if you have some double triodes in stock.

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Old 16th Jun 2024, 11:21 am   #4
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Default Re: MATE Transmitter.

There is some interesting information in that first link David! Thanks for posting that.
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Old 16th Jun 2024, 11:25 am   #5
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Default Re: MATE Transmitter.

Actually I was thinking of using a 6C4 for the Oscillator triode (It's actually half an ECC82) and an ECC82 or 83 for the the other two....I hate unused sections of valves......
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Old 16th Jun 2024, 2:10 pm   #6
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Default Re: MATE Transmitter.

I've got a bucket load of ecc81 valves, so that's going in the front section alogside a Russian 6sn7 in the latter section. This is because my chassis has an octal socket.

The only thing that concerns me (other than the cap RW mentioned) is the heater cathode voltage of the second valve. I am planning to elevate the heaters.

I didn't think this was going to generate so much interest. I just needed a project for my week of annual leave!
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Old 17th Jun 2024, 9:45 am   #7
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Default Re: MATE Transmitter.

I've come across the MATE before. A nice simple cct, but maybe too simple as it looks prone to FMing with the modulation.

However, as I have not actually built it I may be quite wrong! If anyone does, could you let us know how it performs?
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Old 17th Jun 2024, 1:36 pm   #8
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Default Re: MATE Transmitter.

The MATE formed the basis for my pantry transmitter (here). The change I made was to make use of the unused half triode as a cathode follower between audio amp and oscillator. I found that without it there was a large amount of RF signal on the input sockets. There are indeed second order effects.
I found… “with 50% modulation of an 800kHz carrier the frequency was deviating by ±850Hz.”

Vic
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Old 17th Jun 2024, 2:49 pm   #9
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Default Re: MATE Transmitter.

Happy to post it to you Ian for an objective review once it's ready.

From what I've been told it gives a reasonably good account of itself.
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Old 17th Jun 2024, 6:41 pm   #10
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Default Re: MATE Transmitter.

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Originally Posted by Gabe001 View Post
Happy to post it to you Ian for an objective review once it's ready.

From what I've been told it gives a reasonably good account of itself.
Thanks, sounds interesting...
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Old 21st Jun 2024, 11:31 am   #11
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Default Re: MATE Transmitter.

Just to update that I'm making progress on the Mate. Just waiting on some bits and bobs, but should be ready in a couple of weeks.

Ian, I'll post it over to you for some measurements afterwards, if you like. I'm not sure how to test a transmitter other than to check that the modulation trace matches the original, but I can do it myself if given guidance and it just involves a scope and signal generator

I am building it as per the original circuit, but using a bridge rectifier due to the transformer I've got. I've been sent some tips on how to keep hum down - IEC filter, snubber capacitors across diodes and attention to grounding, which I'll implement. Also, I don't have an approriate 250pf cap so it'll be a suitably rated 220pf cap on the output instead.

Happy to implement the cathode follower mod Vic suggested at a later stage, if the original circuit misbehaves

Gabriel
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Old 21st Jun 2024, 12:45 pm   #12
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Default Re: MATE Transmitter.

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I'm not sure how to test a transmitter other than to check that the modulation trace matches the original, but I can do it myself if given guidance and it just involves a scope and signal generator
There are two sorts of things which need doing to test a transmitter.

Firstly you need to check that it does the things it should do:

Does it make the right amount of power and on the right frequency? Can it produce the necessary peak amount of modulation without significant distortion?

These are relatively easy. You can see the AM on a scope trace and gauge the amount of modulation. Bad distortion becomes visible, but yu may want invisible amounts.

A check on coverage (enough but definitely not too much) attends to the power, and if it appears at the wanted spot on the target radio's dial then the frequency is good enough.

More difficult is checking that it does NOT do any of the things it must not do.

This comes down to making sure it emits nothing other than the intended signal. Transmitters can easily produce power on harmonic frequencies which could interfere with other services and get noticed. Short antennae get a lot more efficient on higher frequencies and you could easily wind up with more range on your harmonics than on the wanted frequency. Then there are spurious oscillations which could be on any frequency.

Olde worlde transmitter checking used to rely on 'wavemeters' - simple tuned circuits with a detector and a sensitive meter. You tuned around and checked there weren't any unwanted blips. You could also tune around with a wide range receiver. But spurii could be on any frequency from DC to daylight. Where do you stop? A realistic approach would be to go to the max frequency your active devices could theoretically be used to build an oscillator. VHF for a lot of valves. UHF for a lot of transistors. I know some audio amps with a characteristic spurious odcillation near 70MHz!

The modern approach is to throw a spectrum analyser at it, but this is a lot more expensive than sig gens and scopes. There are now little hand-held ones styled like the little VNAs that are all the rage.

Realistically, once you know the frequency it works on, try tuning around with a receiver to each of the integer multiples of that frequency to find the harmonics and check that the amplitude is negligible so that it cannot come to the notice of anyone who might think it's not negligible...

I've got our local amateur radio society set up with a couple of full-blooded spectrum analysers which can get passed around as needed and I've been doing demos on how to use them. One goes to 1250MHz and has a tracking generator, the other goes to 110MHz and I've now through generosity on this forum, got the matching 110MHz tracker for it. This makes it a great machine for testing and developing things like crystal filters. This seems to be a useful service which a club can provide and gets people out of a bit of a hole, and meets the licence requirements to check transmitter cleanliness "From time-to-time". Not everyone has to have all of the possible test equipment all of the time.

David
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Old 21st Jun 2024, 1:33 pm   #13
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Default Re: MATE Transmitter.

Quote:
Originally Posted by Gabe001 View Post
Ian, I'll post it over to you for some measurements afterwards, if you like. I'm not sure how to test a transmitter other than to check that the modulation trace matches the original, but I can do it myself if given guidance and it just involves a scope and signal generator

Gabriel
OK Gabriel, I'd be happy to oblige. As you may know I'm the designer of the MiniMod and a 15-watt Restricted Service Licence AM transmitter design approved by Ofcom some years ago.

Apart from the features David described, one of the most important display checks on a 'scope is a trapezoidal display, which really shows up modulation distortion - but unfortunately is rarely presented with a design.

The other important feature is frequency stability. Not only slow drift, but the avoidance of the carrier being frequency modulated by the audio signal. If it's only a few tens of Hz, it's not really noticeable, unless there is a real signal already on the frequency, when it will sound a little "gurgley"! On a clear channel (which are becoming more common, even at night) you probably won't notice the FMing, unless it's really bad.

Theoretically any pantry transmitter should be operating on a 9 kHz channel to reduce heterodynes from co-channel signals, but in practice this is unnecessary as a pantry tx signal should easily swamp any weak stations and local noise - that's the whole point! But again, the decline of other stations makes this less of an issue in any case. (Ofcom spec. for frequency stability is +/- 10 Hz.)

Of course harmonics should be considered, and for AM RSL transmitters Ofcom specify that they must be at least 40dB below the carrier - in fact all radiation beyond +/- 9 kHz must be suppressed by at least that amount. In practice a MW pantry tx will be only a hundred mW or so and the antenna will be about 0.1% efficient at best and I doubt harmonics will be noticed - unless that is you have a keen radio amateur living nearby! A simple LPF or basic output tuned circuit will render harmonics and any other spurious output negligible.

Remember, unauthorised transmitting is still prohibited...!

Ian
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Old 21st Jun 2024, 5:49 pm   #14
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Default Re: MATE Transmitter.

The MATE is new to me and looks to be my kind ‘a pantry transmitter… just need a pantry to put it in… my ‘ole Gran had a pantry back in the 1960s...

Terry...
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Old 21st Jun 2024, 6:18 pm   #15
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Default Re: MATE Transmitter.

Pragmatically, I view amplitude modulation of a free running RF oscillator as a Cardinal Sin.

It will invariably lead to some significant FMing, and for certain combinations of deviation and audio frequency this can cause the entire disappearance of the carrier! Which makes the received signal sound horrible.

You also can't get anywhere near 100% modulation before horrible things start to happen.

Things that may draw unwanted attention to your transmitter.

Keep it clean, use an oscillator with good spectral purity and fed by well regulated supplies, then buffer it properly before trying to do AM on the output stage. Then a decent LPF to eliminate all harmonic emissions.
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Old 21st Jun 2024, 8:45 pm   #16
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Default Re: MATE Transmitter.

Wow, thanks everyone for the very informative replies. I must admit that I don't quite understand all of it,but I do get the gist of it. Transmitters are rather new to me!

Ian, I'll drop you a pm once it's ready and I've established that it works. After testing, depending on the shortcomings, we'll discuss remedial action. It'll be an interesting project.

Gabriel
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Old 26th Jun 2024, 8:40 am   #17
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Default Re: MATE Transmitter.

Seeing this post again had me looking at a Wearite PO2 oscillator coil, Radiospares Midget mains transformer and a small salvaged chassis in the workshop…. Crikey, as if I need another project.

Terry
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Old 26th Jun 2024, 10:46 am   #18
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Default Re: MATE Transmitter.

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Originally Posted by G6Tanuki View Post
Pragmatically, I view amplitude modulation of a free running RF oscillator as a Cardinal Sin.

It will invariably lead to some significant FMing [...]
This is true. My MiniMod suffered from this slightly but wasn't really noticeable with good construction layout, but I sorted out one for a guy who made his on Veroboard with lots of uncut and stray tracks capacitively couplingn which did make the FMing much worse. That prompted me to add a buffer stage in the MK2 (see BVWS Bulletin Winter 2023 and on this forum from post 121 here: https://www.vintage-radio.net/forum/....php?p=1529242)

I'm always on the lookout for valve low power deigns and I found references to The Knight Home Broadcaster. Knight was a sort of small brother to Heathkit in the 1950s and 60s so I assumed their 'home broadcaster' (which doesn't really cover a room) was worth perusing. I had reservations as it was clearly intended to be a compromise between complexity for the inexperienced home constructor and performance.

Long story short, but during research to reproduce the design, I found a Knights Home Broadcaster reproduction kit on eBay. They'd tried to maintain the 'nostalgic appeal' of the original design while reducing the chances of it killing its users.

I've made many, many kits in the past 50-odd years and this was easily the very worst, both in construction layout, performance and price. Maybe in the 1950s no one really cared about the distortion and FMing - and from the design you can see why FMing would be a problem for the reasons outlined by G6Tanuki.

The original design runs straight off the mains onto an isolated metal chassis! The reproduction has a blue fibre-glass sheet panel construction bolted together, with a mains transformer (240v for UK version) screwed to the side, with a 1 1/4" mains fuse shoe-horned into the box and other badly considered, cramped alterations.

To be fair, I think it was an attempt by a very small US company (Knight Kits are long gone) to get into the growing pantry transmitter market by trying to update a vintage circuit to have nostalgic appeal. But I think we have higher performance demands these days regarding modulation quality and of course safety. They would have been better much of redesigning the whole thing.

I was going to write a review of it for the BVWS Bulletin, but I couldn't think of anything good to say. It's hiding unused (and virtually unusable) on my top shelf as an embarrassing reminder of my poor research!

The original 1950s circuit shown below.
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Old 26th Jun 2024, 1:07 pm   #19
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Default Re: MATE Transmitter.

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Pragmatically, I view amplitude modulation of a free running RF oscillator as a Cardinal Sin.

It will invariably lead to some significant FMing...

You also can't get anywhere near 100% modulation before horrible things start to happen.

...

Keep it clean, use an oscillator with good spectral purity and fed by well regulated supplies, then buffer it properly before trying to do AM on the output stage. Then a decent LPF to eliminate all harmonic emissions.

I'd agree with most of that!

I have modulated an oscillator (EF91, triode-connected, Hartley oscillator) and got quite decent AM up to >90% modulation.

However, there was FM - I don't know how much, but it does manifest itself as rather curious tuning in the receiver: if off-tune then the receiver can slope-detect the FM and this can add or subtract from the existing AM depending which slope you're on... it's not nice.

However, modulating a crystal oscillator might be acceptable, because the external circuit has correspondingly less effect on the frequency.

Getting a good, pure, sinewave and then applying to a 4-quadrant multiplier (DC-biased AF going to the other input) followed by a Class A amplifier might be an alternative to modulating the output stage.

A low-pass filter on the output is really good practice - a 10-foot wire may be appallingly bad at transmitting on MW - we might find that our signal barely reaches our property boundary (which is the limit for our naughty pantry transmitters), but will be much better on SW if there are any harmonics present. So it's prudent to be sure they're filtered out - and then go walkabout with a portable SW radio seeing if there is any signal escaping.
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Old 26th Jun 2024, 7:16 pm   #20
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Default Re: MATE Transmitter.

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A low-pass filter on the output is really good practice - a 10-foot wire may be appallingly bad at transmitting on MW - we might find that our signal barely reaches our property boundary (which is the limit for our naughty pantry transmitters), but will be much better on SW if there are any harmonics present. So it's prudent to be sure they're filtered out - and then go walkabout with a portable SW radio seeing if there is any signal escaping.
Yes, I always shudder when seeing designs for pantry-transmitters that couple the randomly-tuned antenna terminal direct to the anode/collector of the 'output' stage without any sort of filtering or - sometimes - without anything resonant at all!

Run away! Run away!!

At least do a parallel-resonant tuned circuit on the output, with a winding over it to feed the antenna if you're doing it with valves - or maybe a Pi-network.

And for transistors - use a ferrite multi-hole binocular/quadocular transformer to scale the output impedance down to somewhere you can use a nice LPF using 2 or 3 toroids and similar capacitors before you squirt it into an 8-foot length of wire that *could* otherwise just be optimal for radiating harmonics on 28MHz and clobbering my ability to get 10M DXCC !
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