3 Transistor bug transmitter help

[MiXiN]

Just bought one of these http://www.ebay.co.uk/itm/141783274569? ... EBIDX%3AIT but I'm not getting the quoted range and am only seeing about 60 metres off a 9V supply.

Can anyone suggest any ways of improving the range?

The final 9018 Transistor (not sure what NPN this) gets quite warm. Is there something that would be better suited for the buffer or final Transistor?

In the item description and last image there's a circuit diagram to have a look at.

[sinus trouble]

Hello Mr Mixin!
Thats a very VERY basic but educational kit you have!
The TX part is actually only 2 transistors (Q2,Q3) tuning i believe is achieved by the values of C5 and L1?
The problem with this circuit is that surrounding influences, temperature and antenna will have severe effects on your output! i also suspect that all the power available is not going on your single frequency? rather like you are covering a large range of the band!
I would suggest some extra stages,but i fear youd be wasting your time!
As The Beatles once said! 'Let it be' lol

[teckniqs]

Yeah and if you're using their suggested 50cm wire then it's too short. You want at least 65cm for top band and 80cm for the lower end of the band.

....Also adding another ground wire of same length as a bottom element will also help.

[MiXiN]

Was just thinking I'd be able to replace the final Transistors with something more suited to RF. I used to build these 3 Transistor bugs and sell them when I was in my late teens over 20 years ago, and got about 150 > 200 metre range from them, but I just can't recall the Transistors I used.

Good point, Teckniqs, I used the supplied wire - and it's only around 50 CM long and very thin, 32 awg or thinner, and the bug is currently tuned to 88.2 MHz.

[Gum]

I bought some x-ray specs once, but they just turned out to be sunglasses with a picture of a skeleton stuck on the inside of the shades.

[teckniqs]

Was just thinking I'd be able to replace the final Transistors with something more suited to RF. I used to build these 3 Transistor bugs and sell them when I was in my late teens over 20 years ago, and got about 150 > 200 metre range from them, but I just can't recall the Transistors I used.

Good point, Teckniqs, I used the supplied wire - and it's only around 50 CM long and very thin, 32 awg or thinner, and the bug is currently tuned to 88.2 MHz.

Transistor is SS9018

https://www.fairchildsemi.com/products/ ... S9018.html

Maybe you could try something like a BSX20 or H10 in place.

[Shedbuilt]

Substituting the RF transistors for 2N2219s might well give you a bit more output. Not sure if that's a good thing though. What Sinus said is right for this type of design.

[thewisepranker]

Was just thinking I'd be able to replace the final Transistors with something more suited to RF.

The transistors specified are perfectly adequate "for RF".

The final 9018 Transistor (not sure what NPN this) gets quite warm. Is there something that would be better suited for the buffer or final Transistor?

How warm is "quite warm", above 60°C?

[Shedbuilt]

One other thought. It looks like there are no variable components in the oscillator circuit, so presumably (apart from physically changing components), you can only adjust frequency by adjusting the length of L1 ?
This makes me wonder. Do you have any way of being certain that it's on 88.2MHz. Assuming you've checked that it doesn't appear anywhere else on the band, it could be off band, and you could be picking up either an image or a spurious emission. Depending on your receiver, a transmission on 109.6 could be picked up on 88.2. This would seem to be a very weak signal, and the same could be the case if you are tune to a sprog.
What TWP said is right. The transistors are proper RF devices - suitable for much higher frequencies than Band II. Not particularly high output, but should be enough to achieve the claimed range; if everything is working as it should, and the output is being reasonably well transferred to an antenna.
As an aside, generating FM in this way, is likely to generate a fair amount of AM too. With a circuit like this, it probably won't be the biggest problem, but undesirable nonetheless......

[MiXiN]

Alright necks..

I extended the antenna to 80cm, put the bug near my living room window, and am now seeing around 400 metre range in an area with a few buildings in between - this listening on a FM radio on my phone - so I'd no doubt see even more range on a Car radio.

The frequency coverage of this bug only covers about 85 - 90mhz, so to get it operating on the upper part of the band am I right in thinking that the number of turns on the Oscillator inductor should be reduced? Also, do I have to alter the turns on the following inductors?

[Shedbuilt]

You could take a turn off L1 (suck it and see basis), or reduce the value of C5 slightly. Or, for wider and finer adjustment, reduce C5 to say 10pF, and put a 20ish pF trimmer cap across it. Perhaps see how it performs on a higher frequency - before changing anything else. If it falls off drastically, start off by lengthening the output coils if they can be lengthened further, but any changes here will almost certainly change the frequency too. Possibly quite markedly.

[MiXiN]

Nice one, builtinashed.

I think I'll change C5. If C5 is currently 30pF for 88Mhz, do you know a ballpark value to get me up to around 108Mhz? Do you think a 22pF or 18pF would do?

Need to buy some stuff off eBay later, so will order some Ceramic discs while I'm at it.

[teckniqs]

You shouldn't even need to do that, just solder 2 of the turns together so it's one less turn. That should solve it I reckon.

[Shedbuilt]

If you're going down the capacitance route, I think 15pF to 18pF should put you in the right ballpark.
If you want to alter the inductance, personally, I'd take it out, and either wind a new one, or reduce the existing one - rather than have a shorted turn, but you should be able to short a turn without ill effects. I think taking a full turn off may be too much though. Based on some calculations; partly based on assumptions of coil diameter and length, and a bit of reverse engineering, I think going from 4.5T to 3.5T might put your lowest frequency right at, or just above the top of the band.
It's all a bit finger in the air, because there are unpredictable variables involved. The small range of frequency adjustment you're getting, suggests that you're not changing the inductance, or the effective length of the inductor very much, but that's probably to be expected, because the length at the bottom is fixed by the holes in the PCB; you can only really stretch the top; which is not the same as lengthening the whole inductor.

[MiXiN]

Nice one, builtinashed.

Just ordered some 15, 18, and 22pF off eBay and will see how I get on with them.

Did another range test this morning in my mates car, and the range is even better. Seeing something like 3/4 mile in a semi - built up area.

Gonna plant one of these in a pals house for a bit of a laugh.

[Gum]

Gonna plant one of these in a pals house for a bit of a laugh.

What a great idea! What could possibly go wrong broadcasting your mates private conversations to anyone with an FM radio?

[Maximus]

Gonna plant one of these in a pals house for a bit of a laugh.

What a great idea! What could possibly go wrong broadcasting your mates private conversations to anyone with an FM radio?

Or he's trying to tune into his favourite station and accidentally gets feedback though the radio [FACE WITH TEARS OF JOY]


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[MiXiN]

Gonna plant one of these in a pals house for a bit of a laugh.

What a great idea! What could possibly go wrong broadcasting your mates private conversations to anyone with an FM radio?

I went down to just under 86Mhz and listened on a Scanner set to WFM, so probably best you don't jump to conclusions!

[MiXiN]

Builtinashed,

I found a large pack of 22pF Ceramic discs tonight, fitted one, and this gets me from around 105Mhz upwards - so you weren't far off at all.

Also substituted the final Transistor for a PN3563, and a quick check with a field strength meter tells me it's doing a fair bit more output than before.

It was a bit late to go walking around range testing, but I'll do the test later today.