Is this VFO Transmitter capable of 2 Watts?

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MiXiN
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Is this VFO Transmitter capable of 2 Watts?

Post by MiXiN » Fri Nov 01, 2019 11:05 pm

Just been looking around on eBay and come across this transmitter kit - https://rover.ebay.com/rover/0/0/0?mpre ... 3901184963

Is this capable of 2W or will it be a lot less?

For the price, I'm thinking of buying one as a gift for my 13 year old Daughter who's showing an interest in Electronics - and this will be OK for honing her soldering skills.

There is a circuit diagram here amongst other things - http://wavehunter.co.uk/2w-fm-transmitter

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by s2000 » Sat Nov 02, 2019 2:34 pm

I would think it would struggle to do 500mW tbh. The 2n3866 is a 1 watt device @ 28v. If you run it at 10-14v (as the page suggests), it will never do 2w.

It could be a good project though to practice the solder skills though :tup

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by Albert H » Sat Nov 02, 2019 3:29 pm

This is a truly horrible "design". The 2N3866 can develop about 600mW at 12V at best, in a highly optimised circuit.
  • The diagram for this effort shows a grounded-base oscillator with a collector tuned circuit tuned by a trimmer. It will be impossible to tune to a frequency with any accuracy, and then it'll drift wildly as it warms up.
  • The FM deviation is achieved (as in lots of crude bugs) by applying the audio to the base / emitter junction of the oscillator transistor. This will be highly non-linear, and will give distorted modulation.
  • The intermediate stage has two transistors in parallel. There's no sane reason for this, unless they're using a totally unsuitable transistor type and / or a poorly designed circuit.
  • The output stage has no kind of filtering (but it does have a BNC connector!). The output will be a horrible, drifting mess of harmonic crud and will certainly cause interference in the vicinity if connected to an aerial.
  • The PCB layout is truly awful. It looks like it was laid out by someone who's laid out audio amplifiers and thinks that RF is just the same.
Steer well clear. Save your money.

Is there an actual market for a basic VFO kit that doesn't drift badly, is easy to tune, reasonably spectrally clean and fairly cheap? It's not too difficult to design something like that at about 500mW output. There could be a bolt-on PLL for people who need frequency accuracy.

I'd use a Colpitts oscillator at frequency - perhaps using a FET - a buffer / amplifier stage using a BFS20 (or similar) into a BFG135 final, all running from a 7808 regulator. If laid out sensibly, we'd get a neat little exciter with pre-wound coils (as far as possible) and an output lowpass filter. If it was to be a VFO, it would be tuned with a couple of 10-turn preset pots - coarse and fine - and if it was to be PLL, we'd remove the tuning trimmers and (possibly) add an extra buffer stage to feed the input of the PLL. It would make sense to have it fit into a standard tinplate box......
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Re: Is this VFO Transmitter capable of 2 Watts?

Post by radium98 » Sat Nov 02, 2019 4:43 pm

does this 2n3866 really need all that heatsink i doubt ,and it will deliver at all 0.8w with thoses trimmer not more ,btw i did not see the varicap diode but i like that kind of kit .
another question will tr 2n2219a work in fm as per datasheet FT is 250Mhz but as general purpose transistor ,can it walk @ 205 mhz ?

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by s2000 » Sat Nov 02, 2019 5:51 pm

The 2n2219 works well, Ive substituted it for a 2n4427 when I've not needed as much power as 1 watt. It was often used as a driver transistor for the mrf237 back in the day

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by teckniqs » Sat Nov 02, 2019 8:23 pm

No the 2n3866 won't do 2w on that circuit, as someone already mentioned I wouldn't expect any more than about 0.6 to 0.8w from this device.

...I have seen them doing 2w but that was with about 30 volts but they are not designed to do that much power and will get too hot. :stop

I have used 2N2219 in the past instead of a 1w configured 2N4427 about 20 years ago but I'm pretty sure I didn't get any more than about 0.4 to 0.5w out of it.

I like the 2SC2538, it is a lot smaller in a plastic casing, doesn't require a heat sink and although it's only rated at 0.8w it works nicely for 1w.

Although what Albert says is true and it's not great to be used in any way as a professional transmitter or to be used with an RF power amplifier it would still be good enough to build if your daughter just wants to build the kit and see how it works.
And priced only at 16.99 I think it's allowed to be pretty shit and she might find it leads to her wanting to builld and experiment with better gear afterwards, we all had to start somewhere...

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by teckniqs » Sat Nov 02, 2019 9:24 pm

But.... changing the 2n3866 to a 2n3553 would probably give you better results and produce about 2w or so...

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by Albert H » Sat Nov 02, 2019 10:26 pm

The fact remains that there's no output filter, so it'll be rather nasty if connected to an aerial!
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Re: Is this VFO Transmitter capable of 2 Watts?

Post by yellowbeard » Sat Nov 02, 2019 11:02 pm

Apart from everything else, it's expensive for what it is - another nail in its wee coffin. Look at this:
https://www.amateurradioshop.nl/webshop ... uwkit.html
That's an old school 5 Watt VFO kit without an output filter for about 4 quid more , it's a better design in many ways and it will do what it says. You may need a small fan to keep the output transistor happy mind you...

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by teckniqs » Mon Nov 04, 2019 8:20 am

yellowbeard wrote:
Sat Nov 02, 2019 11:02 pm
Apart from everything else, it's expensive for what it is - another nail in its wee coffin. Look at this:
https://www.amateurradioshop.nl/webshop ... uwkit.html
That's an old school 5 Watt VFO kit without an output filter for about 4 quid more , it's a better design in many ways and it will do what it says. You may need a small fan to keep the output transistor happy mind you...
Yes that would be a lot better but are the building instuctions available in English though or just in Dutch?

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by nrgkits.nz » Mon Nov 04, 2019 8:50 am

Albert H wrote:
Sat Nov 02, 2019 10:26 pm
The fact remains that there's no output filter, so it'll be rather nasty if connected to an aerial!
A great place to put it would be hidden away in the bush somewhere for Ofcom to find, have it triggered by some sensors so it only powers up when it’s dark, cold, and wet.

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by thewisepranker » Mon Nov 04, 2019 9:03 am

nrgkits.nz wrote:
Mon Nov 04, 2019 8:50 am
A great place to put it would be hidden away in the bush somewhere for Ofcom to find, have it triggered by some sensors so it only powers up when it’s dark, cold, and wet.
Image

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Re: Is this VFO Transmitter capable of 2 Watts?

Post by Albert H » Sat Nov 09, 2019 4:29 am

teckniqs wrote:
Mon Nov 04, 2019 8:20 am
yellowbeard wrote:
Sat Nov 02, 2019 11:02 pm
Apart from everything else, it's expensive for what it is - another nail in its wee coffin. Look at this:
https://www.amateurradioshop.nl/webshop ... uwkit.html
That's an old school 5 Watt VFO kit without an output filter for about 4 quid more , it's a better design in many ways and it will do what it says. You may need a small fan to keep the output transistor happy mind you...
Yes that would be a lot better but are the building instuctions available in English though or just in Dutch?
I can translate the instructions for you! The "Stentor" is a great little rig that lots of pirates got their start with. The version shown in the advert has some of the coils etched on the board, and as I remember, it's pretty stable. Building a suitable LPF for the output will be easy enough. The kit is just over £23 at today's exchange rate. Expect to pay a couple of quid postage. It's a pretty good deal!
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"Because it doesn't know the words!"
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Re: Is this VFO Transmitter capable of 2 Watts?

Post by teckniqs » Sat Nov 09, 2019 8:34 pm

Albert H wrote:
Sat Nov 09, 2019 4:29 am

I can translate the instructions for you!
About 25 years or so ago I would have gladly accepted that. Appreciate the offer though. :tup

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