small FM transmitters

[plaka78]

Here are some Pictures of low power FM transmitters that I have made out of scrap bits and pieces. The one that on copper clad board is used to listen to internet radio. They both seem to run forever on PP3. I have given up with the BELKINS transmitters as they are very hard on battery’s (8Hrs). There is no need for an aerial. The coil itself radiate very well an gives a slightly better range than the Belkins.

Tony

[paulsherwin]

Very neat job! Could you draw out the circuit? I have a Linex transmitter and am very satisfied with it, but I may build your design for the fun of it, and a second transmitter would be useful for the computer.

What's the sound quality like? Do you get lots of harmonics?

Paul

[plaka78]

The sound Quality is good as what you put into it (Very high quality) as good as if not better than the Belkin and as for harmonics there are not any, By using a undecoupled resistor in the emitter stops the harmonics. I will post the CCT diagram In the next few day

[Gordon]

Look good, I assume it is mono only.
If you have a suitable receiver, eg a scanner, it might be wise to check for any harmonics.
Gordon.

[plaka78]

We have a set top aerial for the TV and placing the Transmitter near it causes no interference on the TV.

[Sean Williams]

Circuit diagram?

I will build one, then have a nose on the Spec anny - might even post a piccy or two....

Cheers
sean

[arjoll]

How are they for frequency stability? I've built a few kitset ones in the past (going back > 10 years!) both discrete and based around the BA1404 and drift was always a problem.

[plaka78]

The cct is very stable

[plaka78]

C1= 22nF
C2= VHF section of tuning cap
C3= 2.7pF
C4= 0.1µF
C5= 220nF or 100nF or 47nF See Text
C6= 47nF

R1= 100kΩ
R2=100Ω
R3= 100kΩ
R4= 22kΩ
R5= 39kΩ
R6= 270Ω
R7= 1.8MΩ
R8= 1MΩ
R9= 5kΩ Pot ( deviation control )
R10= 68Ω
R11= 47Ω
R12 47Ω

D1= IN4001

Tr1= 2N2646
Tr2= BC108

L1= 5 Turns 8mm diameter
L2= 2 Turns 8mm diameter

C1 can connected to either positive or negative rail

C5 100nf give correct tone for more treble use 47nF for more bass use 220nF

D1 acts as varactor diode

[Tim]

So is the one on the input is the 2N2646 and the one connected to the o/p coil the BC108? This assumption is based on the case tabs of the transistors as seen in the photos. I can see the figures 2369(if this is a 2N2369 may = BC108) on the coil transistor. If this is correct is the diagram marked up correctly, or am I just thick!

Unfortunately the power connections are not marked on the diagram.

Lovely neat job. No wonder it is nice and stable.

[plaka78]

Hi Tim

You are right I have used a 2N2369 in the Transmitter cct. The secret of stability with this cct is to use a aluminium base plate to screw the cct to. The coils need to be about 4mm away from the aluminium base plate because the cct will not oscillate. I have made the cct positive earth (hence C1 shown as going to the positive rail)as this get around the problem of stray capacitances from your hands etc. The front panel is made from an old cassette case.

There is virtually no drift with this cct. I have reposted the cct showing +/-

[Tim]

Thanks for the info re. the transistors Plaka. I won't lie awake now!!

I have some BSX19's lying around. These are high speed switching transistors so hopefully will do.
I noticed the +Ve earth, a clever trick!

I will try to build one of these, I might just get round to it before digital switchover!!

[Sean Williams]

Well, I have just knocked one up here - Third harmonic is only 10db down on the fundamental!, harmonic content present over 500Mhz -85dbm at over 1Ghz! (off the power leads)

-20dbm approx output when std coupling loop applied to power leads (needs an RFC if you are going to use a mains derived supply)

-60dbm at approx 1 metre from device!

A very good Oscillator, and results as expected considering the simplicity!

Cheers
Sean

[paulsherwin]

What did you use as the oscillator transistor Sean?

Paul

[Sean Williams]

I used a BSX20 - only thing I had to hand that was close.

I used a BC548 as the "modulator", but have not made any "on air" tests as yet - no vhf broadcast receivers in the shack.

Will probably try on air stuff during the week

Cheers
Sean

[Sean Williams]

Pretty picture to look at - Well, not really, shaky camera syndrome, but it illustrates the output of the transmitter fairly well!

Centre frequency of sweep is 511Mhz - span is 100Mhz/div

Fundamental is first vertical mark on left that is not full screen

Forgot to mention current consumption is 2.7ma @ 9v

Cheers
Sean

Who really needs to get out more!

[paulsherwin]

Originally Posted by Sean Williams

I used a BSX20 - only thing I had to hand that was close.

I have a bag of MPSH10 VHF transistors - I guess one of those would be OK.

http://www.alldatasheet.com/datashee...LA/MPSH10.html

Paul

[Sean Williams]

Looks fine to me - give it a go

[Skywave]

If I might make one small observation on this Thread, please . . .

Re: Plaka's comment: "We have a set top aerial for the TV and placing the transmitter near it causes no interference on the TV" is really not adequate an assessment of harmonic levels from any transmitter - low-power or otherwise - although I do appreciate that with Plaka's design, we are talking very low power.
On the other hand, Sean's approach - using a spectrum analyser - is the correct & appropriate method of making an assessment of harmonic radiation.

After much pondering, I thought I ought throw this in to (hopefully) prevent anyone who might read this thread (now, or at a later date) draw the conclusion that the method used by Plaka is really adequate for this. As a guide, yes - but for meaningful assessment - no.

No offence intended, Plaka; I hope none is taken.

With respect,

Al / Skywave.

[Sean Williams]

I agree completely, I just knocked one up for fun - Hooked it up to the anny to show what a small transmitter can do.

Certainly wont cause anyone trouble (well, unless you are working in a calibration lab, or in a Radar Reciever with it nearby)

The FM seems quite good - certainly well capable of good wide deviation, I would guess this is where the audio quality comes from.

Cheers
Sean