Help with rf transformer

Thread Starter

GarryO'Keeffe

Joined Jan 28, 2015
13
I have been trying to build a simple 27MHz transmitter and receiver but have failed up to now.
I found this circuit from which I can use just the transmitter and receiver part, it all looks simple enough but I cannot figure out how to make the 9:2 transformer that connects the Arial ( or antenna ) . Could someone please tell me what I should use and how to make it?
There is also an inductor in line with the arial, what value should I use here.


I'm not interested in the demodulation and power amp, I simply want to transmit and receive the carrier.

1597410383083.png


Thanks

Garry
 

Papabravo

Joined Feb 24, 2006
14,424
Ask the author -- you have his call sign which is also part of his URL. It is a Canadian call sign. The generic answer is that you find a suitable core and you wind it with some enameled magnet wire. What the core is made of and the geometry are details that at the moment appear unknowable. If you know enough about RF circuits you might be able to reverse engineer the required impedances and come up with a close approximation. Without some expensive test equipment you won't have a way to verify if your solution is even close.
 

Thread Starter

GarryO'Keeffe

Joined Jan 28, 2015
13
Thanks, that is where I got the circuit.

What I need to know is how to make the tuning inductors, i.e. do I need ferritte cores? what size of core? are they air cores? what size wire?
 

MrChips

Joined Oct 2, 2009
21,877
1597417540876.png
The circuit shows iron core inductor whereas a ferrite core is needed where you see the two lines.

An RF coil former will look something like this.

1597417768908.png

It has pins for connecting the wires of the coil. It also has an adjustable ferrite core slug for tuning. You will need a non-ferromagnetic screw driver (usually made of plastic) in order to make the adjustments.

Core diameter is about ¼ inch. The former is not much bigger than about 1 inch long.
Size of wire is not critical. 24-30 AWG is ok.
 

sparky 1

Joined Nov 3, 2018
337
T37-6. ferrite toroid would also work. The 9 turn inductor is part of the resonant oscillator.
A QRP kit like a pixie would be easier.
 

sparky 1

Joined Nov 3, 2018
337
The video shows a neat layout. The author explain clearly how to construct an AM radio transmit circuit.
With some modification it could oscillate up to 30 Mhz. The antenna transformer construction is shown.
The difference is the crystal resonant circuit.
 

wa3tfs

Joined Mar 14, 2017
5
Thanks, that is where I got the circuit.

What I need to know is how to make the tuning inductors, i.e. do I need ferritte cores? what size of core? are they air cores? what size wire?
After looking at the schematic, the two turns on the inductor resonant at 227 MHz with the 15 pF cap are required to be around 2.3 uH. That would be around 2.6 turns on a FT37-43 toroid core. A T36-6 core would not work as too many turns are required to fit properly. Round off to 2 turns if you have no way to measure it. Then, 9 turns wound on the same core will complete the transformer. Number 28 wire is ok for this power level. The inductor in series with the antenna is there to increase the resonant length of the antenna to nearer that of 27MHz. This is just a loaded short antenna for 27 MHz. You may notice I used "around" a lot for values as the actual values necessary to work properly will vary based on component values, board layout, etc. but they will be close enough to get this working.
 

Thread Starter

GarryO'Keeffe

Joined Jan 28, 2015
13
After looking at the schematic, the two turns on the inductor resonant at 227 MHz with the 15 pF cap are required to be around 2.3 uH. That would be around 2.6 turns on a FT37-43 toroid core. A T36-6 core would not work as too many turns are required to fit properly. Round off to 2 turns if you have no way to measure it. Then, 9 turns wound on the same core will complete the transformer. Number 28 wire is ok for this power level. The inductor in series with the antenna is there to increase the resonant length of the antenna to nearer that of 27MHz. This is just a loaded short antenna for 27 MHz. You may notice I used "around" a lot for values as the actual values necessary to work properly will vary based on component values, board layout, etc. but they will be close enough to get this working.

Excellent, Thank you for the details answer. I'll try and find the parts and have a go at building it.

Once again thanks
 

wa3tfs

Joined Mar 14, 2017
5
Excellent, Thank you for the details answer. I'll try and find the parts and have a go at building it.

Once again thanks
One more thing, if you want to make this transformer adjustable, use a variable capacitor in place of the fixed 15 pF. Something like 10-25 trimmer would work fine and then you just need to peak it for best output level. Also, please ignor the typos in my other replay like 227 MHz and t36-6 instead of t37-6.
Good luck on your project. You can visit my web site for a lot more design information at http://wa3tfs.com
 
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