Winding transformers

[camerart]

Hi,

I am trying to understand transformer winding a little better.

My particular interest, is for tuning HF aerial/antennas. Example: For a radio with 50 Ohm end feed aerial. I understand, that approximately 9:1 windings on a toroid.

My question is, does it matter whether there are 9:1 or 18:2 or 81:9, as long as the ratio is 9:1?

If my question isn't clear please ask me to try again

Cheers, Camerart.

 

[ronsimpson]

Clear!

The ratio is important. A 2:1 transformer will change the voltage by 2:1 and the impedance by 4:1.

A 9:1 transformer and a 18:2 will change the voltage and impedance the same way. The inductance and saturation point will be different. Example if 9t=100uH then 18t=400uH. turns squared

 

[camerart]

Thanks for your reply.

Can you give me a few pointers as to how I go about working out each of those components please.

'Say' I have a suitable aerial wire, do I first find out what the find the impedance? If so how? I have just bought an L/C meter, will this help?

QUOTE=ronsimpson;1105882]Clear!

The ratio is important. A 2:1 transformer will change the voltage by 2:1 and the impedance by 4:1.

A 9:1 transformer and a 18:2 will change the voltage and impedance the same way. The inductance and saturation point will be different. Example if 9t=100uH then 18t=400uH. turns squared[/QUOTE]

 

[ronsimpson]

I think you want to know the impedance of your antenna.

The simple answer is to researcher your antenna type. Examples: 1/4 or 1/2 wavelength, ground plain, reflectors, etc. Each antenna type has a impedance. 300 ohms is a typical number, or 75 but there are many different numbers.

A LC meter probably won't get you there. You need to measure the antenna at a certain frequency. Antennas should come with a plot of frequency verses impedance. (not Walmart antennas) A broad cast band FM antenna will be close to 300 ohms at the center of the band and will drift away from 300 ohms at the top and bottom of the band.

 

[MikeMl]

If you are feeding a 50Ω antenna from 50Ω coax, why to you need a transformer?

A 1/4 wavelength (λ) vertical driven against a conductive ground plane has a feedpoint impedance of ~35Ω. The SWR if 50Ω coax is connected to a 35Ω antenna is low enough that most folks don't bother with a matching means.

A 1/2 λ dipole driven at the mid point has a feedpoint impedance of ~70Ω, reduced if the antenna is horizontal less that 1/4 λ above a conducting ground plane. The SWR if 50Ω coax is connected to a 70Ω antenna is low enough that most folks don't bother with a matching means.

 

[camerart]

Hi Ron,

A little more explanation from me needed. I am experimenting with simple wires, so 'say' 20Mtr long. I am trying to make end feed aerials, so where it might be 75 ohm at the centre of the wire I understand that the impedance at the end of the wire is much more, 'say' 750 ohms. Now I have to match it to a 75 Ohm aerial socket on the radio, so I imagine need a 10:1 transformer.

'Say' this is true, then does it matter whether I wind 10:1 or 20:2 or 50:5 as long as the ratio is the same?

From what you say, I imagine I need a signal generator to alter the frequency, then is there a simple way to read the impedance at differing frequencies?

I have a signal generator also an oscilloscope.

Cheers, Camerart

I think you want to know the impedance of your antenna.

The simple answer is to researcher your antenna type. Examples: 1/4 or 1/2 wavelength, ground plain, reflectors, etc. Each antenna type has a impedance. 300 ohms is a typical number, or 75 but there are many different numbers.

A LC meter probably won't get you there. You need to measure the antenna at a certain frequency. Antennas should come with a plot of frequency verses impedance. (not Walmart antennas) A broad cast band FM antenna will be close to 300 ohms at the center of the band and will drift away from 300 ohms at the top and bottom of the band.

 

[JimB]

...the impedance at the end of the wire is much more, 'say' 750 ohms. Now I have to match it to a 75 Ohm aerial socket on the radio, so I imagine need a 10:1 transformer.

Not quite right.
The voltage ratio of a transformer is equal to the turns ratio, but he impedance ratio is proportional to the SQUARE of the turns ratio.
So to match a 10:1 impedance ratio, you would need a 3.16:1 turns ratio.

then does it matter whether I wind 10:1 or 20:2 or 50:5 as long as the ratio is the same?

For a 10:1 ratio, no it does not matter, all these will theoretically work.

BUT, and it is a big BUT!
Depending on the frequency of interest, a coil with 1 turn may end up looking like a short circuit at a low frequency and the transformer will not work very well, if at all.

I am experimenting with simple wires, so 'say' 20Mtr long.

OK

I am trying to make end feed aerials

OK

so where it might be 75 ohm at the centre of the wire I understand that the impedance at the end of the wire is much more, 'say' 750 ohms.

A half wave dipole will have a feed impedance of about 75ohms (at the centre), but it is not particularly helpfull to try and think of the impedance at the centre of an end fed wire antenna.
We are not connecting to the centre of the wire, so lets not worry about it (Yet).

The impedance at the end of an end fed wire antenna depends on the length of the wire at the frequency of interest.
If the frequency is such that the wire is a quarter wavelength long, then the impedance will be low, about 30 to 100 ohms depending on the configuration of the wire.

If the frequency is such that the wire is a half wavelength long, then the impedance will be high, several 1000ohms depending on the configuration of the wire.

All this assumes that we have something against which we can measure this impedance, usually a "good" earth connection.

Rather than worrying about a transformer, although this may be a reasonable solution to matching a high impedance antenna to a low impedance receiver, your efforts may be better concentrated on making a simple ATU (Antenna Tuning Unit), more correctly described as an AMU (Antenna Matching Unit).

JimB

 

[JimB]

is there a simple way to read the impedance at differing frequencies?

Yes, there are various forms of "Impedance Bridge", some are simple and cheap,
Look here for RX1 and RX2 kit manuals, they seem to have a good write-up on the theory of operation.


Others are complex and expensive.
Have a look here for for an antenna analyser which uses a PC for its processing power and display.


If you really want to spend some money, google for VNA (Vector Network Analyser).

JimB

 

[camerart]

Not quite right.
The voltage ratio of a transformer is equal to the turns ratio, but he impedance ratio is proportional to the SQUARE of the turns ratio.
So to match a 10:1 impedance ratio, you would need a 3.16:1 turns ratio.



For a 10:1 ratio, no it does not matter, all these will theoretically work.

BUT, and it is a big BUT!
Depending on the frequency of interest, a coil with 1 turn may end up looking like a short circuit at a low frequency and the transformer will not work very well, if at all.



OK

OK


A half wave dipole will have a feed impedance of about 75ohms (at the centre), but it is not particularly helpfull to try and think of the impedance at the centre of an end fed wire antenna.
We are not connecting to the centre of the wire, so lets not worry about it (Yet).

The impedance at the end of an end fed wire antenna depends on the length of the wire at the frequency of interest.
If the frequency is such that the wire is a quarter wavelength long, then the impedance will be low, about 30 to 100 ohms depending on the configuration of the wire.

If the frequency is such that the wire is a half wavelength long, then the impedance will be high, several 1000ohms depending on the configuration of the wire.

All this assumes that we have something against which we can measure this impedance, usually a "good" earth connection.

Rather than worrying about a transformer, although this may be a reasonable solution to matching a high impedance antenna to a low impedance receiver, your efforts may be better concentrated on making a simple ATU (Antenna Tuning Unit), more correctly described as an AMU (Antenna Matching Unit).

JimB

Thanks Ron, Mike and Jim,

Very comprehensive answers, I will note them all down.

I started by constructing a FOXX-3 which has a fixed frequency, this is how I got on the single tuned wire idea. I think you are correct, with the ATU, I will be building one.

I have loan of an Analyser, and hoped to use a simpler idea in parallel to it, so as to reinforced my understanding of this 'to me' complicated subject. So, simply for explanation, if there are any tricks, where a signal generator, oscilloscope and simple meters can give me some indication of wire impedance, please let me know.

Cheers, all.

Camerart.

 

[camerart]

I realised today, that I would have difficulty with the signal speeds and my equipment.