Balun for loop aerial

[Boom]

I now have some ferrite rings (15cm) very kindly supplied by Ed Dinning. I am trying to wind a balun as simple as possible for MW use. I.e. N turns to go to the loop and N turns to go to the radios A/E sockets. It seems that no one does things as easily as this. Some of the balun designs seem as complex as the radio itself.

Can anyone tell me if it can be done simply by using two windings and if so how any turns I need for input and output?

[G8HQP Dave]

Can anyone tell me if it can be done simply by using two windings and if so how any turns I need for input and output?

If what can be done? Separate windings for input and output form a voltage balun, which may or may not be what you need. What sort of loop? High or low impedance? Tuned or untuned?

[Boom]

To clarify. I have made a tuned MW frame aerial. It has two windings, the main tuned winding and a two turn coupling winding. I am feeding the coupling winding into the A/E sockets of an A22.

I understand the coupling winding is 50 Ohm and the A/E on the A22 is some 300 Ohms. I want to use a balun to match the two.

What I need to know is how many turns do I wind onto the ferrite ring to go to the 50 Ohm windings and how many turns for the winding that goes to the A/E sockets?

[GMB]

You need a turns ratio of 1:2.5 to go from 50R to 300R.

The number of turns depends on the details of your ferrite, enough to make the reactance well more than 50R.

[Herald1360]

If a two turn coupler is 50R won't 5 turns give about 300R? Take that to the set via 300R twin feeder- job done! any unbalance is unlikely to matter enough to worry about.

[Andrew2]

In my experience of resonant loops using ferrite coupling, I simply passed one turn of the tuned winding through the ferrite ring. I was not using the usual one-turn pickup loop.
On the secondary side I experimented with the number of turns for best results and ended up with 4 turns. Purely non-scientific. However, I did stick a few milliwatts up it from a sig gen via a VSWR meter and at resonance it was a decent match to 50 ohms. A few more turns and you'd be up to 300 ohms.

[G8HQP Dave]

For broadcast reception you don't need perfect matching. The input impedance of the set will vary quite a bit over the waveband anyway. It seems that you don't need a balun, but an impedance transformer.

I would use a 1:2 winding ratio to keep things simple. Actually, I would probably not bother and just connect the thing directly.

[Boom]

Originally Posted by G8HQP Dave

to keep things simple. Actually, I would probably not bother and just connect the thing directly.

Yes that is my thinking. I'm getting a bit baffled by science. I doubt I will spend much more time messing about with it.

The frame aerial works well as it is but I understand that the null may be improved even further by adding the balun. I wonder if just adding more turns to the coupling winding to bring the impedance up would do the same job.

[G8HQP Dave]

You may be confusing two different effects: balance and impedance matching. You improve the null by improving balance; a balun may do this, if the loop imbalance is caused by the unbalanced connection to the receiver. Impedance matching will affect the signal level and bandwidth - there is a compromise to be struck - but not the null. A balun may or may not also perform impedance matching. An inappropriate balun may also reduce the signal and affect tuning.

Does it work coupled directly? If so I wouldn't get too het up on getting a 'perfect match' the A22 won't mind a bit. I feed my A22 from 50ohms works a treat.

[Herald1360]

Surely if the coupling winding to the loop is floating, the loop will be balanced anyway, whatever you connect the winding to??

[Boom]

You may well be right Chris. This is the first loop I have made so I am totally in the deep end. The two loops (tuning and feed) are indeed electrically seperated and the null it has is ample to totally knock out the Spanish stations right next to Radio Caroline so I think it is going to be a case of leave well alone.

[G8HQP Dave]

Magnetic coupling between the small loop and the main loop will be balanced. Electric field coupling will be unbalanced, but this may not matter too much.