A balun for receiving?

[AdrianH]

I hope this is in the correct section, if not mods please move.

First a bit of drivel.

Having done my Eddystone 870 I have it on the bench quite a bit recently listening to various stations.

Initially I used a wire antenna that's in the loft, basically just a wire as long as I could get it which is about 15 metres in total. It suffers a lot from general noise pickup, especially as it just plugs into the 'A' socket and i rely on the mains earth. I also tried a 50 Ohm 20 metre dipole. This has a 1:1 balun at the centre of the dipole and standard 50 Ohm coax back into the room, a choke as it enters the room. I have been connecting the inner and outers together with a clip and again just putting this to the 'A' Socket, again picking up quite a bit of electrical noise.

The instruction booklet mentions use of a doublet which I believe to be a dipole with ribbon feeder ( 400 to 600 Ohms ) back to the receiver.

As an experiment I used a ferrite ring wound 5 turns of primary and 15 turns of secondary on it. The primary going to the 50 Ohm plug on the dipole and the secondary now going to the 'A' and 'A-' on the radio, shorting plug being removed. I have found this a lot better for reception, basically the noise level has dropped considerably but the stations are coming through OK.

I am assuming that any electrical noise that is on the feeder is now being cancelled in the ferrite balun and I am in effect getting a balanced input to the radio? Radio Caroline and others are coming in nicely, I seem to be picking up well up to around 18 MHz, but loosing quite a bit on long wave with RTE now being quite week and R4 down and what I would expect.

So I think my balun still leaves a lot to be desired, can anyone recommend what type of ferrite ring I should be trying and how many turns one thinks I should be using. for Primary and secondary? Perhaps I am asking a lot to get down to the Long Wave band as well as 18 MHz, I am not sure. The ferrite type I am using is unknown and probably came from a switch mode power supply filter.

Sorry for the long post but would like to get to a better design if possible, and figure a way to test it.

Adrian

[Restoration73]

I can't see how a balun can be useful here. A half wave dipole resonant on the preferred band can be connected to a balanced feeder
and fed into the A and A- sockets. Noise cancellation occurs on the feeder. A BalUn is only needed with coax at one end.
Note the dipole will also work on the resonant frequency X3 as well e.g. 7 & 21 MHz.
Unknown ferrites make dubious transformers, so use dust iron toroids e.g Amidon etc.
Reception may be boosted using outboard antenna tuner or preselector unit(s).

[AdrianH]

I am using 50 ohm co-ax which is not balanced, so the balun at the centre of the dipole simply allows the dipole to connect to that without causing un wanted currents down the co-ax, well I believe that is the idea behind it.

With my Eddystone, I am basically using the aerial as a un-tuned wire dipole on other bands, using the transformer at the RX end to give me a balanced input to the receiver. All I know is that the noise level when compared to using it as a going into the end wire connection is a lot lower?

Yes I could do with a better ferrite, but do not know what type to still cover LW to 18 MHz, I could try some ferrite aerial rods?

Adrian

[murphyv310]

Hi.
A good long wire and an earth rod with it wound round round a ferrite toroid of 43 type material and a secondary wound for the radio would help with noise reduction. You can experiment with turns ratios for best results.
You'll never beat a 1 meter active loop though for noise reduction.

[AdrianH]

Hi Trevor, external loop antennas would be a bit of a dream here. I did consider a frame aerial in the loft, but would have to also remote tune and rotate, at the moment I just have two wires, a loft wire, or the 20m dipole.

I will continue to play.

Adrian