Hi Art

The AM Loop antenna kit that Oatley Electronics sells as a kit does perform rather well.
It was featured in the October 2007 edition, I can send you a photocopy if you are interested.
The design lets you remotely control the operating frequency of loop, so you can put the antenna in the best location, and adjust it from another location.
I have built a few of them, they do work OK.

Mark

Hi Mark,
I would appreciate that
I haven't done my second attempt yet.

Your email address is hidden, but mine should not be,
so if you send me an email I'll give you an address.
If you have a scanner, there should be no need for a physical photocopy.
Cheers, Brek.

I just had a look at the Oatley Kit instructions in a pdf available online.

I'm puzzled that the schematic indicates use of 4-core twisted pair telephone cable for the loop.

Every other design I have seen warns that twisted pair will neutralise the coil effect of the loop.

Can anyone explain how a loop can be made effective using twisted pair wires?

Maven

Can't answer your question of course, but there goes mine,
when I do get the right design, I then want to reproduce it with
vintage material I already have.. just don't want to waste it on flops!

Another Oatley design I have built, the Miracle Antenna, about 20 years ago (a varicap tuned design that connected the control to the loop through coax) recommended about 6mm spacing between turns to minimise capacitance between turns. Don't know if this was a requirement of the workings of the control or to optimise Q ie sharpness of tune/selectivity, or both.

If an antenna tunes too sharply it would reduce the bandwidth and cut out the high and low frequencies - not good for fidelity eg music, but wouldn't matter for talk-back radio especially through a 2 to 3" speaker.

Other designs use rainbow cable cut and soldered into a loop - quite close spacing but evidently worked quite satisfactorily for the small transistor radios it was intended for as they are not generally regarded as high fidelity devices.

Twisted pair would presumably be worse than rainbow cable as the wires are closer together given the thinner insulation. Maybe the twists introduce inductance that counteracts the capacitance.

Thinking about it, there would be nothing stopping me from twisting cloth wire together,
after having tested the design.

Well I used flat 6-core phone cable (ie. not twisted pairs) which is the standard internal phone extension cable. I was advised not to use Cat5 data cable because the pairs are twisted.

Apart from obeying those instructions, I don't really understand the physics that determines Q for a loop antenna.

Maven

Hello

I think the reason the recent design used telephone wire was probably because it was cheap, I would have thought that using thick wire like that would have caused some drag in high winds.
I have built both designs, the earlier version was more complicated, but very elegantly was able to feed the tuning voltage up to the remote unit and the power, and send the tuned signal down one piece of coax.
From memory the peak was quite narrow, so you had to have a signal to listen to, before you peaked the antenna for maximum gain.
This design is a lot simpler, and doesn't have such a sharp response, which makes tuning it a lot easier.
Telephone wire is a lot cheaper than coax, I am not sure as to how durable it is exposed to the sun and rain...


Mark

Hi Maven,

Re - "Apart from obeying those instructions, I don't really understand the physics that determines Q for a loop antenna."

.... my best guess
... all aerial systems are LCR resonant circuits. In a Loop antenna, the Loop provides the L. The Q is the same as Q for an inductor.
... keeping wires apart reduces self capacitance, allowing better control by the 'added' variable C.
... twisted wires ...??? I thought that twisting wires was a method for reducing stray interference onto input/output cables ie both wires were then equi-distant from the interference source.

Wikipeda has a good explanation of Q and explains the variables eg keeping R low improves Q etc..

Edit: self capacitance NOT self inductance as in 1st Post

Cheers,
Ian

Thanks for the email
I saw one of these on the weekend a friend has made,
and realise the individual conductors in the wire are used individually.

I just have reservation as the kit says it's suitable for the upper AM band,
and I'm trying to improve 693kHz. if anything other than pure experimentation.

The problem with all of these designs is they don't pass on any knowledge about an antenna by suggesting X turns of wire around a cross of X measurements.

It would make more sense to begin with a length of wire cut to resonate at the frequency you want which varies greatly across the AM broadcast band, and maybe not practical because a 1/4 wavelength is still a very long wire,
but I assume they still aim for an harmonic and go from there.

With a triple gang tuner, you can also incrementally switch gangs in & out of the circuit, and also add capacitors in parallel.

I had some luck here:
Image Link

Where a wire is outside, and this particular tuning gang is being used for 7 MHz band.
It can be tuned for maximum signal, or detuned for a strong station to reduce noise.

But I wouldn't have learned anything about antenna tuners from any AM broadcast design I've seen out there.