Mag loop aerial for the Den

[Radio Fixer]

There was thread some while ago but I thought it best to start anew.

Trevor sent me to a site that gave some theory (with maths which I still haven't got too far in following ) and then I got to this practical site:  http://www.pa1m.nl/pa1m/simple-active-receive-loop/

Well, I have built the amplifier and made an aerial which is a single turn of 20 SWG wire and it works very well.  It does the most, compared to a strung piece of wire, with sets that don't have an RF stage and not much gain but still is better with more upmarket sets.  That's not giving it enough praise as it transforms an Ekco AC97 to closing the eye on so many MW stations   compared to 25 feet of wire strung around the room that doesn't close for any and only picks up a limited number.

   
   
   

Its for the Den and the idea is to mount the aerial as a square backing onto the centre double glazed window frame.  It may have to be spaced off as someone mentioned that there could be metal inside the UPVC.

It will be right behind the work bench and so ideal for testing restored radios.  At the moment the circuit board is on the bench connected by 4 ft of wire and so it might have less pickup when built as part of the loop.  I have tested it with short output leads and 5 feet of coax to more distant receivers.

I also made a transformer (similar to Trevor's design) and it makes a small improvement on 'eye tube' closure but not huge.  The idea was to match (somewhat !) the amplifier output (50 Ohms?) to the variable impedance of the radio.

The next step is to find / buy some 15mm copper tube and do a better job of mounting.  I wonder how much difference it will make?  The aerial output is proportional to wire area so it should.

Hope this is of interest and comments appreciated    Gary

[pwdrive]

Good work Gary, yes the UPVC windows will most likely have some metal inside them...the sides almost certainly.

Jeez...Tidy garden, puts mine to shame.

Lawrence.

[Radio Fixer]

Thanks Lawrence.  I want to put the aerial so its obscured from the outside so it has to be near the sides of the centre frame.  How much effect I wont know until I try it.

Actually, the garden is a mess at the moment as we don't really start on it until about now but of course if you have clipped and cut back in the Autumn it helps a lot.

Here it is when first landscaped by me, in 2010, and a bulky windsurfing mate as labourer !!

   

Gary

[ppppenguin]

I know very little about antennas and propagation so these may be daft questions.

What is so special about these loop antennas that gives them such good performance? How important is the amplifier? Is it dependent on modern transistors or could you design a valve circuit that interfaced well with such an antenna?

[Joe]

There's some more information here: http://www.mds975.co.uk/Content/aerials1.html

[Radio Fixer]

Well Jeffrey the theory of magnetic loop aerial is here and for making a single turn loop probably gives the best information.

http://sidstation.loudet.org/antenna-theory-en.xhtml

Now no where have I read this so it maybe a Gary myth but I think more interference is in the electric field than in the magnetic (mainly picked up by the loop) and so by using that the signal to noise ratio of the loop is better than a long wire and in my case more convenient.    
Yes! the signal goes down and that's where the amplifier comes in.  Typically these loops are made for receivers with 50 Ohm inputs and hence the choice of 100 Ohm resistors for the amplifier collectors.  It might not look as if it has much gain, and I'm sure I'm about to write stuff that many already know but bear with me for those that don't.  The emitter of the long tail pair doesn't move to differential inputs.  As the current goes up in one transistor it goes down in the other and in the ideal balanced set up the emitter has no signal across the 'tail'.  The emitters are said to be at AC ground.  Thus the gain of each half is approximately re (the internal transistor emitter resistance) divided into the collector R.  There is an ancient rule of thumb that re =25 / I where I is in mA.  Each transistor is passing about 30 mA but lets keep it simple and say re = 1 Ohm.  So the gain of each side of the pair is 100 /1 or about 40 dB and for the pair add another 6 maybe (well un terminated).  This is substantial and the loop performance without the amp would be of little use.

My window loop is not going to be as good as my Wellbrook that according to , I believe Trevor, is ferrite filled ? and is also on a rotator( and way down the garden and far from Main)s but for normal MW and LW stations I don't find that too much of an advantage, perhaps because they are mainly omni directional.  I will try rotating the window aerial and see what difference that makes, if I can do it with breaking something; very little room in the Den.

Will add more as I play or try suggestions,  thanks   Gary

[Murphyv310]

Hi.
Nice work there Gary
To clarify, there are two types of loop that we have been talking about here.
The active magnetic loop, & the tunable frame aerial. Both have disadvantages and advantages.
The tunable frame aerial cant be successively mounted any distance from the operator and receiver, simply as you need to track the frequencies your tuning unless you make a remote drive for the tuning gang, Varicap diodes have been tried but these will rectify strong signals and cause addition of harmonics and distortion.
The frame can supply a large amount of signal and will pull in weak signals that a long wire will struggle with, 8 turns round a 1 metre frame equates to 32 meters of a long wire and will perform far better than those 32 metres of wire will stretched out!
So the frame can give a very strong signal, its excellent for weak signals, it falls down due to the need for tuning and can also be noisy.

The active single turn loop requires no tuning, they function better with a larger cross sectional area wire or tube and are pretty well immune to mains derived noise. The signal level from the single turn is low so a balanced amplifier is needed to boost the signal to a usable level. No matter what type of amplifier is used some noise is generated but selecting low noise transistors or chip this needn't be an issue.
The single turn loop can be remotely mounted as no tuning is required, this can be an advantage to get some extra height.
They also can cover from Long Wave to above 30mhz dependant on the amplifier design.

Both frame and loop have very strong nulls and this can be a big plus for DXing adjacent stations.

Have fun

[ppppenguin]

Presumably this is also why a ferrite rod works so well as it also receives the magnetic component of the wave.

Tranistors can give a lot more gain than valves since, as Gary implies, gm (mutual conductance) is proprtional to emitter current. Gain = gm * collector loadresistance. This is just expressing Gary's rule of thumb in a more usual form. It also requires that the fT of the transistors is much greater than the frequencies of interest which will always be true for modern transistors and medium wave. Transistors are also low noise, perhaps not in absolute terms but much lower than the atmospherics etc found at these frequencies. The downside may be if strong signals are present as you can get overload and intermodulation. I understand that this is why RF power devices were often used at the front end of solid state comms recivers.

Am I right in thinking that if you wanted to use this sort of antenna with a valve front end you'd probably need to use a grounded grid stage to get a low input impedance? Or have I got something horribly wrong.

[Radio Fixer]

I cant comment on whether you would need a grounded grid stage before each side of a valve long tail pair Jeffrey but it would be awkward to implement if the single turn loop were remote. Using transistors its easy to arrange to supply the 12V down the coax cable but for vales ... well HT and then heaters ...

Gary

[Murphyv310]

Presumably this is also why a ferrite rod works so well as it also receives the magnetic component of the wave.

Tranistors can give a lot more gain than valves since, as Gary implies, gm (mutual conductance) is proprtional to emitter current. Gain  = gm * collector loadresistance. This is just expressing Gary's rule of thumb in a more usual form. It also requires that the fT of the transistors is much greater than the frequencies of interest which will always be true for modern transistors and medium wave. Transistors are also low noise, perhaps not in absolute terms but much lower than the atmospherics etc found at these frequencies. The downside may be if strong signals are present as you can get overload and intermodulation. I understand that this is why RF power devices were often used at the front end of solid state comms recivers.

Am I right in thinking that if you wanted to use this sort of antenna with a valve front end you'd probably need to use a grounded grid stage to get a low input impedance? Or have I got something horribly wrong.

Hi Jeffrey.
Indeed its the magnetic component of the signal we want to use, the tunable loop falls down a little here in comparison to the single turn active loop. I personally wouldn't consider a valve preamp, faffing about with supplies to the loop is an added complication. Don't forget the transistors in the amp are driven fairly hard (they run warm too) to keep their strong signal handling capabilities up, this reduces intermodulation distortion.
Ideally RF medium power transistors should be used and in fact the humble BD131 works very well in the circuit.
I also think some experimentation on the RF output transformer could yield even better matching and thus performance.