The fractal magnetic loop antenna
Fractal has a unique petal-pattern main loop formed from aluminium flat bar 3 mm thick, 36 mm wide. The overall diameter is about 87cm and it has eight zig-zag spokes or petals. It looks a little like a child's “Crazy Daisy” toy that you connect a garden hose to, hence the name.
The loop is brought to resonance with a “butterfly” split stator variable capacitor at the top which is in a weather-sealed enclosure. The capacitor is connected to the loop via bolts secured by wing nuts.
The capacitor is turned by a small DC motor and reduction gear and controlled by a small lightweight controller that is fed via a plug-in 12 V power supply, similar to a mobile phone charger. The box has a “fast” and “slow” switch, which reduces the voltage to the motor and a rocker switch for moving up and down the band.
There is no “up” or “down” button as such as the capacitor can rotate more than 360 degrees. This means that going “up” in frequency would depend upon where the capacitor position was to start with.
Tuning is done by listening for increased band noise as the capacitor is turned and then fine tuning to give the lowest SWR. The antenna is fed with a single-turn Faraday loop to which your coax is connected.
Victor says that the antenna can take up 400W although in testing I limited this to 100W.
The whole antenna is about three feet wide, relatively light and fits on a single pole, which I supported in the ground using a screw-in support obtained from Clas Ohlson.
The theory of how a magnetic loop works is quite well documented and I have used two different commercial varieties (MFJ and Capco/AAA) on and off for years so know pretty much what to expect.
When mounted vertically they offer a fan-shaped radiation pattern in a line with the loop with nulls off the side. At the horizon the radiation is vertically polarised, although overhead it is largely horizontal – just think about what you see when you look down on a magnetic loop and you can see why.
If built properly a magnetic, or more correctly small tuned loop (STL), can be very efficient, approaching dipole-like performance on 28 MHz (10m) and a few dB down on the lower HF bands. I say “built properly” as you have to ensure that losses are kept to a minimum by using good quality components with very low loss connections, such as welded or brazed. A few milli-Ohms of loss can drastically affect the performance of a loop and many proponents of the loop art advocate using vacuum variable or split stator capacitors to reduce losses.
And if you think that everything is known about the STL and its performance think again. Rich Fusinski K8NDS is currently experimenting with helically-loaded magnetic loops that use copper flashing rather than tubing. When fitted with a vacuum variable capacitor and fed with a gamma match he is getting good reports and can even run 1kW into them without problems.
There is a debate about whether K8NDS's antenna is more efficient than a conventional loop made out of copper tubing, but it is good to see experimentation going on.
Anyway, back to the fractal, on test it was found to tune from 12.4 MHz to 29.5 MHz. The positioning of the Faraday loop was found to be critical, but I was able to get the SWR down to around 1:1.2 or less on some frequencies and less than 1.8:1 across the whole frequency range.
As you would expect the bandwidth once tuned was narrow – about 50 kHz between 3:1 SWR points on 20m (14 MHz), 84 kHz on 15m (21 MHz) and 125 kHz on 10m (28 MHz).
On receive the first thing that struck me was just how quiet the antenna is. Band noise was minimal, which meant that weak signals were received loud and clear. Tuning around it was clear that the loop was receiving just about everything my main antennas could hear. Usually signals were down a little when compared with my dipoles, but the lower noise floor meant that the overall signal to noise ratio was about the same.
This suggests that the antenna would be an excellent choice for a short wave listener who doesn't have the space for a full-size antenna. One antenna could be used to cover everything from 12.4 to 29.5 MHz, including amateur, shortwave broadcast, aeronautical and anything else in the spectrum. If you live in a flat with no access to a garden the fractal magloop could be located on a balcony and enable you to play radio.
So how well does the “Crazy Daisy” work? I decided to test the antenna using WSPR (Weak Signal Propagation Reporter). I used 5W and transmitted for two, two-minute periods with the loop (tuned to the appropriate WSPR frequency). I then switched and antennas and transmitted again.
By going to www.wsprnet.org I was then able to see where my signals had been received and what signal strength. I then took only the reports where the signals had been received from both antennas and calculated a mean (average) signal strength.
The results showed that on 20m (14 MHz) the Crazy Daisy mounted with ther base just four feet off the ground was down about 9.4-9.7dB on a half wave dipole mounted at 25 feet.
To give the full picture, signals from the dipole were received by more reporting stations than thefractal, which is to be expected given the performance outlined above.
Similar results were obtained on 18 MHz (17m) and 21 MHz (15m), were conditions were not too good at the time of the tests.
On 10m the antenna performed slightly better (as you would expect, where its physical size is less small compared with the wavelength in use). Here it was around 6dB down on a 28 MHz dipole at 25 feet.
The different radiation patterns and vagaries of the ionosphere meant that some signals were similar in strength and some were worse.
This led me to believe that for optimum efficiency it would make sense to mount the Fractal Loop on a rotator so that you can align it with the area of the world you wish to work.
Once the tests were over it was time to think about the results and why the antenna performs the way it does. Current wisdom is that one of the critical factors of a magnetic loop antenna is the area enclosed by the loop. For example, for a circular loop of radius 40 cm the enclosed area is Pi x radius squared = 0.5m 2 .
Now, although the overall width of the fractal loop is about 87cm, its design means that it covers roughly the same area as a circle of approximate radius 0.3m, giving a total enclosed area of about 0.28m 2 .
Feeding these figures into the late Reg Edwards G4FGQ's loop calculating program (RJELoop1.exe) suggests that its performance might be about 3dB down on a circular loop of about 80cm diameter.
Either way, I think Victor GM0SDV should be congratulated on experimenting with different loop designs and the antenna enables a ham with little space to get on the air. The fact that the loop received and transmitted signals to the USA, Canada, Israel and Reunion Island and took about 30 minutes to erect says it all.
Victor says that he builds the antennas for fun and ploughs the money that he makes from selling them back into research and design – he is currently interested in fractal antennas, which he feels could offer a lot of performance for their size.
The Crazy Daisy is available from KMK UK Limited (e-mail: firstname.lastname@example.org
or web: www.mixw.co.uk ) for ?200 + P&P.
Fractal reduced size magnetic loop antenna
Reduced size 3 times than conventional magnetic loop (size 1.2X1.2-1x1 meter and 70X70cm) Fractal shape make antenna much broader bandwidth up to 120 KHz on 10 meters band Easy tuning slow motion power dc motor with double gears electrically insulated Capacitor Butterfly type 7mm gap between plates make them up to 9KV insulation allowed use up to 400watts power Ridged design easy assembling with size suitable for portable installation also balcony or loft
FRSMLA LOOP 80-30 meters band Frequency 3.45Mhz-9Mhz
Bandwidth 80 meters band 5Khz with SWR >1.5
Bandwidth 40meters band 18Khz with SWR >1.5
Bandwidth 30 meters band 32 Khz withSWR >1.5
FRSMLA -LOOP 40-20 meters band Frequency 5.600Mhz -15.300Mhz
Bandwidth 40 meters band 16Khz with SWR >1.5
Bandwidth 30 meters band 30Khz with SWR >1.5
Bandwidth 20 meters band 80Khz with SWR >1.5
FRSMLA-LOOP 20-10 meters band Frequency 13.300-30Mhz
Bandwidth 20meter band 70Khz
Bandwidth 17 meters band 80 KHz
Bandwidth 15 meters band 90Khz
Bandwidth 11meters band 90Khz
Bandwidth 10 meters band 120 KHz SWR >1.5