MAGNETIC LOOP AERIAL. Diameter 3Mtrs
Starting at the top, the loop hangs on a rope and small pulley and is raised/lowered easily.
The tuning capacitor is a butterfly wide spaced type, with the seperate stators each connected to the open loop ends. The rotor is turned by motor via a wooden doweling rod.
The main vertical support is 70mm dia plastic downpipe.
It supports a small platform for the tuning motor, and also the coax feed loop. The rotator at ground level engages an extension of the pipe support with minimum stress on the rotator. The pipe, with loop attached, easily rotates within the one only loose guide collar as it hangs down from the rope.
Click on these pics for a bigger view.
The 3 volt tuning motor with plastic cover lifted.
This assembly is a Tamiya planetary gear drive. The 10000 RPM motor is geared 40000 to 1 to turn at 1/4 RPM. The tuning on these loops is NARROW. The little motor using two AA batteries (3Volts) develops nearly 2Kg torque at 1cm at only 400 to 1. It seems ideal for the remote control. Just two wires, low current, and a DPDT rocker switch to tune in either direction.
The tuning capacitor.
Butterfly type. Spacing to manage about 4Kv according to available figures.
This home-brewed one has an effective range of about 60pF. It's ok for 40Mtrs. 80Mtr tuning will somehow need about another 200pF.
It is easy to see why the Vacuum variable capacitors are chosen for this job.
The feed loop is about 1/5 the size of the main loop. It is sized and positioned to bring the SWR down to 1:1 at loop resonance. So far I can only achieve that by squashing the feed loop into an oval. OK - for now that's fine, it works very well.
Once set the SWR holds good for any frequency the loop is tuned to.
Just FORGET about antenna tuners, they are not helpful here.
Another view, progress so far is on 40Mtrs only. Results are far better than expected. I can honestly claim that when the loop is tuned "on the nose" with an SWR of 1, then it performs about the same as my 40Mtr trap dipole. This is confirmed by reports received.
It is certainly a very useful practical aerial and a winner in small spaces.
Changes and observations.......Original comments in italics.
Now - updated f rom experience! (April 09)
The SWR gets worse when it rains and certainly needs investigation.
Update - it is not surprising that wind and rain, (or any movement), had the effect of increasing the SWR a little. It has now been found that the capacitance change to tune from 7Mhz to 7.2Mhz is of the order of only two or three picofarads. So - the big variable C is a waste of time for operation on just 40Mtrs. If tuning to 80 mtrs is to be attempted then it clearly would be easier to apply a fixed capacitance to suit whilst adjusting the small variable.. I note with some dismay that spiders had taken up residence in the variable and most were dead! Also corrosion was beginning to show even after less than 12 months. Better protection would be needed.
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Tuning sure is sharp but is managable with practice at 1/4RPM ! (Only 90 Deg rotation is needed). Tuning may be easier if the motor is placed up top close to the capacitor - mainly to remove the wooden dowel rod and its lack of precision or rigidity. In practice initial tuning for maximum receive noise gets pretty close to good SWR. The final tune-up however invariably needs doing with tx tune power applied and takes a few seconds.
Update. No wonder the big home made variable capacitor needed gearing 40000 to 1. Another problem with such a gear ratio was the accumulated backlash in all those planetary gears. It took quite a while to take up the play when changing motor direction even though the gears are very nicely made. An over-run past the 1:1 SWR point meant changing direction with a time penalty.
Moving the motor to the top to avoid use of a long dowling shaft was nothing but trouble and did not make tuning easier.
So I replaced the tuning capacitor with a fixed capacitance of about 38pF. (rated at 6Kv -and using two 82pF in series). The best SWR of 1:1 fell happily on 7.151 Mhz. The 2:1 SWR bandwidth was 45Khz.
Using the programmes from Reg, G4FGQ, RJELOOP1 and RJELOOP2 it appears that only a few pF is needed to tune the whole 40mtr band. The calculated parameters from those programmes seem remakably accurate in practice.
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The loop rotator is not evaluated yet and not considered essential. It is believed that electrical interference can be nulled out should it arise. If the wanted signal remains within the two main lobes rotation could be useful for reducing any unwanted signal.
Update. The rotator does indeed help remove unwanted noise. Nulls are not as pronounced as I expected but the rotator works well and is useful.
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I should perhaps reduce the feed loop size yet again and see if I can form a circle whilst retaining perfect SWR.
Opinion seems diverse, some users prefer the gamma match. My main loop is 3Mtr diameter (across the flats), so the feed loop should be about 600mm according to all advice.
Update A smaller feed loop was tried - it did not do as well. The oval shape remains and works well.
This antenna was originally designed essentially using software available at W2BRI web site.
The loop size was chosen approaching optimum for 40Mtrs with circumference 9.9Mtrs.
Such a loop was predicted to need about 40pF to resonate at 7.1Mhz.
Update The G4FGQ programmes produce the following.
Results of ZL1BJQ previous enthusiasm!
(Click it to enlarge)
Left is 7Mtr Dish
Right is 8x10el yagi array on 36Ft boom for EME contact with W6PO 1976
Left 7.0Mhz
...............Right 7.20Mhz
(Click to enlarge)
The next logical step is to arrange a means of varying about a 5pF capacitor, slowly, to achieve tuning across most or all of the band. It will be in parallel with the now fixed 38pF, which in turn may need reducing to about 35pF. It should be quite easy to use the same motor to turn a thread which will move the proximity of a very small capacitor plate. For the time being 80Mtrs can wait.
Having tried a smaller feedloop with no improvement a bigger loop will be tried. The calculation says it needs to be 710mm. That is bigger than the original.
To re-iterate, when this loop is tuned "right on the nose" with a 1:1 SWR, it is barely a half S point below my trap dipole which is resonant at the same frequency and 30 ft high. This is also confirmed by reports received. Noise levels tend to be lower off the loop.
