Selasa, 21 Februari 2012

Homemade Magnetic Loop antenna for 27 MHz

Else known as Small Transmitting Loop or STL, this isn't a traditional "electric" antenna, because the "near field" is almost "magnetic". This interesting feature makes the ground plane losses very low and generally this antenna has little dimensions. Besides, STL produces little or no TVI and reception is clear from static noises and man made noises, this has a very narrow bandwidth (50 KHz) and for this reason is little or nothing disturbed from near strong signals (splatters).
If you look for commercial "loops" on the Internet, you'll find "monster" prices for them and if you aren't a Rockefeller, you'll never buy one of this smart antennas.
In 2004, I built a couple of them, a little one of 40 cm diameter and another one of 73 cm for 27 MHz but this last antenna runs from 23 MHz to 36 MHz with the capacitor I've made.
Do you like to build something with your hands?
If so, this is the right place. I'm not a Radio Amateur, I'm simply a radio user, maybe a radio lover, so if I'm able to make this antenna to run, why not you?
NOTE: All my antennas are built by recovered parts, so as to restore to life parts that will end to dump and be sure you'll not spend an arm and a leg!

Below, I describe my antenna, but you can modify near all the parameters with a lot of nice programs that you'll can find @ the links at the end of this page.
  • a piece of copper tube 2.29 metres, 12 mm of diameter
  • a plastic tube, PVC or similar, 1.5 metre long
  • 2 little piece of plastic 5 mm thick, 10x10 cm
  • aluminium sheet 1 mm thick and some brass screws and bolts (probably the only things you'll have to buy)
A very important thing is not to use ferromagnetic parts for this antenna, only copper, brass, aluminium are OK because ferromagnetic parts disturb the magnetic features of the antenna, lowering its efficiency!
I recovered the copper tube in an industrial dump store for 1 €, it had a very bad shape, not linear, nor circular, so I had to straighten the tube before I could use it.
Then I built a perfect circle in a very easy way: I taken a wheel rim of a sport bike, then pressed the tube against the rim's throat and MIRACLE! A perfect circle was done in about 20 seconds, am I like Giotto? :-) homemade magnetic loop antenna for CB
pressed end of my homemade magnetic loop antenna for CB Next I pressed the end of the tube in a vice and drilled a hole for end.
Now prepare the holes to pass the copper tube through...
The plastic tube was a part of a car, precisely an old Fiat Panda...
ehm... what is look like this? Click on the photo ;-)support holes for my magnetic loop antenna for CB
homemade butterfly capacitor for magnetic loop antenna So far, the job was easy to do. The most important and hardest thing to build of this antenna is the Butterfly capacitor. A good build is needed, else antenna's efficiency will be very low.
Notice that even at low power (5 Watts), there is a High RF Voltage ranging from 800 to 1600 Volt.
So, don't touch the capacitor when transmitting!
shock hazard Danger of death! shock hazard
The capacitor I built supports easily 5 Watts but I think I could use it up to 300-500 Watts, nevertheless I never tried power greater than 12 Watts (SSB).
This is all you need to build a Butterfly capacitor:
  • a: upper rigid plastic sheet 5 mm thick
  • b: lower rigid plastic sheet 5 mm thick
  • c: aluminium plates for capacitor's stator
  • d: aluminium "butterflies" for capacitor's rotor
  • e: brass bolts
  • f: brass washers
  • g: threaded brass bar to be cutted
butterfly capacitor's components for magnetic loop antenna
"a and "b" are the insulators and support the capacitor's stators and rotor.
The hardest part is to cut "c" and "d", because the scissors tend to deform and roll up aluminium so, at the end, you'll need to straighten the plates and you'll have to be VERY precise and to file a lot! Don't be in a hurry: do a good job!
homemade aluminium butterfly rotor This is an aluminium "butterfly" ready to be installed.
...and here an aluminium plate for the stator. homemade aluminium plate capacitor
brass washers for magnetic loop antenna brass washers...
brass bolts brass bolts for magnetic loop antenna
insulated support holes for my homemade butterfly capacitor Be precise!!!
Butterfly Capacitor mounted on the loop. homemade butterfly capacitor on the magnetic loop
butterfly capacitor on the magnetic loop antenna another shot of the Butterfly capacitor-Loop...
Here is the coupling loop. The coupling loop diameter is 1/5 of loop diameter and the easy way to build is using a piece of a copper wire, some use RG58 but it's a little bit difficult. coupling loop for magnetic loop antenna
motor and gear for tuning the butterfly capacitor A photo of motor and gear for tuning the capacitor. The next photo shows the particular of gear, ALL made from recovered parts of broken printers, both the stepper motor and mechanisms. The whole thing, gears down exaggerately with a 96:1 (96 stepper motor's turns for a 1 butterfly's rotor turn) that means 18432 step/revolution, very precise movement! :)
The gear... stepper motor for tuning the butterfly capacitor
ready magnetic loop antenna for CB The Small Transmitting Loop completed. This antenna was tested only indoor on the ground floor. Maximum distance reached was 55 km in central band, AM. This antenna has a deep null, very useful when present unwanted signals near the wanted one. VSWR is 1:1 but needs to be corrected when you move up or down 3-4 channels.
Here some nice experiments: when transmitting, you can light up a neon tube without plug it to the current! With light on and... ...then with light off... how to light up a neon tube without current and to see the elecromagnetic field
how to light up a neon tube without current What do you think, isn't there a little resemblance with Uncle Fester? ;-) resemblance with uncle fester
Below, you can "see" the electromagnetic field, approaching a neon tube! display of the electromagnetic field
design of the homemade butterfly capacitor Here is what you have to prepare to build the capacit

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