Small transmitting loop antennas or so called Magnetic loop antenna
It was when I was searching some antenna parts to one of my ham friend in my junk shack, I saw my humble Mag loop lying silently, so I thought of getting it back to my operating shack and powered it! Even the capacitor I made is not very good it worked on 20 meters!!
I thought of writing this small article about the TLA (Tansmitting Loop Antennas) in view of serving our ham community in Sri Lanka with my experiences, constructing it. It all began reading an article I got from a friendly ham when I was domiciled in OZ land! Not forgetting some valuable articles via the net I read. It’s a pity that it is not very commonly used, discussed, or constructed by our local hams!
I am not going to over ride a well tuned three element beam with this wonderful, small transmitting magnetic loop antenna, or any other antenna, constructed, pain taking research done by our worldwide hams! If this loop constructed observing the necessary finer points one can enjoy the fun I am sure about it! So why is that this magnetic loop is not popular? I think it is
(a) the knowhow has been a hidden secret not very much discussed.
( b) Physically looking at it is a very simple circular, hexagonal, or a square loop, considering, comparing, to a very expensive, commercial, or home brew set up!
(c) Magnetic loops are born with very limited band width, and extremely high Q factor.
(d) the most critical, expensive item is the tuning capacitor, the ideal is a vacuum tuning capacitor, or a butterfly capacitor, home brew various capacitors, e.g. trombone type, (coax leads tuned to mono band) a cap made out of printed circuit boards.etc, etc! Also remotely tuning!
(e) The tuning of this Mag loop is normally done remotely, As hand capacity upsets the tuning! these needs motor driven systems, sometimes with stepper motors with sophisticated digital circuitry.
(f) A magnetic loop antenna is very prone to hand capacity, and extremely very high voltages are present at the capacitor, may be K, volts with lots of Amps! Normal tuning caps are not suitable as the rotor is connected with a sliding wiper connection! Even a very small Ormic resistance will affect the antenna. I believe those are the main reasons for not very popular with some of our hams, even knowing it very well!
The other factors in constructing this mag loop, all joints must be welded or silver soldered. Copper tubing with a fairly large circumference must be utilized. Please refer to the web articles found, there are very good mag loop calculators via the net if one wants to construct.
This small transmitting magnetic loop antenna is very suitable for limited space installation, such as apartment dwellers. Not required very high towers, a normal dipole or a G5RV or any other antenna requires at least ¼ wave length in height. So just imagine an 80 or 160M what height one should have! A mag loop does not require ground radials, (some use short radials to make an artificial ground) normally a Magnetic loop antenna does not require such a height from the ground level; approximately loop diameter height is required for this wonderful device! The direction of radiation is with the broad side, with a donut pattern, the articles found via the net say this has a vertical and horizontal radiation too. Installation of this wonderful device must be away from metallic beams, hidden concrete, power lines, or the like! Also this antenna producing high voltages one should realize about children or house pets nearby! Please read the article, http://www.ahars.com.au/wp-content/uploads/2014/05/The-Underestimated-Magnetic-Loop-HF-Antenna-version-1.1.pdf
Very hard to find the required tuning capacitor. Home brewing is possible but it is pains taking. Copper tubing joints should be welded or silver soldered. Prone to hand capacity. Tuning should be done remotely. Extremely high voltages, amps are present with the loop! It must be installed away from children, pets etc. A non metallic small mast must be utilized in installation. In attics fire hazard can occur if installed in attics! Care should be considered when installation place.
Tuning systems using stepper motors, digital electronics utilizing PIC or other devices are not freely available in SL.
The aim of my little article is to draw the attention of our local hams, hope they will be interested in this antenna. Any corrections, any suggestions, will be greatly appreciated!
Please note this antenna is generating very heavy voltages and currents, so please be very cautious!!
The experiments should be conducted with their own responsibility, at their own risk please! RF radiation is lethal!!!!
Kind Regards and Best of 73!