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  "This site is dedicated to the electronics hobby community people that feel something magic anytime they send a radio signal trough the air"
PY1AHD ALEX (Alexandre Grimberg) 		 
 

What is a magnetic loop antenna?
 
Child and Radio 1925              Franz Fiedler (1885-1956)    

There is plenty of written information about the small magnetic loop antenna. I will try to explain to the newcomer in simple words how it works.

I would like to have the reader’s permission to express my personal conclusion after seven years in designing and field testing small magnetic loop antennas.

“The high “Q” factor presented on the small magnetic loop antenna that obligates the user to retune the antenna at minor frequency changing is far from to be an inconvenient, but a fabulous electronic feature to improve the receiving and transmitting”

1) Think on the loop as an RF transformer. The primary is the excited loop and has 1/5 of the size of the main loop. It is constructed with any kind of self supported wire for instance thick copper electric wire.

The secondary is the outer circle conductor closed by the variable tuning capacitor that irradiates the radio frequency energy. Remember that there is no electrical contact between the excited loop and the outer loop.

2) The magnetic loop antenna is a high “Q” device. The band width is narrow and the user is constantly requested to retune the antenna for the lowest SWR in transmission any time there is minor frequency changing.

3) The high “Q” of the magnetic loop works like a front end tuned filter circuit. It acts as a pre selector greatly improving the adjacent channels rejection.

4) The small magnetic loop can be used on the vertical plane or on horizontal plane.

Generally in outdoor activities we use the loop on the vertical position and turning the loop on a vertical pole we can kill sources of electrical noise and other kind of interferences. On the vertical position the loop presents a horizontal “8” shape radiation pattern similar to the dipoles.

On condo windows and hotel balconies we use the loop on the horizontal position. On this case we have an Omni directional pattern, in other words the irradiation is like a vertical antenna.

5) The small magnetic loop works perfectly at low heights, normally a 1 meter diameter loop works perfectly at 1 meter above the soil.

6) And the last and most important advantage of the small magnetic loop:
No counterpoises needed.

Historical references in portable magnetic loop antennas

     
          Balkan’s War in 1942                     Alder-shot England, November 1937

AlexLoop SML 7-30 Portable Small Magnetic Loop Antenna by PY1AHD

The AlexLoop SML 7-30 small magnetic loop antenna is the first commercial antenna designed by PY1AHD to be used with the new generation multiband QRP HF transceivers such as the FT817 and the IC 703.

The AlexLoop SML 7-30 is the result of thousands of “on the air” field tests with more than 800 confirmed DXs and a registered record on the Hall of Fame of the HF Pack group on 17 meters band (http://hfpack.com/hall/).

There is no other portable antenna as efficient for its size and weight.

There is no other plug and play antenna available so easy to use.

You will need no more than 10 seconds to be on the air.

1) Leave the Velcro that accommodates the cable to the antenna.

2) Connect the cable to the RF output and turn on the transceiver.

Imagine those features with all the freedom of a no counterpoise antenna.


To tune the antenna you must adjust the front button to peak the receiving noise and then fine tune for the lowest SWR in transmit on any selected frequency between 7 Megahertz to 29.5 Megahertz.

 
The AlexLoop SML 7-30 can be supported on the field, mountain, beach and picnic table operations on a standard short pole.

The AlexLoop SML 7-30 Small Magnetic Loop antenna will work perfectly only one meter above the soil so you can operate you system comfortable seated on a camping chair, or even holding the antenna on your hand.

Always remember to keep the antenna tuner control knob at your hand level for fast and easy tuning.

The AlexLoop SML 7-30 is the perfect low profile solution for antenna restricted hams and short wave listeners that can use this antenna supported on windows, balconies or even inside his radio shacks.

Mounting the kit AlexLoop SML 7-30 Portable Small Magnetic Loop Antenna



The kit can be built in less than 20 minutes following these simple six steps:

1) Mold with your hands the most perfect possible circle with the supplied soft copper tube and put it aside.

2) Open the tuner box cover taking care with the variable tuning capacitor and put the upper cover aside.

3) Locate the center mark on the middle of the soft cooper tube and fix it to the boom making an “X” with the two smaller supplied plastic ties cutting the plastic excess.

   
Front view                                         Back view

4) Carefully introduce with moderate pressure each extreme of the soft copper tube circle on the left and right holes on each side of the tuning box and fix them firmly with the supplied bigger plastic ties.

5) Solder the wire supplied soldered to the tube to the soldered strip on the variable capacitor.

6) Close with soft pressure the tuner and use the two supplied screws to close the tuner box and finish the antenna kit mounting.

Now you are ready to launch your QRP signals on any amateur HF radio band between 7 MHz to 29.5 MHZ with the antenna that shows the best efficiency per size relation ever offered to the radio amateur market.



PY1AHD ALEX