20 Meter Circular Magnetic Loop
Maurizio Malaspina - IW6DFW
My operating conditions are:
- Radio IC-7000 @ max power (100W)
- 20m of RG58 A/U coax --> more or less 75 percent of power to antenna @ 14MHz
- antenna placed on a camera stand on my 2nd floor balcony. 74cm height from the floor.
- ROS 1:1 (tuned manually... acting on the capacitor)
The antenna is a circular loop built with a copper tube (outer diameter 14mm, thickness 1mm).
The overall diameter is more or less 1m... in the reality, because of the strong wind that made it
fall to the ground, now it's elliptical and 96 x 85 cm. To be accurate the linear length of the copper
tube is 290cm and the air gap where the variable capacitor is welded is about 4.3cm measured
on the axis of the tube, in the middle (this measurement is not critical at all!!!), so if the circle was
ideal the diameter will be 93/94cm (less than 1m!).
As you can see from the pictures below,
the gamma match is a simple 48cm piece of RG58/U with the inner conductor and the shield
shorted together then welded respectively on the central of the SO239 connector (which the metal
casing threaded, directly soldered on the copper tube, inside the circle, in the lower position)
and on a point of the tube, inside the circle (watching from the connector front, on the right).
This simple wire gamma match is unconstrained.
Finally, on the top in picture shown above, is welded a variable tuning capacitor, a butterfly capacitor (it's important to avoid sliding contacts in the perspective to use an ATU in the future, to avoid sparks due to the high voltage generated). Its range is [41,117]pf measured between the two external fixed plates, moving the electrically insulated rotor. Each capacitor (measured between the single plate and the rotor) is [40,190]pF. Follow that a little residual stray capacitance in the range of [20,22]pF, depending on the rotor position. The air spacing between the plates of the rotors and each stator is between 1/1.2mm.
I report the most significant QSO (for QRB and/or QRP) performed from my station of Macerata (Italy) QTH locator JN63RH below:
Note-- All dates below were in 2012. Times of contacts removed for web page width control
and text size.
band width. As I have already anticipated, this is an high Q antenna, so the band within a
1:4 SWR is only 20/30KHz on 20m up to 90/100kHz on 10m.
I've also built a 2m loop, shown below......very simple, cheap and good performance!
- Loop: circumference 77cm, material copper pipe 1/4'' x 0.8 mm
- Coupling with tx: via gamma match
- Tuning Element: homemade variable capacitor dielectric lexan
- Connector: SO-239 panel welded to the loop
- Total weight: about 100g
- Tested with 50W (FM)
Ideal for indoor use (within the QTH) as it fits perfectly in the range 144-146MHz even from
inside the house.
Characteristic of high directivity, high Q and noise rejection
It can work both vertically and horizontally polarized.
In the case of the 2m antenna, the directivity features is more evident. With local repeater you
could go from 9+60dB, down to 2,3dB simply rotating it of 30 degrees. Also it is simple to
compare H and V polarization: try it to believe!!!
I hope my experience will encourage you to experiment with this little
and magic antenna:
you could build one rugged and place it on the top of your roof with the rotor and an homemade
I am proud to share this information with you!
Maurizio Malaspina - IW6DFW
P.S. feel free to contact me for further information about the loops and also on
the F5SWN 20m EH antenna I've built, shown below.
Email iw6dfw AT gmail dot com