QRP 900 GRAMS HYDRAULIC TUNED SMALL MAGNETIC LOOP

For the last four years I am designing and field testing small magnetic loop antennas for QRP HF back pack operation and the loops became my FT817 best friends.


The first step to build a magnetic loop is to find out a variable capacitor. If you have a 50 Pico Farads maximum and a 10 Pico Farads or lower minimum capacitance split or butterfly capacitor in your hand, you just have the passport to the amazing small magnetic loop world.The small magnetic loop described on the text can tune by the home made 5 Pico Farads to 50 Pico Farads variable capacitor any frequency between 10 meters to 20 meters, and with the parallel association of a 180 Pico Farads capacitor it covers the 40 meters band. Working QRP (less then 10 watts) allows me to design some very interesting variable capacitors from metal tubes (piston type) to sliding capacitors made on printed circuit boards.If you want to run 100 watts of power you can look for a big plate spacing split, butterfly or the high isolation vacuum type capacitors. High isolation capacitors are expensive, big, heavie and hard to find.If you design a 100 watts loop you will need to know the risks that the RF exposure generated by the high concentration of RF energy on the loop. So is recommended keep at least 15 meters distance from the loop.


What is a magnetic loop antenna?


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


1) Think on the loop as an RF transformer. The primary, the excited loop is 1/5 of the main loop. It is constructed with any kind of self supported wire (can be thick copper wire, coaxial cable shorting the braid with the inner wire, etc…). The secondary, the main loop is the conductor circle 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 transmission any time there is a 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. When you turn the loop on the vertical plane you can kill sources of electrical noise and other kind of interferences.
5) The small magnetic loop works perfectly at low heights, normally a 1 meter diameter loop works perfectly only at 1 meter above the soil.
6) And the last and most important advantages of the small magnetic loop:
NO COUNTERPOISES.

This antenna was presented to CQ Magazine readers by Dave Ingram K4TWJ on his World of Ideas column on the October 2006 issue and translated to Spanish to the December 2006 issue of Radio CQ Magazine.The antenna including the control serynger, the cable and the connector have a total weight of 900 grams, and when I have the adjusting serynger on the acrylic support on my belt results on a weight 200 grams lower.

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Antenna weight

Excited loop

Water tuned sliding variable capacitor

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The antenna “boom”

The antenna “boom” is made on a one inch diameter water plastic pipe. The tube is 1 meter long and the serynger fits with little pressure inside of the tube inner wall.

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Water tuned loop schematics

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e-mail: [email protected]

p:/s tribute to Mr. Alex PY1AHD. My magnetic loop antenna tutor. http://alexloop.com/