QRP 900 GRAMS HYDRAULIC TUNED SMALL MAGNETIC LOOP

My name is Alexandre Grimberg, my call sign is PY1AHD. I have born in Rio de Janeiro in 1949.I receive the PY1XLS call sign on the year 1978, and in 1987 I became “Classe A” with the call sign PY1AHD.I am a lawyer working on the security electronics business since 1974 on my own company called Radiocom (www.radiocom.com.br). I remember when I was young boy that while all my friends was playing football I was at home developing all kind of scientific experiments. Today after 50 years I am still developing my own toys.

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.

In practice I have only 700 grams of antenna on my back pack.

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Antenna weight

Excited loop

The excited loop is 1/5 of the main loop size. 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... 

Water tuned sliding variable capacitor

To make the sliding variable capacitor I cut two printed circuit board rectangles. The fixed one measure 180 milimetrs by 58 millimeters and the sliding one measure 58 millimeters by 70 millimeters. Do not forget a 10 milimetrs by 10 millimeters extension on the lower right side to of the sliding printed circuit board that will accommodate a slot to solder the coaxial loop braid.

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The copper over the fixed printed circuit board is over half of the board.It is important to cut a small rectangle measuring 2, 9 centimeters by 2 centimeters on the right top of the fixed plate to keep enough room for the right side of the loop conductor (coaxial cable).

The size of the coaxial braid from the end of the loop to the sliding plate slot is 250 millimeters.The coaxial loop cable extremes are fixed in place by three plastic ties on each side on the main base.

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On each side of the fixed capacitor plate I have carefully glued with cyanocrilate (Super Bond) two pieces on plastic” U” channel guides.

This guide is made for the hobby and modeling market in California by Plastruct by the reference “90583”.The fixed rectangle capacitor plate is glued to the acrylic base by double faced Scotch tape.The acrylic base measures 26 centimeters on each side, 9 centimeters on the top and 5 centimeters on the smaller side.

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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I developed a hydraulic system that can control the moving plate position resulting on a variation on the resonating frequency of the antenna between 14 megahertz to 30 megahertz. 

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The seryringer is a 20 millimeters type and the excursion is 7 centimeters.I have made a “U” shaped acrylic support transpased on my belt trousers that keeps in place the frequency calibrated 20 milliliters serynger that permits tune the antenna atached on my backpack.

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40 meters adaptor

A 180 Pico Farads fixed capacitor is connected to an alligator clip and lowers the resonating frequency of the antenna to cover the 40 meters band.

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Keep in mind that working on 40 meters with this antenna we can expect only
4% efficiency compared to a 40 meters dipole, but despite of this supposed low efficiency be prepared for great surprises 

Making the loop

The loop is made on RF 52 ohms coaxial cable. The coaxial cable reference that I use on this type of loops is the RGC-213. This cable have a 0,25 centimeters diameter inner copper wire that will keep the magnetic loop self supported on a perfect circle shape.The total extension of the loop circle is 254 centimeters, resulting on a circle of approximately 80 centimeters in diameter.The coaxial loop cable is short circuited soldering the braid to the inner copper wire.

On the left coaxial loop extreme leave 1 centimeter of the inner copper wire involved by the braid and make a good solder using a clean 100 watts soldering iron.On the right coaxial loop extreme leave 1 centimeter of the inner copper wire soldered to the braid.On this side you must leave 25 centimeters of braid on a semi circle shape to be soldered on the slot of the moving printed circuit board sliding plate.These 25 centimeters of braid will allow the free movement of the capacitor sliding upper plate.

Water tuned loop schematics

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What is the efficiency of this QRP magnetic loop?

According on G4FGQ Reg Edwards software:

Band	Efficiency	Loss in db (ideal antenna)	S-Units
10 12 15 17 20 40	90.7% 84.5% 73.43% 60.50% 37.80% 4.6%	0.4 db 0.7 db 1.3 db 2.2 db 4.2 db 13.3 db	0.1 0.1 0.2 0.4 0.7 2.2

I hope that with this project I can stimulate the imagination of the radio amateurs all over the world.I would like to say that I have more then 500 Dxs over the last four years with a FT817 and one of my loops and many of them with other hfpack stations.One of the most interesting was a pedestrian mobile to pedestrian mobile QSO with G0SBW Tom in England on 17 meters band registered on the Hall of Fame of the HFpack group as a world distance record on this category.Another one that impressed me takes place when I was walking on the sand of the paradisiacal beach of Coroa Vermelha in Bahia with the 817 and my Traveling Loop when I reach VR2XMT Charlie in Hong Kong returning to my call with a 5 by 9 signal report.

e-mail: py1ahd@ig.com.br

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