PA3ECT  Zunderdorp Amsterdam

         

  Subject:

                                  Magnetic loop antenna        PA3ECT@AMSAT.ORG

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Magnetic loop antenna

 

The story starts with getting some remainder pieces of coax cable, that was used in a radiosystem in the Amsterdam Metro underground, for making radiocommunications possible. so called leaky coax.

In the past I was doing several experiments with loop antenna's but they where small sized and ment for the higher HF amateur bands 10-15-20. By a friend of my PA3BKW we have build a larger loop from copper tube 35mm diam. conductor and a loop diameter from 2.6 meter.This loop worked well but I never had the opurtunity to test it real. At my home there is not much room for large wire antennas and I desided to build a magnetic loop antenna, Also in mind that noise and local QRM are lower on a magnetic loop.

On the internet I found a little program "loopcalc" that was a great help for me. Now I can give parameters and found the best size and frequency choises, alswell the voltages, capacitor values, circumference and effectivity.

The first experiment is the loop below on the picture, it must have a rendement of 11% (-10dB) against an ideal open dipole. First try to find out to match practice and theorie.


 

The first loop       

This poorly constructed antenna was introduced for the first time on 80 meters in the dutch amateur net and had a reasonable performance from the start. With fluctuating conditions it is very difficult to check the expected results of what the loopcalculator calculated (-11dB) on the parameters of this loop antenna compared with an ideal open dipole antenna.For a week I collected reports from other stations and that gives me a picture of the performance of this antenna. I was to compare with people having a too short dipole antenna for 80 meters but shurly not the weekest station.

I also had a reference station in the neighbourhood Joop PA2JM and with a lot of reports I could say that he was producing 1/2 to 1 S point stronger signal compared with my loop antenna. Joop was using a Diamond vertical with an extremely good signal on 80 meter for a vertical. At the time I can lay my hand on an other thicker coaxcable (1 1/4"), time was right for the next experiment.

Because I thougt it was a waste to cut the cable my first experiment was to build a 2 winding loop antenna, 4 meter diameter. First I bind the two conductors close together but that result in a resonantion frequency of 1.6 MHz with almost no capacitor value.

To spread the conductors by pieces of wood (11 cm) I can get a resonantion frequency of 3.6 MHz with very little capacity. Disadvantage was the weight and the sensitivity for rain, if the wooden spreaders became wet and the antenna looses performance. The results were not superb but better and with many reports and Joop for reference, our signals became  equal.

 


The second loop


 

The splitstator capacitor

 

In the end I decided to go back to a one turn loop antenna, cut the coax and made the loop 4 meters in diameter.Before cutting I steal an extra two meters and that results in a kind of magnetic egg shape. Later I bend these oversize from the bottom upwarts to the middle and that results in the shape on the picture. An advantage on this shape is that the birds landing on the loop, sitting on the cold side of the antenna having no effect on the performance. Good for me and good for their feet HI. second I could easily mount my vacuum capacitor with remote control in my loop standing on a pretty level. After a lot of connections and reports with other hams and with Joop for reference I can conclude that my signal rise 1 to 1 1/2 S points compare to Joop.This result had a reasonable match with the outcome of the loopcalculator which calculated a rendement of 60 % compared to an open dipole. My capacitorvalue is 125 pF for 3.6 MHz resonantion. This antenna performs equal to most half wave dipole antennas because they are mostly also not ideal.

 

 


The previous loop antenna   

This was my previous loop , in the tube is a vacuum capacitor from 10-1000 pF. tuned by remote control with a small motor. can tune from 1.5 MHz to 7.2 MHz.

 

The actual loop

Worked with the actual loop on 3797 Kcs

This is my vacuum C with motor drive.

 

The principal of the loop.

 


De Harrisses with the Henry 2KD boost the signal up with at least 8 dB transmitting power

 

This was an earlier qso with this loop antenna.

 

 


In between an experiment on 145 MHz.

 

 

Radius 20 cm with beam effect.  Front to back 12 dB, gain unknown.

 

 

From small to big. On the VERON eastercamp this loop experiment. 6.5 meter diameter. Works great.

 

 

 

 

The loop

 

 

 

 

De capacitor section

 

 

 

De capacitor section zoomed

 

 

End

PA3ECT  2007 Zunderdorp Amsterdam