..Magnetic Loop Antenna

2001
02.26

Introduction

Due to my job I have top travel a lot around the world, in order to find a way of making radio I decided to build a Magnetic Loop after some comments of other colleagues for working between 20 and 80 meters mainly. After several tries I built a couple of them and keeping in mind that I was going to work with QRP I decided to design a big magnetic loop, at least 2 meters of diameter.My QRP rig was a DXR20 and TX2000. As my QTH was quite fixed that size was ok, however for travelling I realized that the best antennas are the Indoor ones ).

In any case, the results were quite good and I am still experimenting with them.

Magnetic Loop made with RG213(QTH Madrid)

I built this antenna following the experiences of other colleagues like, for instance:
- KC2TX
- ON4CEG
-
PA3CQR
- TA1DR
- VK3YE
- OK1FOU
- PA3HBB / G0BZF
- G3BGR
- VE3FMQ

in addition to other collaborations by means of e-mail from CT1ETT (Roger), ON4CEQ (Tony), G4FGQ (Reg) , KJ6HZ (John), GW0TQM (Carl)y AA5TB(Steve).

Keeping in mind all the suggestions and recommendations and some formulas that I found sailing through the web I prepared an spread sheet with all the calculation that could help me to build/tune my magnetic loop plus to build another types of loops, please note that the boundary conditions could cause differences among the values obtained experimentally and theoretically.

You can download my spread sheet (Excel 7.0) from here:EA5XQ_MagneticLoop.zip

Construction
The diameter of the main loop was 2,33 cm. and I used the shield of RG-213and a variable capacitor of only one section 24-440 pF placed on the top of the loop and driven manually (this is the worst step because I hadn’t any motor to control it remotely and the tuning was quite hard)and the driver loop made with the inner of the RG213 and with a diameter of 1/5 the diameter of the main loop.The feeder to the rig was RG58-U.

Magnetic Loop

Due to its big size, obviously there are better ways to do it, I used plastic tube PVC of 5 cm. diameter and in order to keep the loop in the same plane in all its points I used a sheet of cork (see the picture)you will see that with this size the RG213 is easy to be folded up.

I was testing the antenna in several places in the apartment trying to find out which is the best place with the minimum noise, at the end in my bedroom was the appropriate place (even though my wife did not think the same). I tested the antenna in the dinning room but the noise reached S+6/7 (most likely due to the fact that the elevator of the building was closer to it).

The rig used was my RIG Howes (DXR20 and TX2000 ) and the output power was 5W. Working QRG 7.0-7.40 Mhz.

Bandwith

The bandwith was quite narrow in 40 meters, for example measuring SWR between points of 1,5:1 the bandwith in 40 m. was 6 KHz. even though the values provided by the formulas were quite different.

Tests Performed

After spending a lot of time trying to contact with someone and taking into account my boundary conditions I was able, at the end, to make the following QSOs:

 

QRA RST received RST sent
SM5NBE 449 599
F5NJF 559 599
EA1VB 229 599

These results encouraged me to follow testing new designs and in this case to build a new one with Copper tube, someone told me that the performance is going to be better and the losses smaller.

Another advices from USA colleagues were related to use RG-214 instead of RG-213 because the shield is double and coating with silver and therefore the conductivity is higher.It is un my to-do list.

 

Magnetic Loop with Copper Tube of 15mm diameter (QTH Monterrey: XE2)

In my trip to Monterrey as XE2/EA5XQ/QRP I built a Magnetic Loop for 20m with copper tube.

Construcción

For building the loop I used copper tube of 15mm diameter (0,55″) and with a lenght of 3m (118″)what it makes a loop of 95cm (37,4″) of diameter. The loop was not a perfect circle due to the fact that I had to give it the shape manually, without any other mechanism. In the picture you can observe the antenna inside home and fitted by a chair … without comments…..

As at the beginning I had not any capacitor, and following the results of the calculations provided by the excel spreadsheet that appears in this page – a little upper-, I used a piece of 50cm (19,6″) of RG58 (which capacitance is 100pf/m) and sliding the shield respect the central I tried to get the resonance in 20m (14.060 KHz). As I could not I added in parallel another piece of 10 cm. (3.93″) as you can see in the picture:

thereafter I was moving the main wire until the resonance in 14060 KHz. with this configuration I was able to contact with K4TWJ.

After a trip to San Antonio (Texas)I was able to acquire a variable capacitor from SURPLUS in KCOMM Communications and that facilitated me the task of moving around the band. Now, I need to find a motor for controlling the CV remotely because manually is quite complex to change the frequency due to the fact that you have to be moving from the rig to the antenna and avoiding to be close to the antenna when you are measuring the resonance.

For feeding the loop I was using, following the rules, a mini-loop 1/5th the size of the bigger made with RG58. Here you can appreciate the “high technology” for fitting the wire to the structure ….without comments either.

Bandwidth

The Bandwidth for this antenna, bearing in mind the SWR between two points of 1.5:1 was of 35KHz and if you use as limit 1.7:1 the bandwidth was 50 KHz, I mean for the whole CW segment for instance.

Tests done

I’m still working with this loop, trying to improve its performance, for the time being these are the results:

QRA RST received DAY QRG SFI K A
K4TWJ 339 20/08/02 14.048 237 3 15
KD5EOO 549 24/08/02 14.055 210 3 10
EA5BM 429 25/08/02 14.045 179 1 7

As soon as I will get more results I will let you know.

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