• 22 Giugno 2024 1:39

IK1TTD

An Italian Radioamateur Station

A new vision for Loop Antenna

Trying to built several kind of antennas, which is by the way so amazing for me, I recently found something absolutely astonishing. To be clear nothing that I have invented by myself, but this project works ! And work very well, so I really feel right to share my experience with other people.

The idea is not new, something that other HAM they have already tried to build, but honestly the mostly project you find around are not super simple to make it, and it’s requires some precisely skills to build it efficiently.

I am in contact with my good friend Riccardo IU1OPK, he involved me in his past project, trying to build an efficient Magnetic Antenna…. We did a lot of test, and some of them was quite encourageous, but at the end him first and me second we was not so happy with the efficience of that antenna.

While I was still testing the MAGNETIC TUBE ANTENNA (see in his blog the details), one evening he called me, and he share with me the new esperience he was doing. He sent a couple of articles from a Ham Magazine, where was showed how to build a LOOP MAGNETIC ANTENNA using a new vision.

At first when I saw the pictures I though was something that I could not realize, as normally these antenna has some specifics features that I do not like it too much:

  1. Magnetic radiantions
  2. Difficulties building some parts of the antenna
  3. Very narrow bandwith

But reading better the articles, soon was clear to me that this building idea was different and pratically super simple to make it… so why not try then ??

The idea it is to use a multilayer tube called PEXAL tube, made of plastic/aluminium/plastic used in the heating system… this tube it is SUPER SIMPLE to shape it … the second part of the antenna is done by a copper tube of specific lenght, with appropriate diameter to go inside the multilayer tube …. The third part is done by normal copper wire to induct the RF signal to the loop.

In this article I will avoid all kind of technical explanation, I will write a separate article for that, or you can still refer to Riccardo’s BLOG…. he is very prolific sharing all technical datas, I want just to convince you to build it and having a lot of fun with this MAGNETIC SIMPLE LOOP ANTENNA… so let’s start.

Buy list for a 20 meter LOOP ANTENNA

  1. Multilayer tube 20mm diameter
  2. Raw copper tube 15mm diameter
  3. Copper Wire 15 mm
  4. Plastic clip fitting the 20mm PEXAL TUBE
  5. Plastic T fitting the 20mm PEXAL TUBE
  6. Wood stick for painter
  7. Eletric Tape (not mandatory)

To start, cut the PEXAL TUBE for a lenght of 285cm, and give it a round shape, it will result a round of 90cm diameter about; don’t get mad to find the perfect round, it will not necessary for now…

Now we need to prepare the condenser, which will be represented from our RAW COPPER TUBE; the idea it is to realize a nice round loop, in order to insert almost completely in the PEXAL TUBE; to make this you can use several technics; I have use a bike wheel without the pneumatic, but I give you a small secret to proceed… as the COPPER TUBE is quite robust but also easy to squeeze it wrongly, my suggestion it is to fill the COPPER TUBE with sand, this will help you quite a lot; with the tube filled of sand, now you can pull it on the bike wheel, making some pression on it to let him curve until the angle desired; try to bend it in the most uniform way, avoiding to make different angle, if you do a nice job here the most is done.

Another suggestion I can give it is to BEND FIRST, and CUT AFTER… as we will use a lenght of 75-80cm of COPPER TUBE, and it is normally sold in 100cm bar, it is easier to bend 100cm as 80cm… so bend the full bar 100cm, and then cut at first 20cm… choose to cut the side less bend, in order to use the best and uniform curved part.

Now mark the COPPER TUBE right in the center (if 80cm, then mark at 40cm, and insert it in the curved PEXAL TUBE, leaving the center mark visible … it should result now a round of 90 cm of PEXAL, and a part of COPPER TUBE should be visible for about 15 cm…. Keep the mark always in the middle. The visible section of COPPER TUBE will change the resonant of the antenna.

Now  cut 1 meter of 1,5 mm wire, and with 2 PLASTIC CLIPS try to make a triangle; this triangle will change the impedance of the antenna, and we are looking for 50 ohm of course. Try at first to make about 12 cm of the wire hanged to the PEXAL COPPER with the PLASTIC CLIP, and let fall to the center of the LOOP the 2 part of the wire remaining… you should have a result like the picture here below. When you have realize the triangle you will use the T PLASTIC CLIP to hange the LOOP. The two terminal of the wire now will be connected to our cable incoming from the transceiver.

Now insert the PAINTER WOOD STICK in the T PLASTIC CLIP to hang the antenna; I was able to fix the LOOP using some tape on the tip of the PAINTER STICK, if you want you can do use different system… As the PAINTER STICK is normally 2  meters high, just use all the lenght, so you will hang the LOOP about 2 meter from the ground, higher is not necessary, lower should give you different resuts….. DONE ! Now start the sperimentation …

I have used the NANO VNA directly connected to the triangle to see how works… to find the right you will act like this:

  1. To adjust the impedance change the size of the triangle, and looking for 50 OHM. At the end I was able to find a value of 48,1 OHM which is almost perfect.
  2. To adjust the resonant frequency enlarge or reduce the diameter of the loop…

WARNING ! Starts from the measure of 15cm VISIBLE COPPER TUBE, and see where is the resonant, and then adjust FEW MILLIMETER and check again… enlarging the diameter (showing more COPPER TUBE) will move up the frequency, decresing the diameter (showing less COPPER TUBE) will move down the frequency.

Here the exaclty measure I have found:

  1. Diameter PEXAL TUBE 92cm
  2. Showed portion of COPPER TUBE 15 cm
  3. Wire Triangle Upper side 12cm

Watch out… these measures can be good to start, but should be not considered as mandatory, with most probably you will find your best fit with different measures; this is becasue the ANTENNA LOOP it is subordinated to the area around.

On 20 Meters Band you should have a Bandwith of about 40 Khz; at first could be very limitate, especially if you make CW and SSB, but you will see how will be fast and easy to move the resonancy. 

To change the band for resonancy will be enough to change the lenght of the condenser (the COPPER TUBE) and the same antenna will work on other BANDS, but this will be subject for another articles !

And please do not be worried, becasue at the start everything will result difficult, you will probaby take a good half hour to start seeing a good result on the VNA, but when you understand how works, istantly will became easy, and you will be able to find the right impedance and resonancy also changing the location of the antenna.

Once you find the right number, plug to the TRX and test it ! Start with 5 watt, and see what happens !

At first impression, this antenna it is super silent, comparing it with a VERTICAL ANTENNA, you will note that signals are overall lower, BUT THE NOISE WILL BE ALMOST INESISTENT; the result will be a nice and relaxing sound of HF Signals. During my test, with 5 watt and decent propagation conditions, I was able to works all Europe in CW, with some replies from Nord America Beacons; 10-30 watt will allow you to cover a larger area in CW and be responded from all Europe in SSB;  if you jump up to 100 watt, will surprise you as good are the results…. But I prefere to use it with lower pwr, safety first !

WARNING ! As the MAGNETIC LOOP ANTENNA will generate a MAGNETIC FIELD, be sure nobody is around the loop for 3/5 meters when you transmit ! Probably a cellphone make much more damage, but anyway I have bough a MAGNETIC FIELD TESTER, to check it, I suggest to do the same, so BE SAFE !

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