Tuesday, September 14, 2021

Magnetic Loop Antenna - The wrong path, questionable. But it ends well...

Tuning box outside
In my last post about experimenting with the MLA I commented to John AE5X that I had to figure out how to get my MLA resonating on 28MHz. I thought a smaller cap (switchable) would be a solution. John gave as answer a link to a calculator to see what value I would need.

https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx

I calculated my antenna as follows:

Antenna length 2.70 meter (notice don't use a comma but a point!), conductor 0.9 cm diameter, 28MHz, 10W.


I would be needing a 23pF capacitor. However in practice I needed a 9pF capacitor to match as good as possible, still a 2:1 SWR. Well, you see the comments: 2.7 m is not ideal. I thought that's why I probabely don't get a good match. The antenna is self resonating since it is a quarter wave long, I should avoid that. The only way is to make a smaller loop. A antennalenght of 2.55 m will do fine. Unfortunately it will cost me some efficiency. But with these kind of antennas it is always a trade off.
So I redesigned it and calculated it as follows:


Notice that I now actually measured the diameter of the coax braid which is 7mm. Tuning capacitance would be approx 29pF but unfortunately even using my minimum of 15pF gives me a max. usable frequency of 22MHz.

So, I was thinking now. I would be pleased with a small MLA that would cover 80m-10m. Normally that is impossible. But why not using an extra loop in series? A so called multiturn loop. Yes, it is not efficient, but always better as a dummyload. So, I found another calculator on the web. This one seems to be more accurate and you can calculate multiturn loops as well.

https://comtech.vsb.cz/mlacalc/

Calculating with the same values as before I found that the capacitor needed would be 12pF which is much lower, actually it is half the value I got with the first calculator. Strange this calculator didn't warn that the loop would be too long, but on the other hand, that's the theory you should know as MLA builder. I constructed another 2,5m long loop and connected it in series with the existing one. The 80m is available this way without any problems tuning it.

But now I was thinking, when visiting the website from OM0ET who heavily influenced me in designing my own loop. Why is he using a too long loop (3m) and still be able to tune it between 40-10m. Someone pointed me to a possible answer. OM0ET only uses the core from the H1000 coax he uses. This is what is written on his website:

The Main Loop is made of an excellent quality coaxial cable BELDEN H1000 (made in Holland) which is 10,3mm thick in outer diameter. Inner conductor is made of solid Bare Copper 2,62mm thick core which allows to hold the perfect circular shape of the loop without deformation of gravity. In all my Magloops is connected only the inner core. Outer shield is disconnected for improve the SWR results and less sensitivity of nearby objects.

Theoretically this can't work very well since the core has only a diameter of 2,7mm. And we all know as experienced loop builders that the larger the diameter the more efficient your loop will be. But you never know, I could be some coupling between inner/outer conductor takes place. At least I found it valuable to experiment with it and I only connected the inner core to my tuning box. Unfortunately tuning the antenna was almost impossible now, and it looked like it was even more sensitive when I came close with my hand to tune the whole antenna. Besides that best SWR was 1:1,7 (R85 X=15) at most peaks and highest good SWR available was at 20MHz. Not really a success. So, I connected the shield again to continue my quest for tuning the MLA to 28MHz.

I measured the capacity of my variable capacitor again. It is 15-525pF approx. The minimum capacity is too large to tune the loop above 22 MHz, that is something for shure. I needed a system to lower this capacity. So I thought of a simple switch, switching a capacitor in series with the existing one or short this capacitor out to disconnect it. As a simple capacitor I used 9cm of RG58 instead of a mica cap used before, it has a value of about 9pF in theory (in practice as well after measuring). The nice thing.....it works! I'm able to tune 24,8-28,8 MHz now. 12m and 10m with a flip of a simple switch. 

However, with this experiment I'm now convinced that the capacitor is the most important thing in the whole MLA installation. Most valuable would be a variable capacitor with a range of 0-1000pF or so. The problem is that such a device probabely does not exist and if needed you have to construct it yourself. Keep in mind that you need some kind of reduction device otherwise tuning would be impossible.

Experimenting with Magnetic Loop Antennas is very addictive. It is simple to build such an antenna and there are a lot of "simple" challenges, practical and theoretical, to explore. To not make this blog post too long I decided to write another post about the practical modifications I made to my loop, including some photos from the experiments so far. I might do some on-air tests in the mean time, because that's were an antenna is made for.

3 comments:

  1. Hello Bas

    Regarding your comment regarding the 0-1000pf capacitor amongst the many design issues for such a variable one problem is going to be the minimum capacity of such a variable another is likely going to be size for anything approaching a useful voltage rating. One way around this is as you found using a smaller variable with fixed capacitors the trick is getting the combination right for the ranges you want to cover. One approach I remember reading about used coax capacitors but instead of having the complication of switching simply used a plug and socket arrangement thus avoiding stray capacity and voltage rating issues.

    Kevin G6UCY

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    Replies
    1. Hello Kevin, indeed the minimum capacity will always be a problem. Since there is always capacity of some sort between 2 conductors. Regarding plug-in coax capacitors....that's a nice thought. Haven't thought about that. I already made a switch in the box when I build this thing years ago and had in mind to switch something with it. It is a heavy duty switch. My loop is only for QRP powers but if using 100W a plug-in coax cap is prbabely a better solution. That's the fun of tinkering with a MLA, you always find another challenge and new things to implement. 73, Bas

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    2. Hello Bas

      I agree Bas they really are great fun to play around with. Within the general requirements and limitations of magnetic loops there is a lot of latitude to play around and try out various combinations of components and circuit arrangements depending on what one has available and ones own specific requirements. They also have the added advantage of handling local RFI well when used indoors which is why I'm interested in them.

      Good luck with your experimentation Bas

      73, Kevin G6UCY

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Thanks for your comment. Bedankt voor je reactie. 73, Bas