View Full Version : Small Magnetic loop impedance transformer question

05-01-2012, 04:52 PM
Hello all,

I am in the process of planning the build of my first magnetic loop. Read all i could stand for right now so im going to jump in. My question is; i have seen some folks use a ferrite core for the transformer to 50ohm coax. How does one determine which is the right core for the job? Do i simply purchase a 1.5-30mhz core with a rating of up to 3kw and go from there?

I understand the transformer is dependent on antenna impedance and power to be used in an of itself, but without an antenna analyzer it would be hard for me to determine this at the moment until i purchase another one. The one i had, i had to sell prior to moving back to the states i needed the money. I will not be running more than 100watts PEP and i plan on making the loop of 1/2 copper tubing 10ft in diameter.

I mostly run CW but on occasion i like to get on phone and or PSK31.

Yes i do have some books on the matte
Yes i have joined a group on the subject through yahoo but no one can answer the question with certainty.

05-13-2012, 10:14 PM
For the mag loops I've played with, the faraday feed loop is a very simple and effective way to feed small magnetic loops. Some of my experiments are here http://forums.qrz.com/showthread.php?208737-Magnetic-Butterfly-Loop-from-Hard-line
Another link that may be helpful is here http://www.qsl.net/mnqrp/Loop/Mag_Loops.htm.

Not quite what you were asking but hope it helps! -Bill-

05-13-2012, 11:26 PM
I have a magnetic loop up and operating well. It's 40 feet in circumference and 10 feet in diameter. Mine is in a square shape with the bottom 6 feet off the ground. I used 1" copper pipe. The suggested size for my loop was to use at least 2" copper pipe but using the calculators available I found the reduction in performance was small. It was about .1 db difference and a world of difference when it came down to price. The 2" copper pipe was very expensive and by using 1" I save about 75%.
I run 500 watts to my loop with no problems. I use a vacuum variable capacitor rated at 25Kv. The value I have will enable it to be tuned from below 80 meters to above 40 meters. The use of a higher value would have given me 160 meter coverage but it wouldn't be very effective and the bandwidth would have been 700Hz. So I get 80/75,60 and 40 meters coverage.
I use a gamma match on mine. It's an easy approach and tuned quickly. I have a SWR below 1.4:1 everywhere I operate.
Tuning the loop requires the varying of the vacuum variable capacitor. I use a small motor with a gear reduction down to 10 RPM. It's a 120Vac reversable motor but I am not using 120Vac on the lines going to the motor. Instead I use a 12.6Vac transformer with the primary side going to the motor and the secondary side going into the shack. There I have another 12.6Vac transformer whose secondary is connected to the secondary of the transformer mounted in the tuning box. I used a 3 conductor #16 wire 100' extension cord to get the 12.6Vac to the tuning box. I also had to have a way to reverse the motor so I used two 12VDC relays. One relay would work with positive voltage and the other worked with negative voltage. So for the 3 conductor cable going to the tuning box has one wire as common, one wire as 12.6Vac and the last wire had the 12Vdc switching. Didn't have to worry about getting a stepper motor arrangement going. I have worked with stepper motors in the past and after you total everything needed for a stepper motor control, the AC motor was way cheaper and just as effective.
As you have already found out there is a lot of information on the internet on magnetic loops. Some of it is very optimistic in their claims so take it in with caution.
There are a number of calculators that will help you with your final design.
I have a lot of references to various articles on the internet but it sounds like you're already overloaded. If you need this information then let me know.
Have fun

05-13-2012, 11:26 PM
I recommend you follow an established design. Do you have the ARRL Antenna book?

05-14-2012, 01:41 PM
I recommend you follow an established design. Do you have the ARRL Antenna book?

Yes i do - 19th edition - but one none the less. Its packed away from our recent move but ill lok into it. Im going to scale down my needs to qrp - max 10 watts. This should help in finding the impedance matching transformer that i need easier - i hope

05-14-2012, 05:34 PM
I have used and built several Magloops. You can see a pic if you follow my links..search for K2TL on QRZ.com for my profile page.
Go here for some good matching info:


Scroll down the page until you get to the part about ferrite transformer coupling. It is pretty clear regarding your question and how to actually use it.
I have lost several years off my life screwing around with a coupling loop. Sometimes it worked perfectly, other times I could not get a match to save my soul, and everything in between. The ferrite transfomer coupling gives me a perfect 1:1 match, all the time, under any weather conditions, and it is just wonderful!
One suggestion..toss the idea of using copper pipe. The weight and multiple joints are major issues, especially in mounting and performance. I use 1 inch hardline. Just use the shield as the loop conductor. It is light, no joints, you can form a perfect circle, usually free or dirt cheap if you can get to a hamfest, and it can be mounted on a broomstick if you really had to. My loop is about 5 1/2 feet in diameter and is about 5 feet above ground on a fence in the backyard, surrounded by trees and neighbors. I use it primarily on 30 meters. It consistently outperforms my reference antenna, which is an 88 foot twinlead fed inverted vee at 45 feet. A magloop can be a total dudd, or an amazing antenna. It all depends on the build. Slop one together or have a lot of joints, and it will be lossey as hell. Oh, and yes, the capacitor needs to be a butterfly type, vacuum variable, or some of the homebrew schemes you will find. You can loose 90% of your power in the capacitor if it is the typical compression/plate type.

Jim K2TL

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