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KG4ULT
06-17-2012, 03:25 AM
Hello All,

Quick question; can one tune a toroid core with secondary windings for a magnetic loop without having it coupled to the main loop using an antenna analyzer?

Thanks
Frank

KO6WB
06-17-2012, 09:31 AM
Actually no you can't or at the very least you shouldn't. The toroid transformer is for wideband coupling of the magnetic loop to your transmission line. The toroid is actually a single turn link and you would use the proper number of primary turns to couple the impedance of the magnetic loop to your transceiver. If you put tuning on that it would lose it's wideband coupling capability and you would also have tuning problems. First you have to tune the loop then the coupling and then the loop again and the coupling one more time. Now you're ready to transmit.
If you try to tune the toriodial transformer without having it on the magnetic loop you will get results and once you put it on the loop you will have a substantial alteration of your previously tuned circuit. The tuning for the loop would have to match the tuning for the toriod transformer to get the best transfer of power. There will also be a difference in impedance that may or may not be tuned out.
Keep in mind that any tuned circuit you put near a magnetic loop will be influenced and may not transfer power correctly.
Also the components that you use would have to be at the rated power present on the primary winding of the toroid transformer. My primary is the same thing as your secondary. I'm thinking transmission of power to the loop and you're thinking of the reception qualities of the loop. So don't get confused by my terminology, it's different but the same.
In short it's more of a hassle and won't really get you anything more. No additional gain, no greater receive and no reduction in the noise level. That's unless the added tuned circuit does load down the system and causes the magnetic loop to perform worse.
Now don't let me stop you from experimenting. It can be fun to see what happens when you make changes. Who knows you may even find the Higg's boson. Seriously if you do this try to document your results and what you did to obtain those results. From there you can expend effort to explain why it did or didn't work and where you can proceed to as a next step (you do know there's always a next step).
Hope you do well and have fun. Let us know how you did.
73
Gary

KG4ULT
06-18-2012, 03:31 AM
Actually no you can't or at the very least you shouldn't. The toroid transformer is for wideband coupling of the magnetic loop to your transmission line. The toroid is actually a single turn link and you would use the proper number of primary turns to couple the impedance of the magnetic loop to your transceiver. If you put tuning on that it would lose it's wideband coupling capability and you would also have tuning problems. First you have to tune the loop then the coupling and then the loop again and the coupling one more time. Now you're ready to transmit.
If you try to tune the toriodial transformer without having it on the magnetic loop you will get results and once you put it on the loop you will have a substantial alteration of your previously tuned circuit. The tuning for the loop would have to match the tuning for the toriod transformer to get the best transfer of power. There will also be a difference in impedance that may or may not be tuned out.
Keep in mind that any tuned circuit you put near a magnetic loop will be influenced and may not transfer power correctly.
Also the components that you use would have to be at the rated power present on the primary winding of the toroid transformer. My primary is the same thing as your secondary. I'm thinking transmission of power to the loop and you're thinking of the reception qualities of the loop. So don't get confused by my terminology, it's different but the same.
In short it's more of a hassle and won't really get you anything more. No additional gain, no greater receive and no reduction in the noise level. That's unless the added tuned circuit does load down the system and causes the magnetic loop to perform worse.
Now don't let me stop you from experimenting. It can be fun to see what happens when you make changes. Who knows you may even find the Higg's boson. Seriously if you do this try to document your results and what you did to obtain those results. From there you can expend effort to explain why it did or didn't work and where you can proceed to as a next step (you do know there's always a next step).
Hope you do well and have fun. Let us know how you did.
73
Gary

Gary

Thanks and understood. I just thought it would make the tuning process easier as i was not having any luck with the coupling loops i was trying until speaking to a guy - Thanks Don - on one of the groups he pointed out something that i thought of at a glance but then overlooked. Well i was using LMR240 for the coupling loop and coax to radio. Well this coax not only has a braid but a tin foil surrounding the foam diaelectric. This obviously was shielding the faraday loop from coupling to the main loop. Hello...

so once i removed the tin the antenna was able to get close to resonance on 40mtrs. Now i just need to adjust the coupling loop tomorrow after work to get the swr down to at least 1:5 or better.

As for the toroid i did not have enough shielded wire to add more turns. I used 3 turns of 12awg solid copper wire. This is a 2" broadband hf core "2-28mhz" or at least that is what the vendor said at the hamfest. That is why i wanted to check it outside of the main loop.

In any case i am def documenting everything and observations for future reference.

Thanks
Frank

KO6WB
06-19-2012, 05:58 AM
Very good Frank, it nice to finally get results but I hope you're trying for at least 1.5:1 not the other value you mentioned. I think that's what you meant.
On my magnetic loop I used a gamma match. Have 1.2:1 maximum on all bands except this morning when I found my SWR went up. This prompted a visit to the said antenna and I had found the Electrical Conduit (PVC) that I used to hold my loop up was getting seriously deformed by our 100+ degree heat. The antenna had moved from it's place where it was doing just fine and was now in close proximity to the stucco walls of the house. The loop tried valiantly to couple it's energy into the stucco wall. Allas it was doomed to failure and I now have to figure out a method of supporting my loop that can stand the summer heat. Stay tuned.
Have fun
73
Gary

KG4ULT
06-19-2012, 12:16 PM
Yeah after some more tuning last night i cannot get any better than 3.1:1. I have two issues - variable cap and the coupling loop. Im going to try another cap i have in stock and if i cannot get it right ill swap to a gamma match. I did learn that i do not like the AA54 and am thinking of selling to purchase the MFJ269B. It is not easy to sweep if you dont know where the antenna is somewhat resonant whereas the MFJ you can just turn the knob until you find that point. Than you can work from there.
Its a nice gadget and i thought it would be cool to use it in conjunction with my laptop but id rather use the MFJ.

Frank

KA5S
06-19-2012, 01:14 PM
I can''t picture your setup. The loops I have here either use a coupling loop straight to coax, with the loop itself tuned with a capacitor, or are untuned, for broadband EMI measurements. In neither case is a toroid involved. How are you using one?

I have found the easiest way to find resonance is with a receiver, as there's almost always enough noise to tell when you're close.

Cortland
KA5S

Yeah after some more tuning last night i cannot get any better than 3.1:1. I have two issues - variable cap and the coupling loop. Im going to try another cap i have in stock and if i cannot get it right ill swap to a gamma match. I did learn that i do not like the AA54 and am thinking of selling to purchase the MFJ269B. It is not easy to sweep if you dont know where the antenna is somewhat resonant whereas the MFJ you can just turn the knob until you find that point. Than you can work from there.
Its a nice gadget and i thought it would be cool to use it in conjunction with my laptop but id rather use the MFJ.

Frank

KG4ULT
06-19-2012, 06:06 PM
I was going to feed the antenna (couple the coax and rig to main loop) with a toroid as some articles have posted. Im currently using a small loop 1/5 the size of main loop. However as i stated i cannot get the loop to go down less than a 3.1:1.

I did juse my receiver last night and listened for the spikes in noise - but boy when folks these things are very narrow banded because of the High Q of the system - they werent kidding. I do not have a motor to turn the rotator on the cap; i came with a long plastice shaft that connected to the capacitor for turning. I understand that my proximity to the loop and cap dont help but i can make small adjustments and check meter. Regardless i cannot go lower than the mentioned swr. This is why i was thinking of revisiting the toroid feed before going to a gamma match feed which form what i hear is the easiest way to couple to the antenna.

The cap i purchased was a differential cap from MFJ as i purchased the wrong one. Its value is from 32 - 350 or so however it doesnt help. Im looking on ebay for a good tuning cap. Im also thinking of trying to use a capacitor made of coax. If i can get one band to tune - namely 20mtrs i can at least go back to my notes jot down my observations and build a new one. I want to expend every option that i have available however without having to purchase more material or gear.

Frank

KO6WB
06-19-2012, 10:42 PM
Hey Frank, you're making progress and the best thing is you're experimenting and learning. This is what amateur radio is about, that and communicating.
Most of those that have built magnetic loops use the Faraday coupling method. Small loop coupling to large loop just about everywhere. The gamma match isn't used often but it can be very effective when done correctly. The gamma match also will give you an indicator that the other methods won't, and that's how well you built your loop. The farther away from the coax connection point the gamma match stub has to be the greater the losses in the loop. So if your loop only requires 8-14" you're good. If it requires 16-20" or more, you have problems. These measurement apply to a loop that uses 40 feet of copper pipe. A smaller antenna the distance would be shorter.
One more tool some of the builders have used is the milliohm meter. The lower your resistance the better the antenna will perform. There are a couple of postings on the internet that tell you how to make a milliohm meter with stuff you may have already. All you need is something to provide a constant current and a DMM that can read mV. Even the DMM from Harbor Freight will work well. So check it out and see how you're doing.
73
Gary

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