Magnetic Loop tuning question

[VE3PCD]

I am posting this thread hoping that someone can help me.

I have just built loop a antenna but is having a hard time getting the SWR below 1.5:1 and stable; the loop is built of the following:

8 feet 3/4'' copper pipe
Freq range 30-10Mhz
DIA 2.4''
octagon shape
0-250pf Cap from an SB-220 amp
using the Faraday feed loop

I have tried several combinations of the feed loop except for the gamma match but without success. May be it's my tuning procedure.
Here is how I am trying to tune it for lowest swr.
- Adjust the CAP for noise
- check swr
- Adjust the small loop to get the swr down
- Change the band and repeat

I cant think of anything else
73
Peter

[KH2G]

Gamma match will work. Other than that I will assume you've tried a cap in series with you coupling loop to tune the loop on down. SWR of 1.5:1 is not bad.
With loops also you may have noticed that proximity of most anything can affect it. Q is very high therefore very narrow tuning.
Have fun
Dick

[KE3WD]

0-250pf Cap from an SB-220 amp

Be advised that the brush resistance of that cap may be keeping you from tuning as well.

That is why we use a Butterfly Capacitor in the magloop (or, if you've got the bucks, a vaccum cap).

One can also build a nice Trombone Capacitor that can handle the power easily and does not have brushes involved...


73

[VE3PCD]

Hi Dick

Actually I made a mistake, the SWR will not come down below 7.5"1. I was hoping for 1.5:1. I have not used a cap with the Faraday loop though I have used many combinations of that. I think the next thing will be to try the gamma match

Note: I have made a loop before and did not experience this problem, seem like the swr stay around 7 and higher and come down.

Peter

[KO6WB]

The losses in the capacitor will limit your loops operation and reduce the effectiveness. Your tuning proceedure is not really the one to use. Once you get the highest noise from the loop you then apply a low power signal to it and tune the capacitor for the lowest SWR. Then and only then do you adjust the faraday coupling loop for the lowest SWR. Keep checking with each movement of the coupling loop that the antenna hasn't changed tuning. When you find that point where the SWR and the tuning agree then you do not touch the coupling loop again. Unless something changes you should not have to touch the coupling loop ever again.
Remember the magnetic loop couples to everything around it and if you are standing close to the loop to tune it then you're also tuning the signal through you. Once you walk away the tuning will change. That's why the use of a remote tuning motor is almost always applied to the loop.
Okay, your capacitor is not the best, so you would need to replace it. The loop has a very low radiation resistance and any losses in the connections impact the performance. Get a Butterfly capacitor or a split stator capacitor. There are no wiper arms connections in those and therefore the losses are very low. One of the most effective capacitors is a vacuum variable and a new one is expensive but a used one that has been tested can be reasonable.
I use a gamma match on my loop and it works very well. Nowhere in the tuning range does the SWR go over 1.2:1. Presently my loop is on the ground awaiting new mounting hardware. It's a good antenna and compared to what I had been using it's fantastic.
There is a fellow selling capacitor kits for reasonable prices and he currently has postings here on QRZ. These have not been proven to be a good choice for a loop they also haven't been proven they are a bad choice either. One thing is certain they will be better than the capacitor out of a SB-220.
BTW a SWR of 1.5:1 is perfectly acceptable. Getting a lower SWR would make no difference in your signal to the receiver at the other end of a contact. If you have one of the fussy rigs that has a problem with such a low SWR then you may need to do some adjusting.
Have fun and let us know how you've resolved your difficulty.
73
Gary

[VE3PCD]

Hi Gary,

Thanks for the info. The SWR is over 7.5 always, guess I was hoping for 1.5:1 HI HI. I have just connected a temporary gamma to test it and it's the same, so it could be the cap. The loop does work, I was actually able work a w5 on 20 watts today using my antenna tuner, so all I really have to do is to that that darn swr down.

Other than a vacuum cap what do you recommend from mfj?
73
Peter

[KO6WB]

MFJ will also sell you the capacitor they use in their loops.They're the MFJ-19 and the MFJ-23. They cost a little bit less than a used vacuum variable. The MFJ capacitors will tune quicker that a vacuum variable but that's not necessarilly a good thing. When the Q of the antenna is high the amount of tuning range to get it right on is as little as 0.07pfd, not micro farad but pico farad. It's easy to jump past the tuning spot with such a fine level of tuning. The air variables usually go through from maximum to minimum capacitance in half a turn of the shaft. The number of degrees you move is so tight that you can not use a stepper motor with a straight through coupling connected to the capacitor shaft. The steps would be too large and again you would miss the sweet spot where it is tuned. This requires some gear reduction. A vacuum variable has about 30-35 turns for it's full span. Mine is coupled to an AC motor that has gear reduction to make it 10RPM. That equals a gear drive ratio of 350:1. The tuning of my loop is still very sharp and easily overshot.
One of the things you should do is to make a simple milliohm meter. All you need is a constant current source and a DVM that can read millivolts. Normally you would use a current of 100ma but I've seen where levels as high as 1 amp were used. You would connect the current source to the loop where the capacitor has split the loop open. Put one wire to each side. Using the DVM measure the voltage present across the loop from the capacitors opening. From there it's just a simple math equation to find the milliohm resistance. Note: if you used 1A as your current source the millivolts reading on the DVM will equal the milliohms. It must be very low in value. My loop which is 40 feet of 1" copper pipe measures at 2.5 milliohms. When I measure the terminals from the capacitor I get 3.5 milliohms. That tells me I have 1 milliohm drop just at my capacitor connections. I'll have to reduce that as much as I can while the loop is down.
Okay so get the milliohm reading from the loop itself and then put the test through the capacitors contacts. One problem with using a high current in the resistance of the capacitor may read really low because so much of the current is flowing through.
I have a commerical milliohm meter here and it uses 10ma as it's current source.
Your loop should read about 0.5 milliohms, that's 500 microohms. If it reads higher than that you will need to find out what is causing that much loss. The solder joints are one target and for all intents and purposes it is the only target.
That should be enough for you to find any of the problems you have with the loop. It will work much better once these problems are solved.
Another source of capacitors is;

VA6POP
Location:CALGARY,ALBERTA,CANADA
Homebrew butterfly capacitor 5-15kv 2

.
73 Peter. Good luck
Gary

[KB4QAA]

You do have all the copper joints soldered, right? (Just checking the obvious).

[VE3PCD]

Yes the joints are soldered though I just noticed that there are a few that are not soldered all the around, I am not sure this could be the issue as they is still contact.

I will re-solder and see what happens.

[KO6WB]

A check for solder joints and other resistive problems can be done with the milliohmmeter. Toss one together and measure the actual resistance of the loop. Anything improper will stick out and it will be easier to repair.
The calculator for magnetic loop antennas written by AA5TB allows you to enter the additional resistance of the loop and all connections. This will clearly show what impact the resistance of the loop has on the over all operation of the loop. You may find the reduction in performance is not that great and you will be able to continue with your project.
The program is a Microsoft Excel one but there is a free download that will run it and even modify it if you want.
The program is aa5tb_loop_v1.21.xls - Microsoft Excel Application (62kB). It's here at; www.aa5tb.com/loop.html.
The free download is for OpenOffice and you can get it here; http://www.openoffice.org/.
BTW there's a lot of good information on the AA5TB site so read as much as you can and you can get the information in a .pdf format. This can be saved to your computer for latter reviews.
Hope this helps
73
Gary