AmateurRadio.com

For some time now I have been trouble shooting my MFJ 1788 loop off and on. I have had a problem with getting 20m to tune. The best I could do was an SWR in and around 12 to 1 and really that is not good at all. Even with that SWR the K3 tuner was able to drop it too a flat match but odd things were still happening....for example
1. Tuning the loop for max noise (as low SWR was not happening) did happen but as soon as the rig was tuned in any way the noise dropped to nil.
2. Once max noise was achieved and I transmitted the max noise was gone again.
3. There was no spots on the Reverse Beacon Network while giving the tuned match a go.
4. When transmitting power levels were all over the place from 1 watt to 5 watts.

I had posted my 20m tuning problem on 2 loop user groups and had some great advice but nothing seemed to even come close to solving the problem. I then emailed MFJ  who very promptly emailed me back and advised me to change the shape of a 12 gauge copper loop that was located inside the loop. Easier said than done this involved removing half the plastic cover on the loop and changing the shape of this loop then

Adjusting the loop

checking the SWR. The hard part was trying to figure out where the sweet spot for 20m was on the loop  was. The loop would read an SWR of 12 to 1 no matter what adjustment I gave the loop. I ended up tuning the loop for the highest noise level I could hear. I figured this was as close as I could get to the sweet spot for 20m. Adjusting this small loop did not really change anything and I was getting frustrated! The best thing I find when this happens is to put the project aside and do some thinking. I did just that over the past few weeks I enjoyed the bands the loop would tune and thought about my next move. The loop consists of a rotating capacitor that tunes the loop via a motor. I had been reading on the loop reflector sites and a reoccurring conversation was how quality control at MFJ was very low. For some reason I wanted to check the spacing of the capacitor plates on the loop antenna . It turned out the plates had a warp to them and at the low end spacing was good but in and around

Some plates to close

20m's  some of the plates were very close and maybe touching each other. After some time of tweaking the plate spacing I was able to have equal spacing for the full rotation of the capacitor. The SWR on 20m is now down to 1.4 to 1 and that was the problem as I put the loop back together I kept an eye on the SWR to see if it changed at all. The piece of  equipment that really helped me out with this process was the MFJ 259B analyzer. This unit made tracking my SWR very easy, bottom line is the antenna is now working great and now it's time to get on the air and see how 20m is preforming from the condo.
Mike Weir, VE9KK, is a regular contributor to AmateurRadio.com and writes from New Brunswick, Canada. Contact him at [email protected].