Friday, July 29, 2011

Z-Match Tuner Upgrade

A good friend of mine - WD8OSU - got me hooked on monoband loops fed with 1/4 wave matching section of 75 ohm coax. They really performed well but I don't have the space for monoband loops, I really only have space for one. About 10 years ago I started using a horizontal loop that was about 520' in diameter & fed with 450 ohm ladder line. It was phenomenal on all bands but really excelled on 20M and above. I first used an MFJ-920 tuner with a toroid balun, but it just didn't work all that great. At times I could hear the thing arcing over - not good. I sold that and started looking around for a better tuner to feed the loop. I homebrewed a Z-match tuner, mostly from parts I had on hand, using this as a starting point: The Simple Z Match Tuner Simplified. Very easy to build, took about an afternoon and it was up and running. I did need to purchase the triple-ganged variable capacitor though. I only used two gangs with the 3rd just hanging out there unused.

Front Panel

Rear Panel - (L to R), Input, Balanced Output, Ground bolt

Internal - Very simple

All in all, this tuner worked very well. Very sharp tuning and with loops inherently quiet, the entire antenna system was a performer.  The loop was built out of 20ga wire that I must have had 2000' feet of the stuff. I found 10 spools at the local ham swapmeet and got it all for $5. This first loop actually stayed up for about 5 years then the local kids started messing with. I had to come up with a loop that would fit entirely within my property and I needed to upgrade the triple-ganged capacitor to something beefier. Certain settings on 160M & 80M would cause the little guy to arc over. Kinda cool to see sparks, but the Icom 7000 did NOT like sparking! If I dialed down the power to about 50 watts, all was good.

I searched high and low for a high voltage dual-ganged variable capacitor. I spent about 6 months, I could find plenty like I already had, but none larger. I decided on a lark to go to the local ham swapmeet again, I haven't gone in over 5 years. I walk in, take a left, see this guy with a table FULL of variable caps and walk right up to the table just like I knew there were there all this time! $20 later, I had one of these:

E.F. Johnson - 13-312 uF

It looked like this new variable was never used! No setscrew marks on the shaft, the flanges were also clean. Huge difference between the old variable and the new one:

The new one on the left is from a transmitter, right one a receiver. About an hour later the Z-Match looked like this:

Upgrade Completed

There's a new loop up too - that'll be another post later. During initial testing, the tuner didn't appear to be working on 40M. That bugged me for awhile because the loop is a 40M loop, it should just work! I couldn't find any settings that would tune the loop below 2:1 SWR.  To find the settings for 40M, I plugged the output of the tuner into a dummy load. Then I went on the hunt for the 40M settings. I ended up finding settings that would get me to 1:1 SWR. I reconnected the ladder line and tried again. This time SWR was 2:1. Better, but not good enough. Very carefully I started rocking the knobs and bingo, it all fell into place. The tuning is incredibly sharp! Going from just one hash mark in either direction would send the SWR skyrocketing.

Initial testing on 40M at night, I've been able to hear the east coast, midwest, and Canada. Canada has me puzzled as this is only 3 degrees off the plane of the loop.

Glad to have an antenna within my yard that no one can yank down plus the upgraded tuner. See you on the bands!

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