Friday, August 23, 2013

A homebrew Tayloe SWR indicator

I've been playing with magnetic loops for a while and I have yet to perfect the science but I'm wanting to do some stuff with my MTR and a small loop on the deck for instance.  The problem there is the MTR's receiver is so quiet, I can't tell when I've got the loop tuned like I can on the KX3 with the pre-amp on.  Knowing this and that the MTR is a little sensitive to high SWR, I've been avoiding too much playing around since I have to take the KX3 to act as a REALLY fancy SWR meter.  Well that all may be over with now.

Dan Tayloe, N7VE, developed a really neat resistive SWR bridge that uses a simple LED to identify the dip in SWR on an antenna.  QRPkits.com sells a nice kit of the bridge for just $15 and I recommend that if you don't already have the parts in your junk box.  I got really lucky and found some 57 ohm resistors in an old shortwave radio I tore apart and I had the rest of the parts in my junk box (minus a DPDT switch).  A schematic of the bridge is below (credit to QRPkits.com)
It isn't a complex design by any means but the unique part is that it gives the transmitter a matched load regardless of the actual impedance of the antenna, protecting those precious final transistors!  Since my implementation uses 1/4w resistors rather than the 1 or 2 watt ones you get in the commercial kit, mine is probably limited quite a bit power wise but I didn't feel any heating in the testing I did at 5w, and you don't really need lots of transmitting time to identify if the LED is on or not.  Here's what mine looks like:
Since I didn't have a 470 ohm resistor, I just used two 1k resistors in parallel.  Not fancy, but it works!  To test it out, I put my mag loop at the antenna end and plugged the MTR into the input.  Sending a few dahs, it was very obvious I was out of tune.  I continued sending dahs into the bridge as I tuned the capacitor on the loop and sure enough I found the dip in luminance on the LED, it was very obvious.  I then swapped the antenna to the KX3 to see how close I actually got to the SWR I wanted and I was absolutely shocked to see that it read 1.3:1.  The dang thing actually worked!  Always a great feeling when something you kluge together on your bench works like its supposed to!

So now I've got a bridge that works but it does need a switch to be truly useful since you don't want to have to plug it in/unplug it every time you want to test SWR.  The coax you see in the picture is unterminated too, they're just going into a couple banana-BNC adapters so I'll have to decide how I want that to work too.  Anyway, it works so you can be sure you'll see an update on this in the near future as it gets some real world use.