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WA6MHZ
09-05-2011, 02:57 PM
At the Swapmeet Saturday, I scored THESE in a box for $2.

http://farm7.static.flickr.com/6194/6115946131_306af1cb8d_z.jpg

We see a 20-120pF variable cap, a 20-360pf variable cap and a military roller inductor.

Not bad for $2! and that included some ICs and other junk.

So, as long as I HAVE IT, I might as well put it to use.

This is the makings of a 300W or so Antenna tuner.

I looked on the web to try and find a good schematic of what I want to build. I saw plenty of ideas, but not quite what I want.

The usual fare is a "T" Match, with a variable cap coming in, the coil from that point to ground, and a variable cap to the output. They want lots more PFs than I have, and alot more turns on the coil.

So what about a garden variety PI Match, like is in the output of most boat anchor radios?. Input: Variable cap to ground. Then through the coil to the output where there is another variable cap to ground. That is the way most transmitters do it.

Seems like it should work, just got to make sure both ends of the coil are well off ground.

Add to this mess a balun and a wattmeter, and I have made myself a MFJ!

In fact, you can buy the dual meters from MFJ, which I just might do.

So I need a Wattmeter circuit. You can't download MFJ Schematics. They let you have the manuals and keep the circuits hidden. So I need to come up with just how they make the little RF Sensors. I see they use a small torroid and sense that. gone are the days of strip lines like in the OLD SWR Bridges.

I know, I coulda bought a MFJ for this will wind up costing me. And I don't really NEED another TUNER as I have 2 MFJs at home plus some spares.

But I have this crazy urge to BUILD, and now that I have the parts, BUILD I MUST!

WA6MHZ
09-05-2011, 04:14 PM
I just found the schematics to the MFJ 949 and 969, and they are both T-Matches too!!!!
Doesn't anyone use a PI NETWORK MATCH????

VK4KGW
09-11-2011, 02:32 AM
Perhaps googling ? with speciic search terms

4L4AGU
09-11-2011, 05:34 AM
73837

If you going to use this tuner for most HF bands, than you need larger capacity capacitor. 20-120pf is not enough for most cases. So, you may add some rotary switch and additional capacitors. It also should be noted, that output capacitor capacity is not that important to be matched exactly for T match tuner, so, you can easily replace it with rotary switch with 4 positions, which will connect fixed capacitors. For example, my homebrew t-match tuner has 12-250pf input capacitor, 0-30uh coil, and fixed-switched output capacitor with values of 440-220-110-55 pf. Tunes almost everything, everywhere. I even tuned inverted L antenna with it on 160 meters!

K5FH
09-14-2011, 06:18 AM
Pat,

You might try an L-match configuration using the large capacitor with a vernier drive and some arrangement to switch the capacitor to either the input or the output of the inductor.

Save the 20-120pF capacitor for another project :)

KB1UMD
09-20-2011, 05:39 AM
Pi Match tuners require larger, more expensive caps and possibly larger inductors to tune the same impedance and freq. ranges. That's one reason many tuners use the C-L-C grounded inductor T-Match design - more range for lower cost. Also the larger Pi-Match components may need a bigger box, adding yet more cost and eating precious shack space. Both designs need 2 caps and 1 inductor. But the T-Match caps cannot ground their frames. You have to mount the caps on Plexiglas or similar technique. I used nylon standoffs.

Or, if you use a Pi-Match, you could tune a smaller impedance range with the same components. Besides some low-pass harmonic filtering, there's not a lot of other advantage to the Pi-Match. If you adjust a T-Match to extreme tuning range, it may absorb quite a bit of internal power. I think the Pi-Match internal dissipation is more forgiving.

I made a homebrew T-Match 200W C-L-C tuner with external SWR meter which works great into a balanced 100-foot dipole. I might try a Z-Match for my next tuner. It's a unique, interesting design which saves one control. Why build another tuner?? I will include the SWR meter inside, possibly based on LED's.

KD5SPX
09-21-2011, 01:57 PM
The inductor shown is the exact same one used in a tuner I recently purchased off of Ebay. It looks like it needs cleaned up a bit but a good score. Can't wait till the next local hamfest in June here in Texas :-)

KD5SPX

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