The AM Forum

Hola,

The other day I fired up the rig to get on the air and my "automatic antenna tuner," which is built into the FT-1000D, smoked. I never trusted that thing. Pulled it apart to try to fix it and realized what a vunerable situation it was. 

Driving a linear with a solid state rig sometimes requires a good match in-between.  At least in my case, unless the swr is 1.3:1 or less, I start to get power foldback.

I decided to keep the auto tuner off line from now on and build up a rugged manual "coax to coax" transmatch that will easily handle 300 watts or more.

This is a time-tested circuit that has been used in many commercial transmatches over the years. I built it on a Plexiglas sheet, but you can use aluminum for a better groundplane.

I checked it with the antenna analyzer from 160-10M using various antennas and a dummy load for the output - and never found a match that wasn't 1:1 or extremely close.

I added a 3A fuse as a safety between the linear and driver. It may help in times of a linear flashback. I found that by attaching the stator side (not rotor side as mentioned in schematic) of the variable caps to the roller inductor, the stray hand capacitance was nil. This is the section that stays still. The caps float, so you need insulated couplings or large plastic knobs.

You may use a fixed coil and rotary switch in place of the roller, if desired.

No need to go out and buy an MFJ tuner for such a simple task.

73,

Tom, K1JJ

** Schematic in next Post **

The Transmatch schematic is posted  below....

Notice the rope around the MFJ analyzer for tower climbing... :-)

T

Very clear.

& translucent

"I'm gonna rebuild the White House outa Plexiglas. Government transparency can't be much clearer than that."

 - The Clear James Obama Bond

True, except I already have 2 MFJ tuners kicking around unused.  They "claim" they are good for 300W, but I think that's overly optimistic. ;D

I believe MFJ rates things like antenna manufacturers.

However, I've found that the MFJ ratings pretty much coincide with input power ratings.

IE, your 300 watt dummy load is good for a final that runs 300 watts INPUT, ICAS :)

Makes a bit more sense, seeing that small resistor and tuning caps now, eh ?

--Shane

Note that with this circuit there are a number of combinations that will provide a match. Go with the match that has the variable caps at their highest capacitance - this will be the lowest Q and least lossy arrangement.   

You can play with this type of tuner and see the effects of various settings on the efficiency at the link below.

http://fermi.la.asu.edu/w9cf/tuner/tuner.html

Start with the output cap at maximum, and try to get a match using the input cap and inductor.  If not, then decrease the output cap in stages and retune.   Repeat until a match is found, then record the positions so you don't have to do that all over again.

I've been running one like that for years!! I like it's wide tuning range.

Mine looks like that after massive doses of raw meat and steroids, it fills an aluminum box the size of an R-390. I use a broadcast roller inductor and 2 wide spaced breadslicers. I have a massive homebrew torroidal balun on the output (yes I said the output) that weighs several pounds. Chris (W2JBL) calls it "the worlds largest MFJ tuner". It will take large doses of high power, long old buzzard transmissions and a low-Z antenna and stay ice cold!

I had no room for it at the operating position, so I cut a hole in the wall and racked it into the wall, dubbing it the "hole in the wall" tuna.

                                                       The Slab Bacon

Frank, you get away with the balun because of the low Z.

Frank,
         I get away with the balun because it was designed to take many times the amount of power I am running just like the tuna. I have also had no problems running it on 20m as well. No wimpy girly tunas or baluns allowed in my shack!!

When building a tuna, overkill is ALWAYS your best friend!!

                                                               the Slab Bacon 

overkill is always your best friend

Yes, of course, I agree.  But the Plexiglas was more for the fun and viewing later. At a low impedance like this and low power, there is not much worry about RF interactions to other gear.

Building it in an aluminum box and using heavy conductors would be better, but I think the difference is probably not enuff to measure on 75M. This is a dedicated transmatch between a ricebox and a linear, transforming 200w pep maximum.

If ya look at the diameter of the inner conductor in the RG-59/U that connects the boxes, and the diameter of the roller inductor wire, you'll see that the inter-connections in the transmatch are about the same. Though, agreed, that there are points in there that may carry heavier current when the match dictates. Also, since the caps are floating, I wanted to simulate a quasi- 50ohm line throughout the box, so I figured a tight twisted pair would be close to 50 ohms. A twisted pair is a decent technique to do this.

But, maintaining 50 ohms inside the box is not critical anyway when you consider that the roller , caps and everything physicaly laid out departs radically from 50 ohms... :-)

I did run a quick heat test with 500w running for a few minutes and could not feel any obvious heating. 

73,

T

How much power is lost from a non-shielded connection that is a few inches long on 80 meters?

Metal boxes do not improve the efficiency. In fact, in some cases, they reduce efficiency by lowerig the Q of the coils.

Update:

I just ran another 500w test to check the wire heat and found an interesting thing....

The last test I didn't feel the actual inter-connecting wires. As it turns out, the wires from the S0-239's to the caps are stone cold. HOWEVER, the one wire that runs between the roller inductor and the two caps was getting slightly warm!  This wire is part of the resonant circuit and is obviously carrying more current inside the L/C  loop.

So, I changed the inside wire to #10 wire and everything in the tuner runs cool now.
It's probably no more than a few watts difference, but what the heck.

I had a feeling about that when I built it but let it go.

Tnx for the heads up, Brian.

Tom, K1JJ