The AM Forum
September 01, 2014, 03:29:10 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Build this simple transmatch for your solid state rig  (Read 4639 times)
0 Members and 1 Guest are viewing this topic.
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7062


"Let's go sailing, Tommy!" - Yaz


« on: January 22, 2009, 08:13:34 PM »

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 **


* Transmatch 006.jpg (320.63 KB, 1280x960 - viewed 531 times.)

* Transmatch 005.jpg (319.14 KB, 1280x960 - viewed 534 times.)
Logged

December / 1964    to    December/ 2014 will mark 50 years in this hobby.   Yikes!  Brrrrrrr.
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7062


"Let's go sailing, Tommy!" - Yaz


« Reply #1 on: January 22, 2009, 08:15:23 PM »

The Transmatch schematic is posted  below....

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

T


* Transmatch 002.jpg (294.12 KB, 1280x960 - viewed 307 times.)
* Transmatch Schematic.pdf (22.93 KB - downloaded 234 times.)
Logged

December / 1964    to    December/ 2014 will mark 50 years in this hobby.   Yikes!  Brrrrrrr.
Steve - WB3HUZ
Guest
« Reply #2 on: January 22, 2009, 08:45:04 PM »

Very clear.
Logged
W3RSW
Contributing
Member
*
Offline Offline

Posts: 2575


Rick & "Roosevelt"


« Reply #3 on: January 22, 2009, 09:22:44 PM »

& translucent
Logged

RICK  *W3RSW*
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7062


"Let's go sailing, Tommy!" - Yaz


« Reply #4 on: January 22, 2009, 10:14:07 PM »

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

 - The Clear James Obama Bond
Logged

December / 1964    to    December/ 2014 will mark 50 years in this hobby.   Yikes!  Brrrrrrr.
WB2YGF
Guest
« Reply #5 on: January 22, 2009, 10:40:19 PM »

No need to go out and buy an MFJ tuner for such a simple task.
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. Grin
Logged
KD6VXI
Member

Offline Offline

Posts: 782


« Reply #6 on: January 22, 2009, 11:31:28 PM »

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

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 Smiley

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

--Shane
Logged
W1VD
Contributing
Member
*
Offline Offline

Posts: 400



« Reply #7 on: January 22, 2009, 11:35:10 PM »

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.   
Logged

'Tnx Fer the Dope OM'.
Steve - WB3HUZ
Guest
« Reply #8 on: January 22, 2009, 11:40:24 PM »

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
Logged
K3ZS
Member

Offline Offline

Posts: 1007



« Reply #9 on: January 23, 2009, 12:08:05 PM »

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.
Logged
The Slab Bacon
Member

Offline Offline

Posts: 3928



« Reply #10 on: January 23, 2009, 12:08:23 PM »

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
Logged

"No is not an answer and failure is not an option!"
WA1GFZ
Member

Offline Offline

Posts: 11156



« Reply #11 on: January 23, 2009, 12:12:25 PM »

Frank, you get away with the balun because of the low Z.
Logged
The Slab Bacon
Member

Offline Offline

Posts: 3928



« Reply #12 on: January 23, 2009, 12:25:43 PM »

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 
Logged

"No is not an answer and failure is not an option!"
WA1GFZ
Member

Offline Offline

Posts: 11156



« Reply #13 on: January 23, 2009, 12:36:13 PM »

overkill is always your best friend
Logged
w5hro
Member

Offline Offline

Posts: 1919


« Reply #14 on: January 23, 2009, 12:52:41 PM »

It would be better to use flat heavy and stiff copper braid or something than using the small gauge red and white round wire.

P.S. If its not going to be in a shielded box then coax probably should be used instead of the wire to minimize interference and power loss. Those cheap junky MFJ, Ameritron and etc. tuners use wire in twisted pair like that, but itís not the greatest way of doing it. They do use shielded enclosures however.


* Transmatch 006.jpg (320.63 KB, 1280x960 - viewed 144 times.)
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7062


"Let's go sailing, Tommy!" - Yaz


« Reply #15 on: January 23, 2009, 01:47:43 PM »

It would be better to use flat heavy and stiff copper braid or something than using the small gauge red and white round wire.

P.S. If its not going to be in a shielded box then coax probably should be used instead of the wire to minimize interference and power loss. Those cheap junky MFJ, Ameritron and etc. tuners use wire in twisted pair like that, but itís not the greatest way of doing it. They do use shielded enclosures however.

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

Logged

December / 1964    to    December/ 2014 will mark 50 years in this hobby.   Yikes!  Brrrrrrr.
Steve - WB3HUZ
Guest
« Reply #16 on: January 23, 2009, 01:49:54 PM »

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.
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7062


"Let's go sailing, Tommy!" - Yaz


« Reply #17 on: January 23, 2009, 02:10:43 PM »

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
Logged

December / 1964    to    December/ 2014 will mark 50 years in this hobby.   Yikes!  Brrrrrrr.
w5hro
Member

Offline Offline

Posts: 1919


« Reply #18 on: January 23, 2009, 03:32:49 PM »

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.

Yep, T-Network tuners work best for 50-ohm unbalanced feed line systems, but the connections in the network can get really hot and cause other problems. Since itís in an unshielded box I would just use some RG8X to connect everything. It would probably require re-marking the resonate points on your roller inductor again, but thatís easy.

P.S. The unbalanced feed line system is the reason the wires in the t-network get hot. Itís like the ďSkin EffectĒ, but with RF current. Thatís why itís better to use flat metal of flat braid for the connections to prevent the effect.

Later...
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2014
Powered by SMF 1.1.19 | SMF © 2013, Simple Machines
Page created in 0.146 seconds with 18 queries.