The AM Forum

I just finished building my parallel link coupled tuner for 75 meters and here's the result.  If I need more capacitance on the input, I can always add a doorknob capacitor. 

Jon
KA1TDQ

Nice!

What size wire is that, looks like about 6 guage?

The primary is #12 (I think) and the secondary coil is 1/4" copper tubing.  You can't really see from the photo, but I used #8 to attach the secondary coil to the bread slicer. 

There's a coolness factor in the way this project came out.  I think it would look awesome on the desktop, but I'll have to be careful during transmit not to touch it (and keep small pets away).

Jon

Very nice construction job Jon!  I am sure you will enjoy using it.
73,
Joe, W3GMS

160m tuner to match Gates BC1-T to 450Ω load.  Link coil is #8 wire and main coil is EF Johnson 1/4" flat edgewound.  833A and 9v battery in photo give perspective.  Doesn't look as neat as Jon's, but still in prototype stage. I have a larger bread slicer if that one continues to arc over during final tests.

By carefully adjusting number of turns on the link, no series variable capacitor is needed. But of course, this is a single-band tuner.

These tuners make one feel like a Marconi !

Is there a rule that governs the diameter of the link coil ?

Todd

Have you had any arc overs on that input cap?  I have a nice vacuum cap for the output L but would be looking for ≈ 2000 pf for the input C but think that I'll have probs finding something that will have the C and have the E rating.

I don't like using fixed caps - never have.  That'll be a shopping item for my next visit to NearFester

Nice looking.  Yeah, don't draw arcs off the thing with a pencil unless you  put a rubber eraser on it

Al

Yes, A big tuner reminds me of how much I want to build a spark rig

Very nice!!! I like your idea for the input coil.
Now to feed the aerial.
Fred

Just that it fits over the coil with a little space in between, or the link can be a smaller coil inside the main coil, as with the BC-610 and the early Gates broadcast transmitters with push-pull finals.  Also, a gap can be left in the middle of the main coil, with the link the same diameter as the main coil, inserted in the gap.

That's a nice job of winding that copper tubing.   the only thing I can suggest off hand is working out some sort of clips for the coil that are not steel and have good contact surface.  What I did won't work on a round coil but maybe you can see in the photo the silver plated knife switch clips I used on the end panels of the Johnson four section cap in my Matchbox.  The mod was to add series vacuum caps (they are way overkill but they were cheap) to match my feedline down to 3500 kc.  The clips were 25 cents each from Surplus Sales and I soldered them to narrow copper strips I cut from a roll of strap.  If you used a rat-tail file you could file away the teeth on your coil clips and increase the contact surface area.

Rob

Thanks for the compliments everyone and the tip on the steel clips.  I didn't think about the composition of the metal (or surface area).  I'll file out the middle teeth like you said.  I won't be running any real amount of power through this right now anyway...  The wife and I are moving to Phoenix in a few months and will be in an apartment for some time.  I'll be relegated to a rice box and a short string off the balcony (nothing like even remotely close to running 1/2 ton equipment and an array on 75).

Jon

Jon:
Your antenna coupler looks very nice.
Chuck

Well....I have a couple rotten comments  ;D 

I don't like PVC to support the coils as a rule as it tends to melt when it gets warm.  Better is to use either FRP (fiberglass) or acrylic (plexiglass).  But if you plan to run low power...no biggy. 

Also I prefer to not use wood as a supporting structure as it can catch fire.  Again FRP or even countertop material like Corian or porcelain.  On Corian, I get scraps from cabinet makers (like Paul KB1IAW).  It can be drilled and tapped and if won't burn. 

My .02...and not worth even that probably 8)

Keep building...that is the fun part. 

Peter

I'm of the same opinion regarding the PVC coil forms.  The stuff is not low-loss at RF.  One way to improve it is to drill the sections of pipe full of holes, as many and as large as you can and yet still retain the form - much in the manner of "window line" where they punch rectangular holes in heavy duty TV ribbon to reduce losses.  I would have used some other material, like plexiglass strips or fibreglass.

Back when most supermarket items came in glass jars, I would wind the coil over the glass with wax paper wound round it, cement the plastic strips in place, then break the jar and let the glass fall out and pull out the wax paper, leaving a low-loss air core coil.

I doubt that mounting it on a wooden base would likely cause a fire.  Since the earliest days of radio, wooden breadboards have been used, even for high power rigs.

If I remember, wood has to be heated to something like 400 or 500 degrees Fahrenheit before it will catch fire. Unless you are having some severe loss, like the whole transmitter output being absorbed by the tuner and not going to the antenna, i doubt if that tuner will even get to 200 degrees.

Nice tuner. 

Funny story.  Each week we log onto Stickam so we can see each other on video while we use our transmitters on AM

KC6MCW was keyed down getting long winded with a big signal into AZ.  On his video I see him facing the microphone and at the end of the table behind him is his Home brew link coupled tuner on fire.  He just kept talking and talking.  Meanwhile, I see the link section burning with a nice flame.  I typed into the Chat window that his tuner was on fire and he figured it out pretty quick from there LOL.  He ran down the table and started putting the fire out. 

Dont use insulated wire. haha

C

That's a funny story assuming only the link was damaged  :D  and it's also good to know that about the insulation.

Not to beat a dead horse here, but I picked up a few parts at the hamfest today and built a smaller series link coupled tuner for my rice box.  When I get to Phoenix I plan on using a physically short dipole at the apartment for higher frequencies. 

I skipped the primary variable capacitor and used just a 3-turn link around the main coil.

Jon

Jon:  Way cool!!  Think building a tuna is my next project--you've got me thinking about it.  Have fun building.  Once you start--you can't stop.  It's amazing what you can do with a little drilling and blasting and some ordinary stuff <hihi>. 

73's Geo

My link coupled tuner.  The link coil is a silver plated steel coil that came from an old Collins tuner.  Colored tape marks the different settings.  Plexiglass panel was added after getting an RF burn while tuning.

here is my tunner. i built it and used it for a bit. was going to finish it up but i got corrupted by some guys here and did a coax fed driven array. 

Todd

You're leaving NE?  That's news to me

Al

Guys:

This has been a good thread.  Lots of nice construction hints.  I think I have all the components for making my link tuner.

Thanks guys

Al

Just as a follow up...

I did end up adding a variable capacitor to the input of my small link coupled tuner and it works!  I tuned an 8' dipole (4' on each side) on 29.1 Mhz and got a 1.1:1 SWR. 

I figured an 8' antenna will be the worst case scenario for my apartment when I get to Phoenix.  If it will tune this, it certainly will tune something bigger. 

Forgive the rice boxes, but it's all I can do on short notice :-)

Jon

Nice looking station Jon.  You are always welcome in my shack when you get to phx.  Come by and operate all you want.

Clark

Thanks Clark!  I'll look you up when we get there.

Jon

Mantenna Tuner

I no longer use this since I switched to coax but it's ready when needed.