The AM Forum

Put up a nice high 80 meter dipole fed with open wire line and need a decent tuner. I couldn't find anything that looked good  so I decided to build one based on The K1JJ design.

The big coil is 1/4" od tubing and the 5 turn coil is 3/16 " od tubing. I have finished drilling the plexiglass spacers and hope to install them tonight. I have a vacuum variable cap to tune the large coil. Hope to send some RF through it in the next week. Waiting for some solid copper clips to come Tuesday.

Good On Ya! That is real ham radio stuff!!

Got the plexiglass spacers drilled and installed and the secondary 5 turn coil fits inside  the large coil.

I'm thinking that it doesn't matter where the 5 turn coil is installed and the end would make wiring it easier. i have some teflon insulated @12 silver plated wire that i will connect it with.

any comments would be appreciated.

thanks

Pat

If you are building a single-ended, or unbalanced tuner, then the link should be located at the "cold" end of the main coil. But if you are building a balanced link-coupled tuner, feeding a ladder line or open wire, then the link should go at the center of the main coil, where the RF voltages are the lowest.  

I made my main coil the same way you did, and the tubing is much better for the main coil, considering the circulating currents.  But for the link, I just used teflon-insulated number 12 wire for a 4-turn link, centered on the outside of the main coil.  You will also want to include a series capacitor, between one end of the link and the ground, or shielded side of the link cable, to tune out the reactance of the  link coil.  Been using mine on 75 and 40 at legal limit, no problems.  I do not have an antenna suitable for 160 at the moment.

Since you have already finished the inside link coil, there should be no problem running teflon-insulated leads between the large coil turns to connect the link at the center.

Thanks. I will mount it permanently in the center and use #12 to connect ti.

Pat

Looking good. I think mine has a 5 turn link coil. It's mounted right in the center to feed a balanced antenna. It takes some time to learn how to use it but you'll have a fine tuner when you're done.

I split the tubing right in the center to switch from parallel to series configuration with a clip lead. I feed a short doublet and had some heating in parallel. Pics are on my qrz page if you need any ideas.

I found there were multiple combinations to get a good match on most frequencies. Use an rf ammeter or build a pickup and find the combination of taps cap settings that give you the most current up the feedline. Mine only seems to work 160 thru 20. I made a balanced L and prefer it on 40 thru 10.

It would also be good to provide a center tap on the big coil that can be grounded through an RF choke to dissipate static buildup on your antenna. You could even use a high voltage, high value resistor as a bleeder.

Using a choke or resistor instead of a direct ground allows the RF currents to balance themselves out without forcing a reference point.

Could you explain this further?

Real-world dipoles and balanced feeders rarely, if ever, appear as truly balanced. You’d see this if two calibrated RF ammeters were placed at each output terminal of the matching circuit. Forcing an RF ground at the center of the output coil of a link coupled tuner, rather than letting it float, reduces the cancelling effect of the feed line.

Since link coupled tuners (and Johnson Matchboxes) lack a D.C. ground for static draining, an RF choke or high voltage/high value resistor should be used and that can go in the center where the impedance is lowest.

I am just finishing up the tuner. may get to test it today. I'll take a photo of the fininshed tuner and also add a choke at the center.

Pat

How do you measure the "cancelling effect"?

What will the ammeters reveal about the feed line and/or antenna?

How can a single point ground overcome physical imbalances in the antenna and feed line (involving distances on the order of 100s of feet and a ground plane of 1000s of square feet)?

What is the effect of grounding the center of the link on common mode current or common mode impedance?