The AM Forum

I had built a homebrew link coupled tuner a while back and have been using it with great results. Today I decided to try something. With the understanding that the link is an unbalanced input, I disconnected the coax and put a W2DU balun in its place then reconnected the coax. Now the W2DU balun is feeding the link and coupling capacitor. I applied a 250 watt carrier and to my surprise, the balun got very warm. So that is telling me that an I3 current is present on the outside shield of the coax. Why is this? Is it due to the link input not being balanced? Or is it due to an imbalance on my open wire feeders to the antenna? Should I make up a coaxial choke to kill that I3 current? The way I see it, my antenna system is trying to use the inside of my shack as part of the antenna thru the tuner. I do have RFI in the shack but I have detected most of it coming in from alternate sources and not so much from the tuner. I have included a few pics of my tuner and antenna system.

its telling you that the W2DU balun is a buncha JUNK and cant take the demands of AM action.  ;D :D ;D

Seriously, ferrites rated for SSB power just dont cut it for AM. That's probably the only thing going on. It works, but the demands of am bring most ferrites to their knees quick like. The constant power beats down ferrites.

Just a gag. maybe some one else not as goofy can be of more help.

My question is why you put the bal-un there in the first place?

I can't see any reason you'd want it in th circuit.

Are you sure its not a W2AU balun?  A choke balun in an unbalanced to unbalanced circuit that is reasonably matched should not much of an effect.

This means there is a ground loop between the rig and tuner ground. The balun is trying to reduce it so it heats up. Hey Derb I can easily put over 5 KW pep, 1 KW carrier through the output transformer of the class e using a ferrite transformer. It warms up just a little.

Put in a station gound!!!!  Make it the project for the day..   


Clark

Your tuner converts balanced to unbalanced. That does not necessarily mean it stops or chokes off all common mode currents. Your tuner, antenna and feedline must be very symmetrical to reduce common-mode currents to a minimum.

Try the same experiment with the BalUn feeding a dummy load.

I have some questions:  What prompted you to do this in the first place?  What is the ratio of the balun? 

hams usually have some sort of feedline ferrite device at the low Z unbalanced input to a tuner for balanced High Z feedline so that rf appearing on the outside of the coax shield is trapped at the tuner.  This RF is often referred to as common mode current because it is unwanted but the path, i.e. "mode" is common with the desired signal, the RF field propagated in the dielectric between the center and shield inside the coax.   This choke is called an unun, common mode filter, 1:1 unun, "line isolator" and maybe other names but it is usually a series of ferrite beads (a "bead balun" yet another name for it), or one big ferrite core with teflon coax on it for several turns or sometimes 50 - 100 feet of coax wrapped on a plastic coil form to make an inductor (it has to be big for 160) so inductive reactance acts as the trap for the common mode RF.  This last type of filter eliminates the problem of core saturation and heating by not using ferrite. 

You need an un-un (unbalanced to unbalanced) that is ratio one to one because your Z on each end of the choke should be 50 ohms unbalanced.  BUT,

sorry darn computer is out of control when I'm in a hurry...

BUT **you may not need a common mode filter at all** because you use a link coupled tuner.  Think of this as an RF isolation transformer.  You may use an audio isolation transformer to break up hum inducing ground loops in ur audio rack gear.  Looks to me like your link coupled tuner schematic has the same thing going on for it at RF.  I would not think any common mode on your balanced feed would make it across the coupled inductors in the tuner.  I have a Johnson KW matchbox that is similar and I think that is one of the good things about it but perhaps someone with more experience with these tuners can confirm or correct me.

tnx & 73 & if you are having a RF feedback prob. I hope you get it resolved.  Might be something else?

Tell us how close ur shack is to ur antenna, your equipment and how it is all connected, how it is all connected to ground...maybe that will provide clues.

Rob K5UJ

Joe, as Bruce said, with the circuit you show for the tuner there is NO need for a balun at the input, because with the link coupling of your circuit you already have an inherent balun. You can't improve on that, which is why the Johnson Match Box is so good. Your only improvement over the Match Box is that you have eliminated the reactance of the input link with the series variable caps in the link circuit, which I don't believe the Johnson has. I have the Matchbox, but it is in Florida and I'm in Michigan, so I can't look to see for sure. The balun effect obtained with the link coupling sure beats having to use a ferrite toroid balun at the output used by every tuner mfgr except the Johnson, as far as I know.

Walt

Instead of a variable cap in series with the link, you can use an adjustable link, either swinging (like the B&W HDVL series coils) or rotatable (like the one in a BC-610 output coil).  You can even vary the number of turns on the link, although that would be a little hard to adjust.

BTW, I would disconnect the ground to the rotor/frame of the split stator capacitor as shown in the diagram.  That ground connection can cause common mode currents to appear on the feedline, so that the feedline acts like a Marconi antenna working against ground, and radiates.

Don, on my home-built tuner the link was swinging, but it still had inductive reactance that had to cancelled to allow a 50 + j0 input impedance. Without the series cap there was always a +j at the input. Had to get rid of that inductive reactance to get all the available power into the tuna.

Walt

OK, my reason for trying the W2DU at the input of my tuner was to only see if it minimized the common mode in the shack. I was not planning on keeping it in service. My thinking was that the balun is ferrite loaded and that it may kill the common mode coming back. Well, the balun got warm, AND the common mode current in the shack was the same. After some more checking, I had discovered that my CAT 5 internet line coming into the shack was hot with RF when I was transmitting a carrier. Also, the coax coming in from my 10m vertical was hot with RF which goes to my antenna coax switch. So I disconnected these 2 lines and coiled them up in the corner away from everything and 65% of the RF was gone. I also noticed all the outlets in the shack and through out the house are hot with RF. I re-posted a new pic of my tuner schematic showing the main coil grounded at the center tap. When I remove this center tap ground and retune, the RF in the shack goes off the chart! Guess that stays grounded, ha. Thanks for the help guys...

Joe, the tuner schematic you show just above is exactly the circuit of mine. Mine uses inductances from the BC-610, and the link coil swings inside the main inductance. Adjustment of the swing determines the degree of coupling, but the variable cap in series with the link is necessary to cancel the inductive reactance of the link.

However, I've never needed to ground the center of the main inductance. If your situation finds RF in shack with the center ungrounded, it may be caused by some unbalance in the feedline or in the antenna itself. You might want to look to see if such an unbalance is present, but unknown. Or perhaps something unseen in the tuner is causing an unbalance. If there is no unbalance anywhere there should be no need to ground the center of the main inductance.

Walt

Joe,

Is your antenna right over the house?

I wonder if you tried not tying the input coax to ground where it comes into the tuner?  Just have the coax end floating and connect to the input link only.

The antenna is located 45' directly over the house, yes. I have discovered that when there is an imbalance, high rf current was measured on the ground rod wire returning the current back into the antenna system. I am able to achieve somewhat of a balance when I move the link back and forth and adjust the caps differently, only then there is low RF current on the ground rod wire. Question, what is the best way to achieve good balance? Make adjustments at the antenna feedpoint? Then it would be off again when I change bands. Yes, I use this one antenna for 160, 80, and 40. It is a 200' wire center fed with 600ohm open wire feeder. It is not perfectly sysmetrical.

Just out of curiousosity....

What are the specs on yer tuna?

Diameter of main coil and link.... lenght of each...

How many turns on that main coil?

Looks like a lot.

Also, depending on the diameter of the inpoot link...   

I dunno, seems like a lot of inductance in both cases...

But might just be my eyes    8)


Oh, by the way.... you may well be aware,
But that big variable cap it looks like you're using as link C.
It only needs to be a 500 or 1k volt broadcast variable.

Not much voltage at that point.

Well, 200' should be long enough for 160 with a good balanced tuner.  So you're saying the feedpoint is right in the middle so you have 100' on each side.  Okay, so that's balanced else if ur f/p is not centered there's part of ur problem right there.  If your feed comes straight down from the feedpoint and continues over ground suspended off the ground a few feet and makes an entrance to your shack being kept at least a few inches away from metal it should be okay.   Here's what I did, mine comes straight down then there's a curve and it runs over the ground suspended by PVC pipes with T sections holding the  spacers every 4 or 5 feet then a wide curve to my basement window.  There I removed one center pane and put in a plexiglass pane with two holes drilled in the center about 3" apart.  I put banana jacks in these and plugs on the feedline sides.  It gets into the shack by passing through the window that way so it stays several inches from any metal frame.  Think of the feedline as a road surface through the air--no sharp turns, you bank it on turns so one side doesn't get longer than the other and keep it a few feet away from any metal like a mast it runs parallel to.   Setting it up this way is a pain unless you like the challenge which I do, but the reward is such low loss.  Anyway, if ur feedline is bent back in itself, or too close to metal, something like that, then that may be where you are loosing ur balance.

Rob K5UJ

I was able to get a perfect 1:1 SWR reading on my 160m. tuner using a "ham radio quality" SWR meter, by painstakingly adjusting the number of turns on the link.  I didn't re-check it with my GR impedance bridge, so I just assumed that the reactance of the link cancelled the reactance reflected back from the line.  Based on other antenna and feedline measurements I have made, I'm not so sure that a 1:1 SWR reading on a meter necessarily always means zero reactance into a perfectly matched purely resistive load; that residual reactance isn't cancelling out the mismatch displayed by the meter to give a "virtual" 1:1 reading.

Good point.. Don, If you are up now, We are meeting on 3880 at 2300.. or 11 pm our time.. I am hoping you can join us.

Clark

Try grounding the rotor of the main tuning cap to help force a balance. My fugly likes the caps grounded. I'm running a 250 foot Vee with about 75 feet of #10 spaced 4 inches.

Geeesh, The tuner design you HAD looked perfectly workable.
Install the "isolator" on the 50 ohm side. I'm using Walt's device on the 50 ohm side of my balanced tuner for the common mode stuff.
Don't want no baluns on the outpoot........no no no

Fred

TOM VU K1JJ
Would love to see your lamps on the outpoot showing a balanced antenna. That's a good thing

fred

??

Back in '88 I had FIVE tuners in the shack for quick switching of the bands -  had the balanced lights and all.

Today I use coax and hardline exclusively. No more tuners here.   Do as I say, not as I do.... ;D

T

Just curious, Don, is your GR impedance bridge a 916A or 1606A? I have several impedance bridges, including a 1606A and a 1606B and several HP types. I've loaned the 1606B to Rick, but I use the 1606A when I want the best accuracy available. It's accuracy beats the HP-4815, especially in the very low impedance areas, less than 5 ohms.

Walt

It's a 916AL.  Its maximum frequency range is 3 mHz as I recall.  It is designed for low frequencies and the broadcast band, but 160m is well within its range.  All I have ever used it for is to measure the base impedance of my vertical, and last time was several years ago.  The process is kind of cumbersome because I have to lug the bridge, a receiver and a signal source down to the tower and supply a source of a.c. power to the receiver and signal generator.

Thanks Don. The GR-916AL was used to measure antenna impedance on nearly all standard AM BC stations all through the 1930's and 40's and beyond. GR improved on the 916A with the 1606A in 1955.

Walt