View Full Version : Balanced tuners
EI6GXB
09-12-2008, 11:25 PM
Hi,
Built the balanced-balanced tuner, which performs pretty well, albeit with some preliminary RF-in-the-shack problems.
Question(s), though :
I've heard that this tuner mustn't be mounted in a metal enclosure due to coupling.
How then, to prevent stray RF?
The leads for the inductors - SWR varies massively if they are moved, I suspect mounting them vertically may overcome that one, so that the wires leave at right angles to the coils as here :
And twisting the leads around each other, an old trick from building valve amps to keep the heater hum down.
http://www.n4ekv.com/BalancedL.jpg
73s,
EI6GXB
KB1KIX
09-13-2008, 06:22 AM
Sorry I can't help you on this one... but I had to comment and say....
That is really nice lookin!!!!
Real nice workmanship.
Jonathan
EI6GXB
09-13-2008, 09:27 AM
That;s not mine lol!!
L
W4HAY
09-13-2008, 12:39 PM
The rule-of-thumb for air-core inductors is to keep the metal at least one coil diameter away from the inductor(s).
EI6GXB
09-13-2008, 06:01 PM
WA-HAY?! Cool call!
Will do, need to do some rearrangement anyhow.
WA7OET
09-13-2008, 07:27 PM
I built that tuner on a piece of plywood with the coils in the horizontal position. Works great. I plan on using a plexiglas front with a rotary switch since I have all that on hand. I see no problem putting it in a metal box and would if I had one laying around.
If that tuner is working correctly it doesn't need to be in a metal box unless you are concerned with someone getting an R.F. burn.
If R.F. in the shack is a problem it could be a sign of antenna system imbalance. Antenna system meaning "tuner" feedline and doublet.
Start with the tuner. Layout and lead lengths are important. The tuner pictured is electrically correct but will have problems with certain loads due to wiring. It should be constructed using a symmetrical layout with no leads close to other leads. The coil pair should be kept away from the input BalUn so as not to couple any energy.
Another R.F. in the shack problem is when the antenna system presents a very high impedance to the "tuner's" output terminals. Tuner layout will not fix this. Adding or subtracting a few feet of balanced feeder OR doublet length should help. 2 feet at 10 meters can make quite a difference. Lower bands require proportionately juggling longer lengths to trim the system away from a high voltage feed situation.
Another thing to look at would be the input BalUn. Certain commonly found types aren't all they are cracked up to be. The "ugly" coil of coax BalUn works very well at certain frequencies but won't cover all H.F. bands by itself. You can put these in series to get better common mode rejection. As an example 1 might have 25 turns and the second might have 6 turns of coax on a similar form. As an alternative, there are excellent toroidal BalUns available for little money these days.
My experience with a number of installations using the same tuner design as you is that the doublet itself can be installed in a compromise situation and be relatively forgiving of imbalance. Small city lot installations can sometimes be OK. Same could be said for any good coupler design.
EI6GXB
09-14-2008, 07:15 PM
I'm wondering where my RF is coming from, what I'll do is have a few feet off the feedline first.
I'd erroneously placed the two coils very close together but now, a lot better. Is it good practise, as in valve amp circuits , to twist all the cables together, ie. the coil clip cables twisted together, and the capacitor to output also twisted, but not near the coil clips?
L
Steer clear of twisting wires together. This practice will leave the circuit with unnecessary capacitance and limit it's tuning range.
Twisted pairs work well for relatively low impedance signal transmission. The tuner in question can be called on to deal with much higher impedance depending on the system load and generator frequency.
Be aware that sometimes "RF in the shack" is the normal, and correct (albeit undesired) result of, well, the antenna radiating!
It this case the only cure is to move the antenna farther away.
Also, unless the "tuner" is built absolutely symmetrical. it will not be "Balanced", and there will be more or less radiation from it.
Rege
EI6GXB
09-16-2008, 04:46 PM
The RF was (in my case), the feedline coupling into the external speaker wiring (<2" away!! my bad!!).
Doubt I'll have any RF in the shack from radiation of the antenna, as the plasterboard in the attic is foil-lined! wonder what that'll do for radiation patterns lol
L
EI6GXB
09-17-2008, 12:43 PM
Found some PVC-foam stuff used for replacing balsa wood in models to make the spacers out of - at least it doesn't break when I try to cut notches in it!
Even if it's not phenomenally successful, it's the experimentation that counts lol.
Those who have built this tuner - what lead layout works best? Input and output at opposite ends of the board....etc
L
CT2JUT
09-20-2008, 09:12 PM
That;s not mine lol!!
L
Beautiful tuner!
Maybe i stared about 5 minutes looking at the picture.
I think it belongs to...
Check here: http://www.n4ekv.com/tuners.asp
What a really nice job! :)
73 from Lisbon!
Miguel
EI6GXB
10-07-2008, 09:55 AM
OK, ropey looking, but it's made me a ZS contact so far!!
I need to cut the base to size and protect the shaft and tidy a little. Even at 100W though, not a tingle from it. There was before I rebuilt it, but the second time I rotozipped the insulation off the copper, and evidently have made a vastly superior connection over jsut scratching it 'off' with a knife. Going on the formula (haven't managed to usurp college's inductance meter!!), handwound coils for ~ 25mH, and the capacitor is a dual-gang Mullard (280pF) from http://cgi.ebay.com/AIR-SPACED-VARIABLE-TWIN-GANG-CAPACITOR_W0QQitemZ5794611103QQihZ011QQcategoryZ96 966QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I have yet to switch the gangs, so as to give more capacitance (parallel) and less (series), and switch the whole unit itself to input/output, but that will change
Right now it will tune a 20m dipole to 40m, 30, 18 and 15 all at 1:1.
L
Hi,
Built the balanced-balanced tuner, which performs pretty well, albeit with some preliminary RF-in-the-shack problems.
Question(s), though :
I've heard that this tuner mustn't be mounted in a metal enclosure due to coupling.
How then, to prevent stray RF?
The leads for the inductors - SWR varies massively if they are moved, I suspect mounting them vertically may overcome that one, so that the wires leave at right angles to the coils as here :
And twisting the leads around each other, an old trick from building valve amps to keep the heater hum down.
73s,
EI6GXB
You know, of course, that you really do not have a balanced tuner? At least not by what is in the picture.
The idea you can move the balun to the INPUT of a floating network and have a "balanced tuner" is popular, but it is wrong.
Read this link:
http://www.w8ji.com/tuner_baluns.htm
The tuner in you picture appears to have a single capacitor and two inductors fed from a balun at the input. It is electrically impossible to have a good balanced tuner with that setup. It would be no different at all than just using a normal network with a balun on the output.
73 Tom
KL7AJ
10-09-2008, 05:43 PM
You know, of course, that you really do not have a balanced tuner? At least not by what is in the picture.
The idea you can move the balun to the INPUT of a floating network and have a "balanced tuner" is popular, but it is wrong.
Read this link:
http://www.w8ji.com/tuner_baluns.htm
The tuner in you picture appears to have a single capacitor and two inductors fed from a balun at the input. It is electrically impossible to have a good balanced tuner with that setup. It would be no different at all than just using a normal network with a balun on the output.
73 Tom
This is why I love my Johnson Matchbox. Now, if someone could make an adjustable link with toroids to make it more compact, we'd really have something. Something like a toroidal tickler coil. I wonder what the topology would be?
:)
eric
KL7AJ
10-09-2008, 05:44 PM
This is why I love my Johnson Matchbox. Now, if someone could make an adjustable link with toroids to make it more compact, we'd really have something. Something like a toroidal tickler coil. I wonder what the topology would be?
:)
eric
It would be TTTT....Toroidal Tickler Topology Technology
EI6GXB
10-09-2008, 09:32 PM
Many have used the W2DU balun with great mirth.
All I did was follow what was set out in the balanced balanced tuner how-to.
It works. ZS6CCY 4-9. 40, 20, 17 and 15 1:1. No RF in the shack.
L
Many have used the W2DU balun with great mirth.
All I did was follow what was set out in the balanced balanced tuner how-to.
It works. ZS6CCY 4-9. 40, 20, 17 and 15 1:1. No RF in the shack.
L
I certainly did not mean to imply your antenna would not "work".
I'm merely pointing out that the only way to build a balanced tuner is to actually build a balanced tuner.
If you have a network that is not a symmetrical balanced network with a ground reference at midpoint, and you move a balun from the output to the input, you don't do a thing for common mode currents. It doesn't do a thing to improve the effectiveness of the balun.
You can easily see why that happens at this link:
http://www.w8ji.com/tuner_baluns.htm
This doesn't mean you won't be happy with a less than perfect tuner, or that you have a bad tuner, or that I am picking on your work. It just means we could all do a better job understanding the problem.
Your circuit may be a correct circuit because there are some that work, but most of them are not correct.
73 tom
EI6GXB
10-10-2008, 07:56 AM
Hmm. Either way, eventually I'll build a single coil Z-match, link coupled tuner, but to find another suitable variable cap :D
