Tuner Question - I am just not knowing

[W1QWT]

Hi,
I am building a balanced tuner to replace my MFJ thingee. It is from a 1948 ARRL Handbook.
I have a coil that I wound 5 turns on the inside of to link couple the unbalanced side to the balanced side. I used the inner conductor of RG8X with its insulation still on it. One of the local HAMs said I should have wound it on the outside. He said that cause I brought the link wires out through the coil it would arc over. If this is true should I rewind it on the outside? There is about 1/8 inch between windings. The primary is only the insulation width away from the secondary turns anyway?
Also I tapped the secondary at 1:1, 1:2, 1:3, 1:4. 1:7 turns ratio.
Will that give me an impedance transformation of a square of those?
1:1, 1:4, 1:9 etc.?

[Steve - WB3HUZ]

Are the wires exiting the inside of the outer coil any closer to the outer coil wires than the wires of the inner coil? Do the wires exiting the inside of the outer coil have any less insulation than the wires of the inner coil?

If the answer to both of these questions is no, I don't know why there would be any arcing, assuming there is no arcing between the inner and outer coils in the first place.

If arcing does occur, it's simple enough to remove a few windings of the outer coil at its center.

What is the wire gauge of the outer coil? What kind of power are you planning on putting through this new tuner?

[K1JJ]

Q,

Makes no difference if the coil is wound on the outside or inside for arcing purposes in your case.  I usually wind mine directly on the top using Teflon wire - no spacing.  

In your case, the wires passing through can touch the big coil w/o a problem when using that  inner coax insulation. I think it is good for 2KV+.

BUT,  the real reason not to worry is that the center of the BIGGER coil has zero volts potential  [in reference to ground - your 50 ohm input ground too) if everything is balanced, and gets progressively higher as you go out to the ends. So the first few turns in the middle are at low enuff potential on 160/75M/40M - not to worry - even at 1KW.  But, the potential BETWEEN each turn is the same, so you don't wanna be bridging too many, even at the center.

BTW, some guys even ground the center of the big coil.

Oh, one last thing... on the higher bands, like 10-20M, the bigger coil uses less turns, so that  the voltage can rise quickly even out a few turns from the center, assuming you are now tapping in from the ends making the coil smaller..

Later -

T

[W1QWT]

Well The gauge is about 12 and the most power I expect to run is about 150 watts carrier maybe 200.
Based on what Tom said I guess I don't have too much too worry about.
The bread slicers should be ok fine at that power level.
Way more better than that MFJ thingee.

 :ctf:

[W1QWT]

Oh yeah and I also want to keep the co-efficent of coupling (k) high because I am using this on receive also. THat is why I wound the link coupling coil right next to the secondary coil. Does this seem reasonable?
I don't care if I loose a few watts on transmit but I don't want to loose any microvolts on receive!

[Steve - WB3HUZ]

Twelve gauge should be FB for your power level, unless you are feeding a very low impedance antenna (like a 75 meter half-wave dipole on 160 meters).

I wouldn't worry too much about the coefficient of coupling. Unless you have a very deaf receiver it won't make any difference. And if you are using a cap in series with the small/inner coil, changing the value of that cap (which is what you will do when "tuning" the tuner) has the same effect as changing the coefficient of coupling between the two coils.

[W1QWT]

Well I guess changing the input cap tuning will change the coeffient of coupling but it will change it from what  it  was to something less.
Therefore don't I want to start out with  the highest coefficient of coupling?
But then again if I consider the actual impedance presented by the antenna on various frequencies.
So if I start off with something higher then things should  be better on receive?

[Steve - WB3HUZ]

It will change the coefficient of coupling. Whether it's greater or less, I can't say. Lots depends on the load you are trying to match. After all that's the point, getting an impedance match so you can transfer max power from your TX to the feedline.

There is a good explanation of how a link coupled tuner works in nearly all ARRL Handbooks since the 50's. It goes into great detail regarding Q, coefficient coupling, etc. I think it also explains how the impedance transformation works and how to calculate it, if enough data is known. Hint: it isn't as simple as the turns ratios you previously asked about.

As far as receive, it doesn't matter. In nearly all instances on the lower HF bands, the received SNR is defined by the noise external to the receiver - band noise, QRM, powerline noise , BPL, etc. Unless you have a very insensitive receiver, the coupling provided by the tuner will in no way affect what you can hear.

Here's an easy way to check. Tune your receiver to a quiet spot on the band (one where no QSO is in progress). Now disconnect the antenna and set your RX to max RF gain and a comfortable AF gain where you can clearly hear the noise coming from the receiver. Now connect the antenna. If the level of noise goes up, you know you have more than enough signal/voltage coming from the antenna and tuner.

When tuning up the tuner, always start with the taps on the big coil set on the outermost settings. Then work your way in towards the middle until you get the right match. If you have to tap WAY in towards the middle, you should consider using series tuning, that is instead of having a cap across the big coil (in parallel) you have two ganged caps in series with the feedline (one in each leg of the feedline). The problem with tapping too far in towards the middle in the parallel configuration is that the Q of the tuner is very high. This  results in very touchy tuning, and large circulating currents. The coil will likely get hot and you will be losing too much of your signal in heat and instead of radiating out the antenna. In other words, you run the risk of being PW and I know you don't want that!

[k4kyv]

When tuning up the tuner, always start with the taps on the big coil set on the outermost settings. Then work your way in towards the middle until you get the right match. If you have to tap WAY in towards the middle, you should consider using series tuning, that is instead of having a cap across the big coil (in parallel) you have two ganged caps in series with the feedline (one in each leg of the feedline). The problem with tapping too far in towards the middle in the parallel configuration is that the Q of the tuner is very high. This  results in very touchy tuning, and large circulating currents. The coil will likely get hot and you will be losing too much of your signal in heat and instead of radiating out the antenna. In other words, you run the risk of being PW and I know you don't want that!

I had that experience when I first tried to tune my 80m dipole to 160.  It turned out that the length of the feedline was such that the feedpoint of the feeder was midway between a voltage node and a current node, probably the most reactive situation possible.  I was able to tap down on each end of the coil to get a 1:1 match on the swr meter, using a BC-610 split stator plate tuning condenser (7000 volts, 150 pf/section).  Problem was, the 7 kv cap would arc over on modulation peaks at only 150 watts DC input to the transmitter final!  

I ended up adding an additional 1/8 wavelength of feedline so that it came out to a voltage loop, and it then would match with parallel tuning and no tapping down (but required a dual 300 pf 7kv tuning cap plus a 50 pf fixed vacuum across the coil), and it would take a kw fully modulated with no problem.  I used that antenna for a couple of years until I got my radial ground system installed in order to use the tower as a series-fed quarter wave vertical.

BTW, I have found an effective method for running series feed on a balanced link-coupled tuner using a conventional split stator tuning cap.  Just split the coil in two at the midpoint and connect the feedline across the gap.  The split stator cap goes across the ends of the coil in usual fashion, and the link goes over the middle of the coil in usual fashion.  In addition, this is more convenient to tune (requiring only one knob),  than having two separate caps, one in series with each end of the coil.

[Steve - WB3HUZ]

Good stuff. Yes, adding or subtracting (if possible) some line can make a big difference. I saw a "tuner" design (mated to a particular antenna) on the Web 5-6 years ago that was nothing more than a cap and an arrangement to switch in/out various length of ladder line. The guy had the thing working on all bands 80-10. Pretty neat.

I've seen the tuning arrangement you describe below in some older handbooks. You are right, this is the superior mechanical approach.

BTW, I have found an effective method for running series feed on a balanced link-coupled tuner using a conventional split stator tuning cap.  Just split the coil in two at the midpoint and connect the feedline across the gap.  The split stator cap goes across the ends of the coil in usual fashion, and the link goes over the middle of the coil in usual fashion.  In addition, this is more convenient to tune (requiring only one knob),  than having two separate caps, one in series with each end of the coil.