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Author Topic: Smoking tuner  (Read 2527 times)
VA7ICW
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Posts: 16




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« on: May 15, 2010, 11:47:08 AM »

I just put up a folded dipole antenna I made from 450 ohm ladder line the other day.  Due to space restrictions I had to cut the antenna at about 25 feet but did feed it with some more ladder line, another 15 feet into a 4:1 Balun and then a couple of feet of coax into my LDG Z-11 pro tuner. 
I was running about a 100 watts on 30 meters and I noticed the tuner starting to expel smoke. Undecided
I immediately stopped keying and turned everything off then had a look inside the tuner.  All is
well other than stinky but not burn marks noticed and after turning it back on and reducing power
to 50 watts it runs just fine.
What I find interesting and one of the reasons for this post is that the tuner certainly doesn't give
much information out that something might be wrong does it?  It was showing a solid green light on
the tuner with no indication that anything was going awry?  Just wonder if anyone else has experienced anything similar?  Thanks.

Sean VA7ICW
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WX7G
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Posts: 6035




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« Reply #1 on: May 15, 2010, 12:34:16 PM »

But it did send you a signal that something was not right; a smoke signal.
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VA7ICW
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« Reply #2 on: May 15, 2010, 04:34:26 PM »

Yes indeed  Cheesy Cheesy
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KA5N
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« Reply #3 on: May 15, 2010, 04:57:57 PM »

a folded dipole is a single band antenna and should be about 46 feet   (468/freq in MHz) feet in length.  You can't just cut it any length and expect it to work and when tuned properly and fed properly it won't need a tuner.  If you had used just one wire instead of 400 ohm lead you might have gotten it to tune without the tuner smoking.  Most likely you have burnt the coil in the tuner up.
With limited space, the best compromise antenna is either a vertical or an inverted V. 
Allen
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WB2WIK
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« Reply #4 on: May 15, 2010, 05:06:12 PM »

25 feet is way too short, the current in the tuner was excessive.

A 30m dipole is 46 feet long.

If you need to experiment with such short antennas, a manual tuner (not automatic tuner) would be a better choice.
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N3OX
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« Reply #5 on: May 15, 2010, 05:54:00 PM »

Just to expand a bit on what Steve said:

A short folded dipole like that fed with 15 feet of ladder line into a 4:1 balun is getting pretty close to a short circuit.  The dipole has an impedance of about 51+j1300 on 10.1MHz, according to EZNEC.  15 feet of Wireman 551 transforms that to about 10-j400.  A 4:1 transformer will take that down to 2.5-j100

Plugging that impedance into W9CF's tuner simulator  (http://fermi.la.asu.edu/w9cf/tuner/tuner.html) with default settings (which may even be a tad optimistic for an autotuner) leads to a tuner loss of 2.7dB, 46.6% loss.  So if that's an accurate estimate, you would be dissipating about 47W in the tuner (in the coils in particular) at the 100W level.  Increasing the maximum output capacitance on the W9CF simulator to 100,000pF (which basically causes it to autotune itself into an L network) is a bit better at 34W dissipated, but still too much.

Tuners can't do everything, autotuners with small coils especially, and if you have to use any kind of doublet /dipole that's quite a bit shorter than 1/2 wavelength , you should probably consider winding some loading coils to insert in the antenna.

What you were doing is really hard on a tuner.  Running 50W will probably only increase the time before the smoking starts. Undecided

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K7KBN
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Posts: 2801




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« Reply #6 on: May 15, 2010, 06:12:21 PM »

I've smoked salmon, but never tuna... Grin
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73
Pat K7KBN
CWO4 USNR Ret.
VA7ICW
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Posts: 16




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« Reply #7 on: May 15, 2010, 06:42:25 PM »

thanks for all your input and your good advice fellas i am just trying to find something that will work in my small townhome lot.

73, sean
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N8EKT
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Posts: 371




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« Reply #8 on: May 16, 2010, 02:29:54 PM »

Ohm's law strikes again lol

Even at 100 watts, it is easy to exceed either the voltage or current ratings of tuner components.

Many a ham has smoked their 300 watt tuners with less than 100 watts.
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WB2WIK
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« Reply #9 on: May 16, 2010, 02:43:08 PM »

I've smoked salmon, but never tuna... Grin

I know how to tune an antenna, but how do you tuna fish?
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WB2WIK
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« Reply #10 on: May 16, 2010, 02:45:45 PM »

BTW to be serious, it always pays to use a BIG tuner, even at low power levels: Tuners with very LARGE inductors (made from big, fat, large-gauge wire) always have less loss than ones made with smaller inductors.

I use an Ameritron ATR-30 (3kW rated) tuner for QRP work.  In fact, it's almost demanded for QRP work because the tuner will have far less loss than a "small" or "QRP" tuner will, in almost all cases.

It's funny at Field Day when I bring my FT-817 (5W battery operated portable rig) and sit it atop the ATR-30 tuner, which is about ten times bigger than the rig.  However, that's what works.
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KT1F
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Posts: 58




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« Reply #11 on: May 25, 2010, 10:36:42 AM »

I had a similar experience with my Z-11Pro. I have a doublet antenna which is about 40m long center feed with about 30m of ladder line. I smelled and saw the smoke during a 100w CW contact on 20m. It still worked okay after it cooled down but one coil looked seriously burned. I contacted LDG and they assured me that it should be able to handle what I was doing and said to send it to them for warranty repair which I did. They replaced the coil and everything looks and works fine now but I haven't really used it since then. I now use an old MFJ manual tuner which feels a lot more solid.

I still think it's quite a good little tuner but I'm skeptical if it can really handle my 20m situation at 100w. I haven't been willing to try it again. I'm keeping it for more QRP operation. The problem of course is that they only publish a rating in terms of transmitter output power because that's the only practical number most people can relate to their transmitter but that's not really what matters. It's power dissipation in the tuner that generates smoke and voltage that causes arcing so I guess it's difficult to give a power rating that is safe for all impedance.

73
Ross
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WX7G
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« Reply #12 on: May 25, 2010, 11:27:16 AM »

If you wish to know what is smoking run the tuner at 50 watts with the cover OFF. After a few minutes switch the RF OFF. Feel around with a finger for hot components.

Or, run the tuner at 100 watts with the cover OFF. Observe which component emits smoke.
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KT1F
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Posts: 58




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« Reply #13 on: May 25, 2010, 12:05:52 PM »

Or just tune for the first sign of smoke and then back off a bit  Smiley
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N3OX
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« Reply #14 on: May 25, 2010, 12:15:54 PM »

IIt's power dissipation in the tuner that generates smoke and voltage that causes arcing so I guess it's difficult to give a power rating that is safe for all impedance.

A really complete specification would provide safe operating area charts in the R,X impedance plane with the "safe" impedance boundaries drawn for full rated power, half rated power, quarter rated power, etc.   You would need one chart for each band because they'd have different shapes as you got to max/min component ratings and because of changing inductor Q.
It would also be useful to have safe operating area boundaries drawn for peak vs. average power, because that says a lot about voltage vs. current/dissipation limits.

 It would be a lot of information, a fair amount of work to incorporate in the manual, and there's not a whole lot of upside for the manufacturer. 

Not everyone has a lot of impedance estimating resources at their disposal, but for simple antennas it can often be looked up in a book or calculated with rudimentary modeling skills in 4nec2 or something, or you can ask someone who's handy with modeling software.

It'd be nice if we could say in this thread: "well, the impedance your tuner is seeing is 13-j650 and that's outside of the safe operating area for 100W shown in the tuner manual... here's what to do to fix it"

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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