mikefox whiskey
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Tuner Arcing
I have a Decca KW107 Supermatch balanced tuner, recently while transmitting on the lower bands I have found the tuner to be arcing/clicking and the SWR shooting to the top of the scale for a brief second. My antenna is a full wave loop for around 80m and is fed with 75ft of 600ohm feeder, just wondering if anybody is familiar with tuner's arcing. Thanks
Last edited by mikefox whiskey, 11/Aug/2009, 10:34 am
--- Patrick - MFW / 68 FB750
kenwood TS870s - Astatic 1104c Microphone - 54 meter long doublet - Multiband Delta Loop.
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6/Aug/2009, 9:11 pm
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crimso
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Re: Tuner Arcing
The KW is a Z match & can thus work, depending upon load impedance, at very high Q's even at 100 watts & it has no DC earth to dissipate the static off.
Every time you double the Q, you also double the circulating current, so it does not take a very big Q to get a very high voltage.
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7/Aug/2009, 8:27 am
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Hot Fusion
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Re: Tuner Arcing
A guess:
You may have a dodgy connection on the antenna, and when it goes open, a high voltage is presented across a capacitor which arcs. If this is the case, you should also be aware of momentary dropouts on receive - does this happen?
If not, then it is either that the feedpoint impedance is high enough to cause the cap to flash over, or you are simply getting a static buildup, although I doubt this would cause the SWR twitch you have noticed.
I'll go for the 'high feedpoint impedance' option, and to alter that you would need to alter the length of your loop to change the feedpoint characteristics to something the tuner can handle without flashing over. Does this happen on any other bands? I suspect not.
let us know how your investigation progresses.
---
Andy - 26 FB107
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8/Aug/2009, 11:08 pm
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mikefox whiskey
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Re: Tuner Arcing
Thanks for the info and advice, I've checked the connections which are fine and a complete electric circuit all the way round.
This only happens as far as I know on the 86m band, There is still static on the wire but not enough to cause a problem, there is certainly less static than when I had a doublet antenna up.
I think Andy you are right and the problem is a flash over from a high impedance on the line, I have tested the impedance with an MFJ Analyser and found the impedance to be off the chart on the said band, I remember having to work with rapid impedance changes in the past with 600 ohm line, lengthening or shortening the feed line will probably cure the problem somewhat so I will get to it. Cheers
Last edited by mikefox whiskey, 11/Aug/2009, 10:35 am
--- Patrick - MFW / 68 FB750
kenwood TS870s - Astatic 1104c Microphone - 54 meter long doublet - Multiband Delta Loop.
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9/Aug/2009, 10:47 am
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crimso
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Re: Tuner Arcing
A full wave loop antenna should present an impedance of 100 Ohms with a SWR of around 1.7 across the band if a 75 Ohm coax matching stub & 50 Ohm coax feed is used.
I would thus guess that the use of the 600 Ohm feeder means that you are using it on other bands.
My understanding is that a balun should not be used, but I have never read why that is. I also understand that if used multi band antenna then ladder line with an impedance of 450 - 600 Ohms should be used, so you sound to have that right.
I assume that the loop is a correct match at the full wave being used, if not ignore the following. By a correct match I assume that at the frequency being used that you have matched it & not just made a basic mathematical calculation that does not include velocity factors for the wire used in constructing the antenna.
Assuming the above is correct then what you are describing sounds like what happens on a kite antenna when using a Z match, due to static (DC) flash over & that can end the front end of your radio. And as I said the KW has no DC earth.
The lack of a DC earth added to an already potentially problematic form of ATU were static build up is present & this could very well be adding to the potential for problems, if this is what is happening.
If I am corrct then the solution is simple, just put a DC earth on both sides of the ladder line using a bleeder resistor. Or fit an earth to the KW.
My guess for the problem is the total lack of a DC earth on the KW.
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9/Aug/2009, 11:19 am
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mikefox whiskey
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Re: Tuner Arcing
I have since changed the length of the feedline and it has dropped the impedance on around 80m to an acceptable level and no more arching down there, the problem is now the impedance has rocketed around 40m and the tuner is arching a bit there, I think I read somewhere the impedance on balanced line double's every 1/4 wave length, could someone verify this for me so I can adjust the feedline accordingly, at the minute the feedline is about 66ft (around a 1/2 wave on 40m) and the impedance is off the chart on the analyser around this band.
--- Patrick - MFW / 68 FB750
kenwood TS870s - Astatic 1104c Microphone - 54 meter long doublet - Multiband Delta Loop.
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20/Aug/2009, 10:43 am
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John193
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Re: Tuner Arcing
Patrick just use a feeder that is not harmonic on any of the bands your using, that will give the tuner a chance to just tune the antenna and not the complex combination. if you're feeding the loop through a balun, why? the feeder is already balanced. for the loop to work with that feeder you should be running.
1 balun from coax to 450 ohm feeder
2 non harmonic lenth of feeder
3 balbal from feeder to loop (4:1 ei 450 to 108 ohms)
hope that helps
as an aside I have a multi band loop supported by bungee at the corners fed with long feeder line (not ladder) and that joins the antenna at pulleys so i can tune the antenna without even a tuner just by moving the 4:1 balun up and down thus shortening or lengthening the loop and feeder combo.
it tunes to less than 1.5 on any freq between 95m and 35mhz. You will need very high supports to get the variable length to that of mine though, I use very tall trees
john193
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20/Aug/2009, 11:56 am
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mikefox whiskey
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Re: Tuner Arcing
Thanks John for that info, I had the feeder length a non resonant length before (75ft) and I was still having problems around 80m with arching, originally the feedpoint used to have about 100ft of balanced line and I had no problems anywhere on any band but I've moved the antenna closer and don't have room to coil up that much feedline. I'll work something out soon to fix it.
There is no balun or coax on the loop, just a direct connection from the loop to 600ohm feedline which is directly connected to the Z match. The loop is supported by two trees and a pole on the house.
--- Patrick - MFW / 68 FB750
kenwood TS870s - Astatic 1104c Microphone - 54 meter long doublet - Multiband Delta Loop.
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20/Aug/2009, 12:27 pm
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crimso
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Re: Tuner Arcing
quote:
mikefox whiskey wrote:
I had the feeder length a non resonant length before (75ft) and I was still having problems around 80m with arching.
Don't forget the velocity factor when calculating wavelength, 600 Ohm feeder is typically around .86% but that’s not a certainty.
On Echo Charlie at 3.47MHz is 86.45 meters, if that is divided by the coax length it works out at 3.31% of a wavelength so you would have been very close to an almost perfect third of a wavelength, if it was spot on 0.86% velocity factor & third of a wavelength antenna resonates very nicely.
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20/Aug/2009, 3:54 pm
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