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Pellet stove and tuning 40 meters

Jan 10th 2012, 03:34

K2ADK

Joined: Jun 12th 2011, 20:32
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My rig is set up in the same room as the pellet stove is in. I never have a problem tuning 30, 20, 17, 15, 12, or 10 meters, but when I try to tune 40, I get wild readings on forward and reflected power. My output is 100 watts, and sometimes it falls to 10 watts while reflected power jumps to 8:1. No amount of tuning seems to keep it level. When the pellet stove is turned off, I get normal readings.

Is there anything I can do to mitigate this problem without moving my equipment?
Jan 10th 2012, 14:45

W1VT

Joined: Apr 4th 1998, 00:00
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These problems are caused by the ground system changing, particularly when you are using vertical or limited space antennas. What happens is that when the device switches on, additional wiring is added to the ground system--this is the result of everything being connected via 3 wire AC systems with a safety ground.

With a pellet stove designed for efficiency, it is quite possible that your ground system is being modulated by the electronic circuity of the pellet stove--no autotuner can take this sort of abuse. While there may not be a direct galvanic connection to the switched circuitry as the ground connections are not, there is likely to be capacitive coupling to the switched circuitry.

The solution may be to add additional grounding, to mitigate the influence of the pellet stove on your ground system

Zack Lau W1VT
ARRL Senior Lab Engineer
Jan 11th 2012, 02:58

K2ADK

Joined: Jun 12th 2011, 20:32
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Thanks, Zack, I'll give that a try and will post back with my results.
Jan 11th 2012, 03:34

WA0CBW

Joined: Apr 4th 1998, 00:00
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Along with what Zack said be sure you are using a single point ground system and that all your grounds are connected together per the NEC. I would also check the electrical connections to the stove to be sure you don't have the hot/ground/neutral reversed or mis-wired at the outlet.

Bill - WA0CBW
ARRL Technical Coordinator - Kansas Section

Jan 11th 2012, 13:52

W1VT

Joined: Apr 4th 1998, 00:00
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Total Posts: 0

Gread advice Bill!

I bought a Greenlee electrical test set that includes a diagnostic device with lights for making sure that three wire electrical outlets and light fixtures are wired properly--whenever I work on electrical wiring I use it before and after the job--just in case someone made a mistake.

Zack Lau W1VT
ARRL Senior Lab Engineer
Jan 12th 2012, 02:19

AA6E

Joined: Apr 4th 1998, 00:00
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Total Posts: 0
I'm not sure I understand Zack's point about the ground system. The grounds (3rd/green wire) should never be switched in or out in a permanent installation. What gets switched is the AC "hot" and possibly the AC "neutral". It seems to me your hot/neutral lines, when switched on, are forming a line length that is resonant on 40 meters, and it is coupled to your antenna system.

The worst case might be if the hot/neutral weren't run together (in a single cable) but were somehow forming a loop that might be coupling strongly to your antenna system. A loop could form if you have multiply connected grounds (i.e. not using the single point ground design). But it has to be a loop that is switched with your furnace somehow. One thought: your furnace (stove) probably is controlled by a thermostat some distance away from the unit. That is an opportunity to create a loop, separate from the grounding wire. Try adding some Type 31 ferrite cores (multiple turns) to your thermostat wiring near the furnance.

Reworking your grounding connection might help, or you could simply insert extra power cabling to move the resonance below 7 MHz. You might also try to find a way to separate your 40 M antenna and/or feed line so it doesn't couple with the furnace AC circuit. (Physical separation, re-orient at right angles, etc.)

Another line of attack is to be sure your coax line to the antenna does not carry RF on the outside -- i.e. your antenna is properly balanced. Imbalance would increase coupling with your AC wiring. It's hard to know what to recommend first, because we don't know much about your physical furnace wiring, antennas, and transmission lines.

Good luck!

73 Martin AA6E
Jan 12th 2012, 23:13

K2ADK

Joined: Jun 12th 2011, 20:32
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Total Posts: 0
Thanks to all of you, and especially you, Martin. As it turns out, my thermostat wire is exactly 1/4 wave for 40 meters. Ferrite cores have totally eliminated the problem. I'm glad I asked the question here!

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