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Author Topic: Dipole in the attic has an interesting side effect.  (Read 829 times)
N0CRN
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« on: April 19, 2019, 06:58:22 PM »

I'm not sure this is the right forum to ask, but I decided to start here and repost elsewhere if necessary.

After decades as a Technician, I finally did the paperwork to "instant-upgrade" to General (I got my license back when the Tech exam was the General written and Novice CW).    I've never had an HF rig until a week ago, when I bought an IC-7100 because interfacing as as simple as plugging in a USB cable and it also covered VHF/UHF bands.

My wife wasn't happy with my original idea of a long wire antenna hanging above our pool, so I put an 40/20/10 dipole in the attic (a G5RV junior).   It's slightly inverted, as the 51 ft length was just a bit longer than our attic, corner to corner.  It's working well enough for PSK31 on 40m and 20m, at only 20W.  PSKReporter monitors reported hearing my signal across the continental US.    And, I just made an FT8 contact on 20m with only 5W.  I looked at PSKReporter, and I was stunned at the number of stations that heard it.

But, there's an interesting side effect:  when transmitting, the fan on our AC/heater starts running.   The AC/heater is in our attic (normal around here), and the dipole doesn't quite straddle it, but it's 3-5 feet away (horizontally).  Another AC/heater (for the other part of the house) doesn't run, but it's about 10 feet farther away.  I haven't yet determined if the AC compressor unit (outside) turns on, as I'm limited in what I can see/hear from the control point. 

I'm not sure where to start.  I suspect it is the control line from our Nest thermostat.   But, I was hoping that someone else dealing with antenna restrictions had encountered a similar problem.  Any suggestions?
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KE6EE
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« Reply #1 on: April 19, 2019, 08:38:32 PM »

RF getting into AC/heat control circuit(s)?

Put chokes (ferrites with several turns of wire) on all wires connecting to the AC/heat unit.
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W9IQ
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« Reply #2 on: April 20, 2019, 05:06:09 AM »

On what band(s) is the fan problem occurring?

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N0CRN
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Posts: 8




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« Reply #3 on: April 20, 2019, 07:06:11 AM »

RF getting into AC/heat control circuit(s)?

Put chokes (ferrites with several turns of wire) on all wires connecting to the AC/heat unit.

This was my suspicion.  Do you think I need to put chokes on the power (120 VAC) as well?
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N0CRN
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« Reply #4 on: April 20, 2019, 07:07:30 AM »

On what band(s) is the fan problem occurring?

40m and 20m.  It might be happening on others, but these are the only bands I've transmitted.
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K0UA
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« Reply #5 on: April 20, 2019, 07:12:12 AM »

Don't forget that disc ceramic capacitors across control leads can fix a lot of these kind of problems too.

I had a serious problem on 80 meters with 100 watts setting off our alarm system. a single .01 microfarad disc ceramic across its AC power leads fixed it when a ton of ferrites did not.  This was from a full size antenna about 25 foot above the roofline of the house and 100 watts.
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W9IQ
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« Reply #6 on: April 20, 2019, 08:46:12 AM »

On what band(s) is the fan problem occurring?

40m and 20m.  It might be happening on others, but these are the only bands I've transmitted.

Then I would recommend type 31 ferrite on the thermostat wire close to the HVAC unit. Try to put a couple of turns through the center of the core. If the wire doesn't have slack, then clamp on 3 or 4 cores. You don't need cores with a large diameter. Pick type 31 cores that work with your wire and have the highest impedance (see the Fair Rite catalog).

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
K5LXP
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« Reply #7 on: April 20, 2019, 11:39:26 AM »

 
Just to offer the most obvious solution, having an antenna outside that isn't coupling into the house wiring (as much) can resolve this as well.  As an added bonus, it will probably work better too.  Domestic complications aside, running an indoor antenna and addressing RFI is akin to standing in a swimming pool and complaining about the rain.  I get that sometimes compromise includes deference to the contending party but physics doesn't abide by that covenant.  Point being, it might be easier and more straightforward to work out an outside antenna than isolating affected equipment.  It's just a wire in the yard.

Mark K5LXP
Albuquerque, NM
 
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N0CRN
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« Reply #8 on: April 20, 2019, 05:46:14 PM »


Then I would recommend type 31 ferrite on the thermostat wire close to the HVAC unit. Try to put a couple of turns through the center of the core. If the wire doesn't have slack, then clamp on 3 or 4 cores. You don't need cores with a large diameter. Pick type 31 cores that work with your wire and have the highest impedance (see the Fair Rite catalog).


Thanks, that did the trick!   I put ferrite cores around the lines for the thermostat, the 120 VAC, and to the compressor outside.  To be sure, I also put one around the line next to the compressor.

I still need to experiment with higher (than 10W) power levels, but that solves my problem for the FT8 mode.
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N0CRN
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« Reply #9 on: May 18, 2019, 08:55:35 AM »

A final follow-up for anyone that is interested:

As I reported, adding the ferrite cores solved my problem, but after about two weeks it began again.  By this time, we had warmer weather and the A/C compressor outside was also running.  Then, the A/C came on and didn't shut off... at all.  I removed the Nest thermostat from the base, and the A/C still ran.  The only way the A/C shut off was to turn off the power.

I feared the worst:  the control board in the A/C had been somehow damaged.  I called in our A/C tech, and explained the history.  But when he disconnected the thermostat at the A/C, the A/C shut off.  I told him I had removed the thermostat, and he explained that the base of a Nest thermostat has a large amount of electronic circuitry, including some solid-state relays.  He also said that it wasn't unusual for a Nest thermostat to fail after about 5 years, and especially if there have been lightning-related surges.

Fortunately, we had another Nest thermostat in another zone of the house (which had never been affected), so I quickly swapped the bases while he did some needed periodic maintenance.  That fixed the problem, so I bought another Nest thermostat and installed it that night.  I also put more ferrite cores on the wires next to the thermostats, as I had not done so originally (yes, that was an obvious oversight on my part).

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