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HF Background Noise

Jun 19th 2015, 22:54

KB0CUR

Joined: Jun 19th 2015, 22:05
Total Topics: 0
Total Posts: 0
We are an NGO Mission Aviation Group operating in PNG. We are dependent on HF for our communication not only with our aircraft but with our 50+ mission stations. We are having problems with high levels of background noise present at our base of operations making communication difficult. We operate on 6.7mhz and our antenna is a broadband Moonraker 3 wire inverted V antenna. Our antenna height is 7 meters above the edge of our tin roof hanger. Total height above ground is 20+ meters.

We have recently grounded the individual sheets of tin on our roof but that does not seem to help the noise level. Our hangar building is 70 x 80 feet with a shallow peak of 33 feet with the support pole for our Moonraker at the peak.

I would like some help to try to get our noise levels lowered if possible. I have grounded the tin at the lower edges of our roof. Do I also need to ground the other adjoining roofs under the edges of the antenna as well as grounding the tin at the peak of our building?

Before connecting the ground wire from the tin roof to a ground rod, I measured 150 ohms of resistance.

73s

Jerry
KB0CUR
Jun 20th 2015, 19:44

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Can you describe the noise? Is it new or have you always had it?
Does it vary in any way? Weather, time of day, time of season, ...?
Does it vary as you tune across the spectrum? Can you run a test when the power is off to the building?
Jun 21st 2015, 09:48

KB0CUR

Joined: Jun 19th 2015, 22:05
Total Topics: 0
Total Posts: 0
It appears to be a high level of white noise. We have 3 phase 415 50 hz coming in at rooftop level. The noise level is 50% of the bars on the radio meter. Seems to be steady all day long. We just changed from a 2 wire broadband wire antenna to a 3 wire Moonraker hoping that would solve our problems. It has gotten progressively worse over the past 6 months.
Will try turning the power off and checking noise level.
73's
Jun 22nd 2015, 12:13

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
http://www.arrl.org/power-line-noise
Section IV describes finding RFI sources that are often confused with power line noise.

Zack Lau W1VT
ARRL Senior Lab Engineer
Jun 23rd 2015, 07:35

KB0CUR

Joined: Jun 19th 2015, 22:05
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Total Posts: 0
Found that our AC circuits are not the problem. Power was cut and noise remained the same. Signal is 96 Mhz and close to 24mV rms on the scope.
Jun 23rd 2015, 14:39

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
You next step is to use direction finding techniques to locate the source. Most of the time power line noise varies with the weather--since yours does not--it may be inside a building and radiated by powerlines.
http://www.arrl.org/power-line-noise
V. How to Locate Power-Line Noise

Zack W1VT
ARRL Senior Lab Engineer
Jun 23rd 2015, 19:43

KB0CUR

Joined: Jun 19th 2015, 22:05
Total Topics: 0
Total Posts: 0
We tried moving our antenna to the south of our building between us and another hangar, the signal was still at 96Mhz but much stronger. It may be our neighbors. My scope is portable so will try some direction finding later.

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