Estimating and Mitigating 60-Cycle Interference

Discussion in 'The Projects Forum' started by GregTuve, Dec 6, 2012.

  1. GregTuve

    Thread Starter New Member

    Nov 9, 2012
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    Exploration showed that there was only one place I could install a parabolic antenna on my roof in order to aim it at a cell tower 3/4 of a mile away. Unfortunately, the line segment so drawn will go right past a garden-variety step-down transformer mounted on a utility pole. The anntenna's "beam width" suggests that a substantial amount of the signal concentrated at the parabola's focus (about 10%) will be radio interference from the transformer. (That's my best SWAG.) The antenna itself is optimized for frequencies in the 1700 MHz to 2100 MHz range ("AWS band").

    I'm planning to install some kind of lighting arrestor. The only equipment I'll be protecting is a 184.00 battery powered Sonic "hotspot" at the other end of the antenna feed. It works quite well--probably because it has its own frequency filtering capabilities.

    1) How much do I need to worry about interference from the transformer?

    2) Assuming the answer is that it will introduce significant noise, can I assume that a "polyphaser" (the "DSXL" model lighting arrestor popular among ham radio operators) will filter out that interference? -Or does this "RF-filter-coaxial-lightning-protector" begin filtering only when lightning is hitting nearby?

    3) The device supposedly passes only signals between 700 MHz and 2700 MHz. A description is found here: http://www.protectiongroup.com/Mark...Type/RF/RF-filter-coaxial-lightning-protector. One attraction of the device is its short "clamp time."

    4) I'm considering using the DSXL instead of the more common "gas discharge tube," which can handle larger currents but doesn't filter anything and clamps slowly. Would I do best to use both a gas discharge tube and a polyphaser on the same run of coax or would the fact that they'll share the same earthing network cause unpredictable behavior during a lightning event?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    The interference from the AC transformer has a frequency of 60Hz.
    This is way outside the pass band of the 1.7GHz receiver.
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The transformer may still have some corona discharge that will make RF noise, if possible I would get the transformer away from the line of sight.

    What I'm worried about is why you need a "high gain parabolic antenna" pointed directly at a commercial transmitter 3/4 of a mile away that transmits with tons of power?
     
  4. GregTuve

    Thread Starter New Member

    Nov 9, 2012
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    It sounds like what Mr. Chips is really saying is that almost any receiver would have a built in frequency filter (one which should easily filter out a 60 Hz signal). I also worry about radio signals picked up by the power lines feeding the transformer and harmonics of the base frequency caused by real-world imperfections in impedance matching of the power lines (just read up on it).

    The next worry is noise caused by coronal discharge (according to RB). Unfortunately, moving the transformer is not practical. Most of the coronal discharge noise would be near the 60 Hz range, I guess. To answer THE_RB's question, there are a lot of trees in my neighborhood so the signal I'm harvesting from the cell tower suffers from general attenuation caused by foliage (especially wet foliage) and destructive interference (behind swaying tree trunks). When the trees are nearby, a small antenna can be completely immersed in the antinode (a shadow caused by destructive interference). This causes internet access to come and go as the trees sway to and fro'. The problems can be bad enough to interrupt my access to the internet (4G), especially on rainy days, when I may have to forego internet access all day. A broad antenna will (hopefully) be concentrating signal from both antinodes (useless) and constructive interference so that some signal gets through. A simple Yagi Uda might be sufficient, but the parabola is more fun and much less susceptible to ambient radio noise (I think), which is a big problem in this neighborhood. With a loud, clean signal, I should be able to access the net at 42 Mbps instead of the measly and unreliable 5 Mbps I get now. Surprisingly, the commercial antenna transmits with only a few watts. The engineers work on them all the time without even turning them off. I initially thought they would have been boiling their brains.

    Based on your answers, I'm leaning towards the polyphaser DSXL. Can anyone comment on its suitability as a lightning arrestor and filter?
     
    Last edited: Dec 13, 2012
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    A good quality powered (amplified) antenna for that band should have it's own pass band filtering included, and may be all you need?

    As for designing a 1.7-2.1 GHz filter from scratch that is outside my skill set.
     
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