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Author Topic: Noise reduction using 2 antennas?!  (Read 2128 times)
VU2NAN
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« on: October 19, 2012, 07:05:55 AM »

At my location I receive static-type QRM across the 2m band with either of my antennas ( a 1/4λ Ground Plane 15m high & a Slim Jim 10m high). As it's not present at night and on weekends I presume the source is commercial.Very strong signals override the QRM but with weaker signals its a nuisance.When I connect both the antennas to the rig through a 'T' connector, the 'S' Meter reading drops but the QRM disappears and the audio level goes up considerably.SWR is 3:1 with both the antennas connected.73Nandu.
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N4CR
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Posts: 1757




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« Reply #1 on: October 19, 2012, 04:30:25 PM »

It's possible that you've happened upon a configuration that the interference combines in a way that the interferences from the two antennas are out of phase and self cancels. If so that won't hurt anything for receive but can cause an issue with SWR on transmit. You could put a relay in one of the antenna lines and open it for transmit.

Obviously, the best thing you could do would be to track down the source of the noise and try to eliminate it at the source.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
K5LXP
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« Reply #2 on: October 19, 2012, 04:56:10 PM »

I wonder if by teeing the antennas you're not causing a reduction in the amplitude of interfering signal(s).

Something you could try is to put 10 or 20dB of attenuation inline when you're experiencing the interference.  If it's receiver overload you're experiencing, the attenuators will make it disappear.

Mark K5LXP
Albuquerque, NM
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WB6BYU
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« Reply #3 on: October 20, 2012, 08:23:28 PM »

Since both the ground plane and the Slim-JIM (basically a J-pole) are prone to common mode
currents, my guess is that, in connecting the coax shields together you changed the relative
impedance of the coax shield and thereby reduced the common mode current.

With common mode current on a vertical feedline or mast, the radiation is often concentrated
at high angles rather than at low angles, so reducing common mode current can make a
significant improvement in signal strength.
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KC4MOP
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« Reply #4 on: October 21, 2012, 04:38:08 AM »

May I suggest that you try to find where this interference is coming from? Static on 144mhz should not be a very bad problem. Whatever it is, must be very close to your QTH. Plasma TV?? Switching power supply somewhere? Cable TV modem?
It reads that you are complicating your problem trying to cancel out the noise.

fred
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