Reducing noise on an Antenna, when my other antenna is full quieting

[KD4EBL]

I am having a problem that I should know the answer too, but do not. I have three antennas currently. One is a 20M Dipole, the second is a 10M dipole (at around the same vertical height, but positioned differently) and the third is a 10M vertical. Yesterday I was on 20M (and the 20M dipole) and there was a lot noise, but when I switched antennas to the 10M dipole, the signals were coming in but full quieting, when I switched to the 10 meter vertical, there was some noise, but less than the 20M dipole. Any suggestions? All coax comes into the house at the same place and I have a 6 position switch to go between each one.
73
Scott Johnston KD4EBL

[PA1ZP]

Hi Scott

It could be that your 20 mtr dipole is very sensitive for a local source of QRM .
I have the same experience with my rotary dipole for 10- 40 mtrs.
If I turn the antenna so that it looks into the houses of my neigbours (I live in a block tht hs 10 houses under 1 roof that is bout 60 mtrs long) the antenna will pick up a terrible lot of noise.
if I turn it 90 degrees it becomes much more silent , stations however still come in with about the same signal, but I can copy them way much better.

73 Jos

[WB6BYU]

It also may be a difference in noise pickup on the outside of the coax
between the antenna and the shack if there is significant common mode
current flowing on them.  For example, if one coax runs next to a switching
power supply, battery charger, neon sign, motor, video game, or other
source of noise.  If this is the case, adding a balun at the antenna should
help to reduce the noise.

[N4JTE]

OR; your 20 meter dipole is your most efficent antenna, noise is a signal also!
Bob

[WX7G]

I believe N4JTE is correct. As an experiment listen on the 20 meter dipole and switch the preamp OFF and/or switch in the attenuator. I suspect you will hear the same effect as receiving with the 10 meter dipole.

[KC4MOP]

listening to a 20M signal on a 10M dipole would still quieter and still receive, but the reduced noise is from the mismatch to the radio.
I get the same results listening to BC stations on my 40M rotating dipole.

[KD4EBL]

Thanks all! I will dig out the manual and learn how to turn off the preamp and switch in the attenuator. I will also re-examine the path of each coax and see if they cross a RFI source. In fact I will switch two of the coax cables and see if that makes a difference.
Can I measure common mode current running on the outside?

[KB6HRT]

Noise!
 Reducing noise can be delt with by reducing gain at the receiver 99% of the time the incoming signal will always come over the white noise. The better the radios receiver the more control you have over white noise. A good balum designed for your antenna SYSTEM can also reduce the white noise.  Most operators that run mobil have a better signal to noise ratio because the antenna is low,
 Aftermarket screwdriver antennas that are rated for a full KW use a number 12 awg conductor or larger, these antennas relies on good SWR and being resonant to pull in coming signals.  Have played with this stuff for 35+ years this is some of the stuff do remember that has worked for different antenna systems I have installed an was happy with the results.  The higher the antenna the more white noise your antenna will pull in an transversely the lower your antenna the less noise you pull in, so sometimes you have to find the sweet spot where your antenna meats your need an pull in the right amount of stations to suit you.  Most of the time a more costly transceiver means it has better control over its receiver.......
KB6HRT 

[W2WDX]

Noise can also be attributed to static build-up on a particular antenna, from things like wind or snow or even rain. Just because one is building up static doesn't mean the others have to be.

A good DC to ground static suppression scheme can be had with some impulse suppressors. Not the Polyphaser unfortunately, they are just gas discharge and only work if charge build up is substantial making its own noise BTW. So the PolyPhaser stuff only is connected to ground on one leg of a dipole (depending on your balun type). Also DC shunts are not necessary if your tuner inductor shunts to ground (many don't) and it's actually grounded. However the idea of having arcing or high voltage running through my tuner isn't something I like (unless its coming from my transmitter). So I tend to use DC shunts anyway.

Products like the I.C.E. 300 series (now made by Morgan Manufacturing) have an shunt inductor to ground that continuously bleeds off static, as well as the normal gas discharge tube.

Here in the NE, I always use some type of DC shunt to ground to bleed off static on my dipoles. Whether its on coax or ladder-line fed antenna systems. It does reduce the noise quite a bit for me on receive. On my ladder line, in addition to large horn gap suppressors, I also use large inductors on each leg to ground. They have a high enough reactance to be invisible at RF but pass static to ground. They are about 8" long and about 2" in diameter 16awg enamelled wire wound on teflon cores. I carefully match them and mount them carefully to avoid creating any imbalance on the feed-line. I never liked the shunt resistor method. No matter how high the value RF ends up going to ground.

Man-made noise is another thing and can usually be reduced slightly through prudent use of the directivity of the antenna, as mentioned in an earlier reply. However, if the orientation of your dipoles are similar, man-made noise sources may not be the issue here since this would show up on all the antennas on the same or nearby bands, most likely.

John W2WDX