Common mode noise

[NJQA]

N6GN says:

“…….In studying noise sources and in particular the coupling of these sources to the KiwiSDR I’ve come to realize that one of the dominant coupling mechanisms of signals that degrade receiver noise floor is via common-mode signals. These can exist almost everywhere in the system; they exist on the antenna
itself, within ground systems, on the feedline and other connections to the antenna and through the KiwiSDR itself. I’m presently of the opinion that for most amateur stations and even most KiwiSDRs that the dominant source of unnecessary SNR degradation, QRN and QRM, is due to common mode currents. Near-field coupling to a variety of types of local sources, interference which is attenuated as a function of distance faster than the inverse-square field of a radiated plane wave emanating from a distant source dominates a majority of amateur stations.

Although resignation to the existence of “all those noisy digital devices” and the mindset that interference has to be accepted seems to be the prevalent wisdom within the hobby, I’ve found that this is not the case. I have come to believe that the vast majority of amateur receive systems are not limited by either propagated noise, which would be the desired condition, or by radiated noise from local interferers, as is commonly espoused, but by coupling to near-field sources and in particular common mode noise…..”

source:
https://forum.kiwisdr.com/uploads/Uploader/5f/492f44efb5a272d715c5219da67b44.pdf

This pdf has further thoughts on this. (The active antenna he mentions was in the Oct 2018 issue of QST, not Sep 2017 as he states in the pdf.)

[Ray Lalleu]

The common noise on the antenna line is either on the outside of the coax screening, or via the unbalanced currents on a two wire line.

The common mode carries the noise from your house to your antenna. If the line is exactly at mid-distance between the arms of a dipole, for at least a quarter wave or more, then the induced currents in the arms of the dipole are nulling one another. The arms of the dipole need to be of exact equal length and placed at equal height above the ground to ensure the best balance. If there is any unbalance, then a part of the noise is carried back by the line (yes even inside the coax) in balanced form, back to your receiver. Of course, the OCF dipole is a VERY bad idea, and the EFW is the worst !

The noise in your house is your own noise, from your own sources, including the receiver itself, any connected device, any device connected to any wire around, and of course the noise from all the neighborhood can come trough the power, phone and CATV lines up to your own house.

Of course, don't forget that all that mess of wire is an antenna by itself, above a ground connection. The same for all your neighbors. The common mode on an antenna line is also an antenna above a ground return. Something must be done on the receiver/home end of the line to get rid of the common mode.

My own choice is for balanced lines, with a balanced antenna tuner. On the antenna side, the circuit is a tank, variable capacitor in parallel with a taped coil, and as this is not screened, the coil is in two parts placed head to foot (with the switch between the two coil parts). I even tried a balanced probe to pick up the signal towards the receiver, but that was never better than a single probe to either side of the coil. As the characteristic impedance of the line does not matter (in a tuned doublet antenna system), a part can be twisted or closely paralled and coiled in a ferrite ring, as a common mode choke.

Of course, your choice can be a coax line, preferably a buried line, with  ground connections just below the center of the dipole and at the house wall. Not practical here, so I choosed the other way.

As a secondary benefit, a balanced antenna design with horizontal arms is quite inefficient on medium wave signals (or any noise in the lower frequencies), helping the receiver work. No MW stations nearby here, but that may be useful in many US cities with the suburbs mushrooming around medium wave (AM) transmitter sites.

About the doublet antenna : that antenna is also called Levy antenna. In the US, that antenna largely mis-understood, and often misnamed as double Zepp antenna. But the length of the arms can be any length and the tuning is at hand on the ATU near the RX. See "Practical Wire Antenna 2" edited by Ian Poole, published by RSGB.

[NJQA]

Your comment about OCF dipoles and end fed wires is interesting.  I had never considered them as being worse for common mode noise pickup before, but your logic makes sense.

A decade ago I had a 80 meter dipole with a balun, fed with coax.  The coax went straight down and then entered an underground pvc pipe that led to the house.  At the house it connected to a grounded metal plate for entry into a basement hamshack.  I remember that antenna as being very quiet for RF noise, even though I was surrounded by neighbor homes in close proximity in all directions.  I see now that it was well situated to minimize common mode noise pickup, though that wasn’t done knowingly.
 

[Ed H]

I realise the conversation is a month+ old, but one thing that has really worked for my (receive) antennas is the use of a transformer balun. In this way, there is a very good barrier to common mode signals. The method was inspired by reading W8JI's website, particularly in relation to connection to beverages. However, the approach has worked nicely with a dipole, and an 80ft random wire, both connected with coax.

One can demonstrate the common mode noise that would result without these simple transformers, just by touching the receiver's antenna input to the braid of the coax. In normal use, no or very little conducted noise is experience throughout most of the HF range. For LF work (500 kHz and below), I have another transformer that can be placed in-line, which provides an audible benefit in increased CM isolation e.g. for 136 kHz (2200m) beacon reception. It really helps tame the household noise from switch mode PSUs.

Ed