Topband: Testing for common mode noise

[n4is]

Don Kirk

The only way to eliminate Common mode noise is first of all understand the
basic concept what it is. Common mode noise is not a noise, it is actually a
signal generated by a current called common mode current because this
current flow in the outside of the feed lines. Not a big problem if it does
not get inside the feed line. 

Let's start from the very beginning. A signal reaches the RX antenna and
generates some XZ uV voltage at feed point. Then we connect a feed line to
carry this XZ uV signal to our receiver. The receiver amplifier its and we
can hear the signal at the speaker. This is quite simple in theory but
difficult to implement. 

Here are some basic considerations with a flag and 100ft of feed line.
Assuming all impedances match and a low attenuation cable, the XZ uV will be
almost the same at the output of the feed line. The feed line itself is
connecting to the radio and both connected to the ground in some point.
Well.... this 100 ft feed line connected to the ground is an antenna by
itself. We can transmit with a 100ft of wire and the flag helps with some
load on the top. So this "feed line antenna" sure has a vertical part and
can receive signals very well as any long wire antenna does. Also this
vertical TX antenna is very close to the flag, so close that it is connect
with the Flag. At this point we can expect signals from this feed line
transmit antenna to radiate or re-radiate signals into the Flag, both
antennas are very close and have the same polarization. 

The only way to eliminate this interaction is eliminating the feed line.
However we need the feed line to carry the XZ uV signal to feed our receiver
antenna. This problem was fixed long time ago using twisted pair feed line.
Using a twisted pair feed line makes the current cancel itself each twist in
the wires. Using a coaxial cable you need a very good choke to electrically
disconnect the common node current at both ends of it.. 

If you read this far you can take this next one concept. How do you know the
signal you are receiving is coming ONLY from you RX antenna? This is the
hard part, you never know. What we can do is to test if our receiver is
properly shielded to receive signals with no antenna connected. 

The test we can check common mode current is quite simple. Disconnect all
antennas and tune the AM band, a good shielded receiver will listening
nothing, nill. Zipp. Not even detect any carrier.

If an AM BC signal can get in without antenna it is getting in via common
mode noise, it could be you AC line, your rotor cable, other radios
connected to your ground, the ground itself if no properly implemented.
After your receiver passed this test you can move to the next step
connecting a resistor at the end of you feed line and to the same test
again. You should hear nothing again.

Not square yet. How do you know the signal you want to hear is coming only
from the direction the DX station?  Again you don't know. Any resonant wire
or structure nearby can reradiate signal back into the RX antenna and
destroy the directivity we need to improve signal noise.  That's how any
Yagi bean works, each resonant element reradiate into each other to provide
directivity and gain.
 
On VHF long yagi have proven great performance, 3 or 5 or even 9 wave long
boons are quite common. A Director 5 wave long reradiate signal back and
forward interacting with all other elements, For 160m. 5 wave long is 800 m
long or 1200 ft away. 

One simple way to destroy any RX antenna, in special a Flag, EWE, or loaded
loop is to install a low resonant dipole nearby. The dipole will reradiate
noise into the Flag, EWE or any RX antenna destroying the directivity. Sure
there are some directions where the dipole will work like a director or even
as reflector and improve the signal noise in the receiver, however this is
not what we want.

Just to finish, I remember people complaining about problems with the RX
port, some radios are well known sensitive. Figure this out, your Inverted
"L" TX antenna with 1 KW and a resonant low dipole connected to the RX port.
You can expect over 100w return RF to burn the RX input. A low dipole can be
a quite antenna and it receives quite well. BUT a low dipole it is not a
receiver antenna and cannot be connected in the same way the RX antennas are
connected to the receiver.  Any low band TX antenna needs to be neutralized
during RX, the TX antenna reradiate noise and signals back to the RX
antenna. It is just an electrical and magnetically coupling between two
conductors.

I hope my comments can help your tests.

Regards
N4IS
Jose Carlos