[RFI] COMM0N MODE NOISE OBSERVATIONS AND REPAIRS

[Jim]

Jim's comments below.

----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "Jim" <jvpoll at dallas.net>; <rfi at contesting.com>
Sent: Sunday, December 26, 2004 6:46 PM
Subject: Re: [RFI] COMM0N MODE NOISE OBSERVATIONS AND REPAIRS 


> > I never tried performing any 'intermodulation' tests to
> > measure the 'dynamic range' of that old coax; your 
> > post brought this little observation back to mind and 
> > just what problems like ohmic, rusted, oxidized 
> > copper conductors can create.
> 
> Repeaters are an entirely different case.  In the case of a

I don't see it quite that way; I'm sure others may not
either. This opening seems quite dismissive of an actual, 
bona-fide observation I made without providing any suitable 
explanation for said observation.

> repeater duplexing from one antenna, you have all of the
> transmitter power conveyed on the system that is also
> receiving at noise floor, and noise floor at VHF is normally

You will find, given suitable practical experience  on this
subject that many items, objects (incl rcvr and xmtr), in 
the vicinity of said repeater 'system' can provide suitable 
'mixers'/mixing sources for the creation of intermodulation 
products, including the coax itself (already demonstated),  
disimilar metal contacts in the vicinity of the transmit RF 
(steel to brass to aluminum connections and the like). 
I have see where 'wiggling the chain link fence' creates 
noise in a system (six meters on a terrestrially urban low 
site).

I have 'played' extensively in this field and seen many, 
many such effects including "digital intermod" involving 
bread-boarded digital circuitry which provided suitable
'mixing' given a strong RF transmit source (such as 
from a repeater transmitter) through the digital circuit
and back into the repeater receiver. 

> very low compared to HF when the bands are open.

Perhaps in ideally located, well-situated sites, but not
quite the situation in non-ideal, real-world cases as I 
have attempted to describe in short form above.


> 
> When my business maintained multiple repeaters in large
> communications systems, we would even have problems from

Systems, no doubt, placed far from interfering consumer
devices, away from one;s own 'development lab', away from 
those energy efficient 'switcher'/active ballast fluororescent 
light bulbs (yet another GOOD source of 'digital intermod'
I will add).

Systems designed by somebody else, operated for 
profit and never intended to be operated solely for the 
purpose of observing all manner of phenomonon
for the purpose of advancing 'the radio art'.

> loose hardware on or near towers causing noise like you
> describe. Any less than perfectly solid connection would

I'm describing a little different phenomon in the case of coaxial
cable noise as caused ostensibly by the motion of the 
braid, yet I am dismissed on this point.

(Been there, seen that on the 'loose hardware' bit.)

> "noise up" a repeater. Virtually all of the problems were
> connectors or other pressure contact points such as guy line
> to tower contact areas. We always used standard braided
> cables like RG-8 in various parts of systems, but never had
> a problem like that that I'm aware of.   While I've never

Re-read my observations; I didn't make this part up. (Is this
a challenge as to the authenticity of my observation? Then
consider the challenge met.)

> seen the effect you describe, I'm certain it can happen if
> the shield is tarnished or if the lays are not compacted to
> make good connections. I know that climbing past a repeater
> antenna with metal tools or even car keys rubbing will noise
> up a repeater while it is transmitting.
> 
> This has little to nothing  to do with Ray's problem, which

It has, according to me reading, everything to do with Ray's
problem in his matcher (may a re-read of his original post 
is in order?); to wit, "high circulating tank circuit currents on
old braided cable" - I see this as QUITE a recipe for a mixing/
intermodulating environment. I suspect he had AM broadcast
energy providing some some of the RF for this mixing process
too.

> apparently is that someplace in his system he has severe
> common mode ingress into the cables.

I think he did a remarkable bit of troubleshooting and 
concisely summing it up.

I think you may have failed to realize that some of the 
'ground loops' he was desribing were 'noise' ground
loops through some of the wall-warts - I would like
further clarification on this point myself.

> 
> I have a fairly complex system in a very quiet rural

"in a very quiet rural ..." so you have total control of
your situation.

Some of us don't, like me. I live on a city-sized lot in an 
urban, consumer noise laden area with a LOT of AM 
broadcast RF around - perhaps Ray does too. 

Did you consider this last aspect? I did ... but we need to
hear more from Ray on this.

> location and have had some fairly noisy switching supplies
> and lights in my shack without undue problems. Any RF
> ingress is virtually never "ground loops". It is some other

I would like further clarification of this point, as I believe
you are discounting mains-coupled noise through the
wall-warts.

> flaw such as poor antenna feedline or feedpoint construction
> or engineering or bad connections at RF connectors that do
> the deed. I don't use any special cabling. I do make good
> shield connections.
> 
> 73 Tom
> 

I find it very difficult to argue with the success Ray had
in his venture, and I am not going to be as dismissive 
of his results although I do have a few lingering questions
on a few points. 

I, too, have seen many, many oddities regarding noise, and 
cured my fair share both in a professional role as well as 
'an amateur'.


Regards,

Jim WB5WPA