[DogParkList] Noise Part 2

[Judd Sheets]

If the noise comes and goes with connecting and disconnecting the  
antenna, the interference is very likely radiated and being picked up  
by the antenna. If the noise is being radiated by common mode  
currents on the USB cable, then placing a ferrite core on the cable  
at the source of the current may help assuming the core is suitable  
for the frequencies in question. It may take several cores or several  
turns through a single core to provide enough common mode inductance  
to prevent current flow and prevent radiation. Try listening for  
reductions rather that total removal to see if you headed in the  
right direction.

It is also possible that the radiation may be coming from the signal  
conductors inside the cable, although most cables are shielded and  
theoretically should not radiate. I would imagine there are big  
differences in shielding quality between cables.

While Macs are better shielded than most computers, they all produce  
a lot of EMI and the whole HF band can be affected. Anything that  
uses AC signals can generate these signals can make EMI, even USB- 
serial converters and the like. If chokes don't work, you may want to  
try "sniffing" for the source of the RF by using a coax probe hooked  
to the HF receiver. Strip off a couple inches of the jacket and braid  
on the other end of the coax to act as a tiny antenna and run it  
around and see where you find the most garbage. This test can be  
misleading, but it can also give you insight into what is doing the  
radiating. It's never an easy problem to diagnose.

Judd W4BUS


On Aug 26, 2007, at Aug 26, 200711:20 AM, ltrranch at vtc.net wrote:


> Dear Friends, I did a simple experiment based on Chris's inputs. I  

> disconnected the antenna at the transceiver and did the "on/off"  

> test of MacLogger. There was no noise on 20 meters. Not to confuse  

> things, but I did the same test by disconnecting and connecting the  

> USB cable, switching "on/off" MacLogger with the antenna connected  

> to the transceiver. Again, there was no noise. On my radio there  

> must be something unique about 20 meters and the use of MacLogger.  

> I guess the noise is coming down the antenna. Greg, I saw your  

> input. The module you speak of, does it go in-line with the USB  

> interface? Thanks Tom  

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