[RFI] RFI Signature

[Jim Brown]

On 11/26/2025 5:46 AM, Jeff (W4DD) wrote:
> We can see the BW change as we command the antenna to tune so the narrow BW is due to the antenna.  Flex BW (front end) is set to wide which I think is 14MHz.

There is a HUGE difference between SWR bandwidth and the bandwidth of an 
antenna for receive. An antenna may have matching networks and 
components that affect its response.

There is HUGE value in a very wide (at least 10 MHz) waterfall. Look at 
what NK7Z posted almost ten years ago as a method to chase RFI. He used 
an inexpensive (about $125) SDR RX to make 24 hour waterfalls that show 
multiple noise sources turning on and off. The importance of this is 
knowing more about what we're looking for and when to look for it. I 
have one of those RX, and W6GJB and I used it to chase noise at his QTH 
when we were evaluating a spot on his property to do 6M EME.

https://www.nk7z.net/i-have-rfi-now-what-locating-it/

Perhaps you can set up a simple vertical feeding one of those receivers. 
Ham Radio Outlet was selling them. Don't know if they're still available.

We need to re-think instrumentation. I've been doing this sort of 
instrumentation since 1982 in the world of pro audio with hardware that 
cost what it took to be the down payment on my first house ($12k in 
1982). $125 for an SDR that blows away the capabilities of $10K HP 
spectrum analysis gear is a no-brainer! About 10 years ago, 
then-neighbor NR0V, who gave us Pure Signal and did software for ANAN, 
showed me even better capabilities of the QRP ANAN rig that then sold 
for less than $1K. It had frequency resolution of fractional Hz and 
about 120 dB of dynamic range.

To use these inexpensive solutions, all we need to do is keep them out 
of overload and understand aliasing.

73, Jim K9YC
> 
> We can hear it on other bands, but much weaker, although it adds to the noise floor.
>