[Elecraft] [Elecraft-K4] I need a Sherlock Holmes (weird spurs on 40m)

[Alan Bloom]

Yes, the modem router and a second router in the granny unit both run on 
AC power, so they were off.  There is no wired LAN to either the 
computer or the K4 -- both connect to the Internet via Wi-Fi.

Alan N1AL


On 6/7/22 17:41, Ron wrote:
> Sounds like wired LAN noise.  Did you shut down your router and any switches as well as any device connected to the LAN?
>
>         Ron
>        N6IE
> www.N6IE.com
>
> Member:
>                    ARRL
>       Redwood Empire DX Assn.
> Northern California Contest Club
> Northern California DX Foundation
>      DXCC Honor Roll - 337/345
>    Society of Broadcast Engineers
>
> -----Original Message-----
> From: Elecraft-K4 at groups.io <Elecraft-K4 at groups.io> On Behalf Of Al N1AL
> Sent: Tuesday, June 7, 2022 4:22 PM
> To: Elecraft Discussion List <elecraft at mailman.qth.net>; Elecraft-K4 at groups.io
> Subject: [Elecraft-K4] I need a Sherlock Holmes (weird spurs on 40m)
>
> As part of christening my new QTH/antenna/rig here at N1AL, today I did the test where I recorded all off-the-air spurious signals on all bands and then threw the main circuit  breaker for the house and did the measurement again, powering the K4 from a battery.  This is to identify any spurs that are coming from my house so I can do further sleuthing to figure out what is causing them.
>
> One spur (or set of spurs) has me mystified.  It is a series of harmonics, with very stable frequencies, spaced at precisely 24 kHz, that extend from roughly 6.6 MHz to 7.4 MHz.  Each spur consists of a main carrier and a secondary carrier approximately 150 Hz lower in frequency and approximately 8 dB lower in amplitude.  The spurs are all the same amplitude, around -90 dBm (S6), dropping off as you approach
> 6.6 or 7.4 MHz.  I don't see these spurs on any other band.
>
> The spur amplitudes did not change when I turned off AC power, so it can't be the rig's switching power supply or any other electronic device in the house.  It's nothing internal to the radio because if I switch to a dummy antenna the spurs go away.
>
> So it's coming in through the antenna.  The antenna is a 6-band trap vertical about 30 feet from the house, with the coax coming underground to the shack.  We're on a large lot, there is a canyon (i.e. no houses) behind the property, and there is a vacant lot on the side where the antenna is located so the nearest houses in the neighborhood are about
> 150 feet away from the antenna.
>
> The electric utility power lines switch from overhead to underground at our property line, about 150 feet away from the antenna. Internet is via cable, which is underground also.  Both power and Internet enter at the far end of the main house, which is over 100 feet from the shack, located in a granny unit.
>
> I believe the exact fundamental frequency is 7007.03 kHz / 292 = 23.9967 kHz, in case that's a clue.
>
> Anyone have any ideas of what could be causing this?
>
> Alan N1AL
>