[Lowfer] Possible noise problems?

Tue May 1 18:36:43 EDT 2018

While this may not be directly relevant to your question, it may be of 
interest to others.

I have DSL (via underground) and do LF/MF listening and the only 
problems that I've seen from the DSL modem are from its switching power 
supply:  I can't say that I've seen anything from the switching 
regulators within the modem itself.

I cracked the case of the DSL modem's wall wart (I'd done it anyway - 
the original electrolytics lasted only about 24 months, anyway) and 
inserted some small >=100uH molded coils in series with the AC input 
leads (they now function as additional fuses!) and also cut the DC cord 
very close to the wall wart and inserted a bifilar choke (a pretty hefty 
one of several hundred uH in each winding to handle the amp or two that 
it pulls at DC - but winding lots of turns of the DC cord on a hefty 
ferrite core would have also worked) and some large-ish, low-ESR 
capacitors across the DC to help enforce common-mode and suppress 
residual differential RF currents.

The result is that I don't really hear it on LF/MF unless I tune 
directly to the SMPS operating frequency - but even then, it's weak:  
About the only place that I hear it is on HF - very weakly now, if the 
band is very quiet - due to direct radiation from it as this wall wart - 
and the DSL modem - is mere inches away from the window line feed of my 
HF antenna and the balun/antenna tuner.

* * *

I Can't "hear" the DSL modem's carriers on any LF/MF/HF frequency and I 
don't bother the DSL modem (other than a few lost packets) when I run 
full legal power on 80/40 meters - but I can knock it down with >50 
watts on 160 meters with the antenna running in "top-hat Tee" 
configuration (again, feedline is inches away) but it's not bothered at 
all when running 50-80 watts of RF on 2200 or 630 meters from a loading 
coil just inside my window (e.g. several kV of RF on the wire that runs 
within a foot or so of the modem): I'm thinking of moving the modem 
somewhere else in the house other than inches from my big tuner to help 
mitigate its effects when I run 160.

* * *

The biggest improvement to LF/MF noise mitigation is the fact that the 
ground of the coax on my LF-400B E-field whip is bonded to the metal 
roof of my house and there are two large ferrite chokes on the coax 
feedline:  One of several hundred uH near the antenna, with the 
antenna's coax shield bonded to the roof on the "antenna" side of that 
choke and another choke of several mH just where it comes inside the 
house (RG-58 on a TV flyback core) with the coax shield of the "outside" 
portion of the (e.g. "before" the choke) bonded to a (very!) nearby, 
"quiet" Earth ground.  This seems to be very effective in preventing 
"house noise" from being being conducted to the E-field whip - and the 
metal roof with its overlapping seams (not intentionally bonded, but 
possibly touching in a lot of places - and many, many square inches of 
overlap to capacitively couple to each other) screening the whip from 
the inside of the house doesn't hurt, either!

* * *

Finally, when I first got DSL many years ago I was running 256k on a 
28k-foot loop - right at the edge of where they would offer service.  
Originally, the service was marginal, but I was able to get a few dB 
better S/N when I got the phone company to install a "whole house 
filter" in the network interface (NI) box outside to avoid issues with 
stubs and taps of the internal house wiring.  I also home-ran a short 
length of CAT-5 from the NI to the DSL modem.  I was then able to get a 
few more dB when I wound a length of that same CAT-5 on a large piece of 
ferrite (a discarded TV yoke) near the modem which seemed to slightly 
improve its longitudinal balance and prevent it from being knocked down 
easily when I run HF.  Almost 20 years later, I now have 40 meg DSL from 
a nearby DSLAM, but it still uses that same home-run CAT-5 from the NI 
on the TV yoke choke.

73,

Clint

KA7OEI

P.S.  I doubt that the under-hung wires are shielded, but if there 
happens to be AC power distribution on those same spans, I'd worry about 
that.  FWIW I read somewhere that the FT-817(ND) is deaf as a post below 
about 250 kHz - and performs worst down there with the preamp on (e.g. 
IPO off").

On 5/1/2018 2:02 PM, Zack Widup wrote:
> I only have a couple places in my yard where I can put a receiving longwave
> loop antenna. The antenna I am building is 5 feet on a side with a
> supporting cross made of pvc pipe. The top of the loop including mounting
> pipe is about 9 feet off the ground.
>
> One place I can put the loop is fairly close to a power line. I'd like to
> avoid that spot if I can. The other location places the top of the loop a
> few feet from an AT&T DSL phone/internet line (not directly under it).
>
> I am wondering if anyone has any experience with placing an LF/VLF
> receiving antenna close to a DSL phone line. I don't know if the wires in
> that line are surrounded by a shield, but I suspect they're not.
>
> I suppose one possibility to test the locations(s) is to take my FT-817ND
> with a small antenna and put it near the phone line. That radio will
> receive down to 100 kHz. But if anyone has had problems with some similar
> arrangement in the past, it would help.
>
> 73, Zack W9SZ
>
>
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