[time-nuts] WWVB: measuring local 60 KHz noise

Bob kb8tq kb8tq at n1k.org
Sun May 6 14:28:31 EDT 2018


Hi

First off, I don’t think there *is* an ideal antenna that “just works”. Maybe a proper set of EMAG 
probes that come with calibration sheets come close. For a home built this or that …. there are
a lot of variables. 

First up is very much part of receiving WWVB in the first place. Coax to an antenna can have currents
on the outer shield. If they meet up with everything else at the antenna, you are not just measuring 
the antenna output. Equally, if reception is the goal, you may have a ton of noise that you didn’t really
want to have. Of course, the coax might act as a really good antenna …. who knows. 

Something like a 6” diameter single turn  loop with a good choke at the antenna end of the coax would
be my first choice. Not super sensitive. It’s not the ideal reception antenna. For chasing down noise, smaller
is often better. As mentioned earlier we are after stuff that may be in the millivolts per meter range. 

Classic data:

ftp://ftp.ngdc.noaa.gov/ionosonde/documentation/CCIR%20-%20Characteristics%20and%20Applications%20of%20Atmospheric%20Radio%20Noise%20Data.PDF <ftp://ftp.ngdc.noaa.gov/ionosonde/documentation/CCIR%20-%20Characteristics%20and%20Applications%20of%20Atmospheric%20Radio%20Noise%20Data.PDF>

puts the atmospheric noise at 120 db above KTB in the vicinity of 60 KHz. That would put it in the roughly
-54 dbm / Hz range. If your spectrum analyzer has a 1KHz bandwidth, that’s 30 db relative to 1 Hz. Your
SA should read about -24 dbm ( with an efficient antenna).  Coming up with a 1/4 wave vertical at 60 KHz
may make getting those numbers a bit difficult :). Bottom line is still — there’s a lot of noise at 60 KHz. Also
note that the report came out *long* before the modern era of 60 KHz switchers …..

Bob 

> On May 6, 2018, at 1:08 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Hal wrote:
> 
>> I assume the problem is noise.  Is there any simple way to measure the noise
>> around 60 KHz?  How about not so simple?
>> 
>> Extra credit for a way that others nuts can reproduce so we can compare the
>> noise at my location with other locations.
> 
> For any location near a city, the noise level (QRM and QRN -- mostly the former unless there is storm activity within a few hundred km) is shockingly high.  High enough to be clearly seen and measured with a good spectrum analyzer.  So the *simplest* way (but not necessarily the cheapest, depending on what is in your lab already) is to use a good spec an with noise integration over the band of interest (e.g., HP 3585A or B).  You get noise density readings in volts per root Hz.  Divide by the antenna length and you have volts per root Hz per meter.
> 
> Lacking a suitable spec an, any receiver with a reasonably narrow rx B/W and a calibrated, input-referred detector can be used.  Wave analyzers (frequency-selectable voltmeters, e.g., HP 3586) are good candidates, as are some commercial receivers with calibrated "S" meters (e.g., Ten-Tec RX340).  It would also be pretty easy to design a simple "sniffer"-type receiver (input op-amp, active filter, logarithmic detector feeding a standard 1mA meter movement) that could be calibrated by design from first principles and that everyone interested could build for, perhaps, $25-30.
> 
> In the suburbs of a fairly large US city with aerial electric service, I generally see noise densities measured in tens to hundreds of uV per root Hz per meter below 100kHz.  In other, similar locations I have seen as much as hundreds of mV or more per root Hz per meter.  It depends on local factors (whether the electric service is buried or aerial, how well the power utility maintains its equipment, how far away the nearest industrial neighborhood is, how far between dwellings, how much noisy technology the neighbors use, etc, etc.).
> 
> In order to compare with others, everyone needs to use the same antenna.  There are lots of possibilities, but for the sake of universality I recommend a 1m vertical whip.  Everyone can make one of those.
> 
> Note that this sort of antenna is NOT the best type to minimize received noise and maximize received S/N ratio.  For that, you generally want a balanced, shielded loop.
> 
> Best regards,
> 
> Charles
> 
> 
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