[600MRG] Determining noise floor in my location

Sun Oct 21 15:23:26 EDT 2018

Ed's point is a good one. Knowing what your noise floor is vs a termination is a very good indicator of the amount of external noise one is hearing.

In the case of an active E-probe antenna as what the OP said he was using is a bit of a twist.  Placing 50ohms to ground on the antenna pre-amp's input is going to yield a different noise indication than what is real.  I wonder if this isn't a good application for a 10Meg ohm resistor as a termination.  Even then, the whip might be a higher impedance than that. Maybe no whip at all is best, although I expect the shield of the 50ohm line will act as a antenna and couple some noise into the receiver system.

Any other's have thoughts on the topic?

-Brian, WA1ZMS

> On Oct 21, 2018, at 2:12 PM, Edward R Cole <kl7uw at acsalaska.net> wrote:
> 
> Steve,
> 
> Excellent advice.
> 
> One thing you can do is compare noise level with no station signal.  Connect 50-ohm load to receiver and note noise level on s-meter or better by observing a spectrum display using a program like Spectavue.  Spectravue will use digital output of most soundcards which you can connect speaker or headphone output from the receivver.  Spectravue has a display mode called "Continuum" which shows the spectrum amplitude directly in dBm vs time or vs frequency up to 190 KHz bandwidth.
> 
> Then connect receiver to antenna and compare noise display on Spectravue.  I get -130 dBm noise floor with my SDR-IQ connected to load and -115 to -105 dBm when connected to my inverted-L (43H by 122L).  Obviously I am seeing a lot of local noise.  
> 
> I can tune in BC stations with this system to compare signal level.  Local am station KSRM on 920 KHz about 15 miles from me runs at about -90 dBm.  Not sure what they output in power but they use a 1/4-wave vertical.  I can hear stations from Anchorage 70 miles north as well.  They typically run 50-100kw so are almost as strong.  I have a RGPS station less than a km away on 310 KHz and it is -30 dBm (super strong).  An ADIS station on 395 KHz 12 miles away at our local airport runs about -60 dBm.
> 
> Short of using a calibrated signal generator that is about all one can do is view relative readings.  To do a proper MDS calculation one would have to insert a known level of RF into the antenna feedline to see actual SNR.  For commercial repeater servicing this is commonly done.  Bird power meters have an isolation slug which can be used to insert a signal (typically 50 dB coupling).
> 
> 73, Ed - KL7UW
> 
> At 08:57 AM 10/21/2018, Steve WD8DAS via 600MRG wrote:
> 
>> There are a lot of uncontrolled variables in your calculations.  For example, the actual signal level of the broadcast station could easily be different from the predicted field strength at your location.  Maybe the station is not running at licensed power.  Maybe the ground conductivity today between you and the station is not what was shown on the FCC chart from many decades ago.  Maybe their antenna system has lost performance, or is operating at variance from the licensed parameters.  Maybe increased urbanization between you and the station has increased the "path loss".  What about the bandwidth?  What about...  And so on...  
>> 
>> I think you are on the right track with the idea of relative comparison of known signals to noise, but I think it would only be meaningful when comparing one reception situation to another.  For example, you could use ratios of broadcast signal to noise to show that reception at your house is more troubled by noise than at a nearby ham's house.
>> 
>> I've found it very difficult to put numbers on noise, but what really matters is practical experience.  When you tune a receiver to 630 meters (one that is known to actually work below the broadcast band) do you hear any stations?  Try an HF antenna - one of my best receive antennas is a 75m inverted vee.  There are quite a few stations active, mostly using the digital modes like WSPR and JT9.  My WSPR station transmits every few minutes, for example.  You can visit http://wsprnet.org/drupal/wsprnet/map anytime to see who is running WSPR.  
>> 
>> It is certainly possible you have a high noise level.  What have you done to minimize noise coming from your own home?   This will improve your experience on HF/VHF/UHF as well as MF.   
>> 
>> Here's a link to a presentation I made on the subject of tracking noise.  It is oriented toward broadcasters but applies to any users of radio.  I used a comparison of the relative noise to show how many locations have more indoor noise than outdoors at the same address.  it also shows tips for finding noise sources.
>> 
>> http://www.wd8das.net/Tracking-Radio-Noise.pdf
>> 
>> 
>> Steve WD8DAS
>> 
>> sbjohnston at aol.com
>> http://www.wd8das.net/
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> 73, Ed - KL7UW
>   http://www.kl7uw.com
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