Receiver sensitivity parameter


Phillips
 

A receiver's LW and MW sensitivity is often specified in mV/M.  What does this actually mean?  I have tried to find out on the web but have yet to find an answer that I can understand.

I presume it means millivolts per metre.  Millivolts I understand but to what does the metre refer?  Does it refer to the wavelength of the received signal?  Does it mean that a receiver, with a MW sensitivity of 1mV/M tuned to a frequency of 1Mhz, has a raw sensitivity of 300mV?  Or does it calculate in some other way?

Can anyone please enlighten me?



josephrot
 

In trying to "simplify" an answer, engineers may get more "complicated"...

Signal strength is typically expressed in voltage per length or signal power received by a reference antenna. High-powered transmissions, such as those used in broadcasting, are expressed in dB-millivolts per metre / meter as the unit of length (dBmV/m).

In the case of low-power systems, such as mobile phones, signal strength is usually expressed in dB-microvolts per metre (dBµV/m) or in decibels above a reference level of one milliwatt (dBm).

Now, we have reached: In broadcasting terminology, 1 mV/m is 1000 µV/m or 60 dBµ (often written dBu), where again, m refers to metre / meter as the unit of length.

Hope this helps...but please don't throw anything at me...:)...

Joe Rotello / Knoxville, TN / USA


mediumwavedx
 

This can be a confusing subject, so please allow me to offer this explanation.

The common unit used in measuring received field strength is volts per meter, or usually, millivolts/meter 'mV/m' (one-thousandth of a volt per meter).

Volts or millivolts per meter expresses the voltage that would be induced in a one meter long wire placed parallel to the lines of flux of the received signal (remember the electrical flux from a mediumwave tower is vertical, the magnetic horizontal). This induced voltage results from the movement of the flux across the wire.

There is some confusion which persists about the term dBµ, which is related to mV/m.

dBµ, a contraction of dBµV/m, using the Greek letter µ ['mu', meaning micro, or one-millionth], is commonly and usually written nowadays as dBu [letter 'u'] and is the figure used in recent years by the FCC for measuring electric field intensity of AM broadcast stations at prescribed distances. dBu is electric field intensity and directly related to mV/m, always measured in decibels above (or below) one microvolt/meter. The decibel measurement is a logarithmic ratio as you may know.

dBµ ['mu' again] is also sometimes used erroneously as a shortened form of dBµV, or voltage expressed in dB above (or below) one microvolt into a specific load impedance, commonly 50 ohms. Important! Here we have voltage measured across a specific load impedance like a tuned circuit or reactance or resistor.

The modern DSP receivers like the Tecsun PL-380, 310, etc. which employ the Silicon Labs chips, measure and display dBµV as received at the tuned front end across a load, not dBu-dBµV/m. They call it the RSSI indicator, and erroneously mark it as dBµ on the display [Greek 'mu' again]). It is not the same measurement as dBu-dBµV/m, which is derived from volts or millivolts/meter.

You cannot convert dBuV as shown on the DSP radios to mV/m or dBu! The value is not interchangeable.

The FCC offers a conversion calculator to convert from dBu [letter 'u'] to mV/m and back.

http://transition.fcc.gov/mb/audio/bickel/findvalues.html

Or, you can figure it yourself by using the following formula:
dBu = 20 * Log(mV/m * 1000)
To reverse the computation, converting dBu back to mV/m:
mV/m = (10 ^ (dBu / 20)) / 1000

(Log is base 10)

Bill
RADIO-TIMETRAVELLER


Bruce Conti
 

On Wed, Sep 3, 2014 at 5:48 PM, mediumwavedx@... [ultralightdx] <ultralightdx@...> wrote:

The modern DSP receivers like the Tecsun PL-380, 310, etc. which employ the Silicon Labs chips, measure and display dBµV as received at the tuned front end across a load, not dBu-dBµV/m. They call it the RSSI indicator, and erroneously mark it as dBµ on the display [Greek 'mu' again]). It is not the same measurement as dBu-dBµV/m, which is derived from volts or millivolts/meter.

Now I'm confused.  Nice description of dBu and dBµV/m, but you lost me by introducing dBu-dBµV/m without explanation.  What is dBu-dBµV/m?

--
Bruce Conti
B.A.Conti Photography www.baconti.com
¡BAMLog! www.bamlog.com


Richard Jones
 

Whatis.com says it better then me! "The standard unit of electric field (E-field) strength is the volt per meter (V/m). An E field of 1 V/m is represented by a potential difference of 1 V existing between two points that are 1 m apart. 

The volt per meter, or some fractional unit based on it, is used as a means of specifying the intensity of the electromagnetic field (EM field) produced by a radio transmitter. Although an EM field contains a magnetic (M) component as well as an electric (E) component, the relative field strength of radio signals is easier to measure in free space by sampling only the E component. The magnitude of the E component from a distant radio transmitter is often much less than 1 V/m, and in such cases, fractional units are preferred. One millivolt per meter (mV/m) is equal to 10 -3 V/m; one microvolt per meter (? V/m) is equal to 10 -6 V/m; one nanovolt per meter (nV/m) is equal to 10 -9 V/m; one picovolt per meter (pV/m) is equal to 10 -12V/m.

The magnitude of the E component of a radio wave varies inversely with the distance from the transmitter in a free-space, line-of-sight link. If the distance is doubled, the E-field intensity is cut in half; if the distance increases by a factor of 10, the E-field intensity becomes 1/10 (0.1 times) as great. The E component of an EM field is measured in a single dimension, so the intensity-versus-distance relation is a straight inverse rule, not the inverse-square law."






Sent from Samsung tablet


mediumwavedx
 

Hi Bruce,

Sorry to confuse.

I used dBu-dBµV/m to try to show that they are the same thing.

Should have stated dBu (equivalent to dBµV/m).

Corrected paragraph:

The modern DSP receivers like the Tecsun PL-380, 310, etc. which employ the Silicon Labs chips, measure and display dBµV as received at the tuned front end across a load, not dBu (equivalent to dBµV/m). They call it the RSSI indicator, and erroneously mark it as dBµ on the display [Greek 'mu' again]). It is not the same measurement as dBu, which is derived from volts or millivolts/meter.

73s,

Bill
 


Michael <michael.setaazul@...>
 

Confusion may also arise from the use of u for µ
when the latter is not readily available on the keyboard.

Michael

----- Original Message ----- From: mediumwavedx

Hi Bruce,
Sorry to confuse.

I used dBu-dBµV/m to try to show that they are the same thing.
Should have stated dBu (equivalent to dBµV/m).
Corrected paragraph:

The modern DSP receivers like the Tecsun PL-380, 310, etc. which employ the Silicon Labs chips,
measure and display dBµV as received at the tuned front end across a load, not dBu (equivalent to
dBµV/m). They call it the RSSI indicator, and erroneously mark it as dBµ on the display [Greek 'mu'
again]). It is not the same measurement as dBu, which is derived from volts or millivolts/meter.
73s,
Bill


josephrot
 

Thank you, Bill... very comfortably confusing. Actually, pretty nicely explained.
 
Joe Rotello / Knoxville, TN / USA


Bruce Conti
 

On Thu, Sep 4, 2014 at 6:20 AM, mediumwavedx@... [ultralightdx] <ultralightdx@...> wrote:
  I used dBu-dBµV/m to try to show that they are the same thing. Should have stated dBu (equivalent to dBµV/m)...

My bad.  Sometimes my OCD gets in the way.  I initially read it as dBu minus dBµV/m and my brain immediately got stuck.

--
Bruce Conti
B.A.Conti Photography www.baconti.com
¡BAMLog! www.bamlog.com


Phillips
 


I am still trying to get my cotton wool stuffed brain around this parameter.

Does this parameter only apply to receivers with ferrite antennas?  Is this the "missing" piece of the puzzle that has eluded me?  It would explain why a barefoot short ferrite antenna receiver generally has lower specified sensitivity than a long ferrite.

If I follow this line of logic then it would suggest that a ferrite antenna receiver operating at 150KHz would need to be in a field 10 times greater than the same receiver operating at 1,500KHz to deliver the same degree of performance.  This would, in turn, suggest that a ferrite antenna receiver will be roughly three times more effective at the top end of the BCB than at the bottom when operating in fields of the same amplitude.

Am I reasonably correct in following this line or am I talking through a cocked hat?





To: ultralightdx@...
From: ultralightdx@...
Date: Wed, 3 Sep 2014 22:17:13 -0400
Subject: [ultralightdx] Re: Receiver sensitivity parameter

 

Whatis.com says it better then me! "The standard unit of electric field (E-field) strength is the volt per meter (V/m). An E field of 1 V/m is represented by a potential difference of 1 V existing between two points that are 1 m apart. 

The volt per meter, or some fractional unit based on it, is used as a means of specifying the intensity of the electromagnetic field (EM field) produced by a radio transmitter. Although an EM field contains a magnetic (M) component as well as an electric (E) component, the relative field strength of radio signals is easier to measure in free space by sampling only the E component. The magnitude of the E component from a distant radio transmitter is often much less than 1 V/m, and in such cases, fractional units are preferred. One millivolt per meter (mV/m) is equal to 10 -3 V/m; one microvolt per meter (? V/m) is equal to 10 -6 V/m; one nanovolt per meter (nV/m) is equal to 10 -9 V/m; one picovolt per meter (pV/m) is equal to 10 -12V/m.
The magnitude of the E component of a radio wave varies inversely with the distance from the transmitter in a free-space, line-of-sight link. If the distance is doubled, the E-field intensity is cut in half; if the distance increases by a factor of 10, the E-field intensity becomes 1/10 (0.1 times) as great. The E component of an EM field is measured in a single dimension, so the intensity-versus-distance relation is a straight inverse rule, not the inverse-square law."





Sent from Samsung tablet


mediumwavedx
 

Hi Phillips,

Millivots per meter (mV/m) is a way of defining a station's expected (or measured) field strength at a receiving location. It matters little whether that signal is ultimately impressed on a ferrite bar or rod, or a long wire, in that the receiver will take whatever tiny voltage induced and convert it into intelligible audio if it is strong enough. The iron core ferrite rod is basically a signal concentrator. The longer the rod and thus the more iron ferrite, the more the concentration, and the greater signal voltage, at least to a point.

A mediumwave station's expected field strength at a receiving location (daytime hours) also depends on other factors, one being the ground conductivity between the transmitter and receiver path.

I did some articles on ferrite antennas and signal measurements on my blog a couple of years ago. Maybe they will help with introducing some of this material.

Field Strength Calculations (3 parts):

An Unassuming Antenna - The Ferrite Loopstick:

Field Strength Calculations: A History:
Bill
RADIO-TIMETRAVELLER
http://radio-timetraveller.blogspot.com

---In ultralightdx@..., <phillicom@...> wrote :


I am still trying to get my cotton wool stuffed brain around this parameter.

Does this parameter only apply to receivers with ferrite antennas?  Is this the "missing" piece of the puzzle that has eluded me?  It would explain why a barefoot short ferrite antenna receiver generally has lower specified sensitivity than a long ferrite.

If I follow this line of logic then it would suggest that a ferrite antenna receiver operating at 150KHz would need to be in a field 10 times greater than the same receiver operating at 1,500KHz to deliver the same degree of performance.  This would, in turn, suggest that a ferrite antenna receiver will be roughly three times more effective at the top end of the BCB than at the bottom when operating in fields of the same amplitude.

Am I reasonably correct in following this line or am I talking through a cocked hat?





To: ultralightdx@...
From: ultralightdx@...
Date: Wed, 3 Sep 2014 22:17:13 -0400
Subject: [ultralightdx] Re: Receiver sensitivity parameter

 

Whatis.com says it better then me! "The standard unit of electric field (E-field) strength is the volt per meter (V/m). An E field of 1 V/m is represented by a potential difference of 1 V existing between two points that are 1 m apart. 

The volt per meter, or some fractional unit based on it, is used as a means of specifying the intensity of the electromagnetic field (EM field) produced by a radio transmitter. Although an EM field contains a magnetic (M) component as well as an electric (E) component, the relative field strength of radio signals is easier to measure in free space by sampling only the E component. The magnitude of the E component from a distant radio transmitter is often much less than 1 V/m, and in such cases, fractional units are preferred. One millivolt per meter (mV/m) is equal to 10 -3 V/m; one microvolt per meter (? V/m) is equal to 10 -6 V/m; one nanovolt per meter (nV/m) is equal to 10 -9 V/m; one picovolt per meter (pV/m) is equal to 10 -12V/m.
The magnitude of the E component of a radio wave varies inversely with the distance from the transmitter in a free-space, line-of-sight link. If the distance is doubled, the E-field intensity is cut in half; if the distance increases by a factor of 10, the E-field intensity becomes 1/10 (0.1 times) as great. The E component of an EM field is measured in a single dimension, so the intensity-versus-distance relation is a straight inverse rule, not the inverse-square law."





Sent from Samsung tablet


Richard Jones
 

To expand on that nice description: Field strength and receiver sensitivity and receiver meter readings are different animals.

Receiver sensitivity is just one way to rate how a receiver will "hear" a signal arriving at its input. Besides your own ears, there at least 8 ways to determine sensitivity. Most have to do with taking background noise into account.

 A receiver's signal meter (more or less) reports on how much signal it is receiving from what ever antenna is attached to it. For example: S9 = 50.2uV (in a 50 ohm system) = -73dBm. If you were to hook up a signal generator to the antenna input and have it output -73dBm, the s meter would read S9 or 50.2 micro volts if so marked.

The reason why a tabletop receiver can not give you an actual field strength reading is due to the antenna.  Hook up a good or crappy antenna to a receiver and of course the "S" reading will reflect the antenna's ability to extract the signal and get it to the radio. Two antennas, two readings, same location. The difference between the actual field strength (as measured by calibrated equipment) and what the radio reports is called the antenna factor. Most of the time you won't know the antenna factor (AF) of your antenna so the readings your radio is reporting are relative readings only. To use a radio to make actual field strength readings at a location requires an antenna with a known AF and usually a calibrated receiver as well.

Finally, I'm sure it is possible for the manufacturer of a portable radio with a fixed antenna to include AF values in a look up table so that it will display a corrected field strength reading. I would probably want to verify the accuracy before relying on the measurements as absolute values.

That's how I see it. Please correct me if I'm wrong!
Rick







Phillips
 


Thank you to all who responded to my question.  I have a clearer picture now.

It came about because I live within 5Km of a number of high power transmitters, including the famous 5AN, and a lot of the band is denied me because of their proximity.  I would like to use a "conventional" antenna so that I can use traps to minimise the flamethrower signals but the ferrite antenna still adds its signal to the mix.  I could put the receiver in an earthed iron box but this prevents access to the controls and so I need a way to minimise the ferrite signal. 

What I am thinking of trying is to find a receiver with good sensitivity at the low end of the short wave bands to give me an indication of the active performance of the receiver but which has a poor performance on the BCB.  Then I can interface in my own antenna.    I am thinking that I need to look for a receiver with a short ferrite but with great 2MHz performance. 


 Thanks again to all.



To: ultralightdx@...
From: ultralightdx@...
Date: Fri, 5 Sep 2014 11:49:46 -0700
Subject: RE: [ultralightdx] Re: Receiver sensitivity parameter

 
To expand on that nice description: Field strength and receiver sensitivity and receiver meter readings are different animals.

Receiver sensitivity is just one way to rate how a receiver will "hear" a signal arriving at its input. Besides your own ears, there at least 8 ways to determine sensitivity. Most have to do with taking background noise into account.

 A receiver's signal meter (more or less) reports on how much signal it is receiving from what ever antenna is attached to it. For example: S9 = 50.2uV (in a 50 ohm system) = -73dBm. If you were to hook up a signal generator to the antenna input and have it output -73dBm, the s meter would read S9 or 50.2 micro volts if so marked.

The reason why a tabletop receiver can not give you an actual field strength reading is due to the antenna.  Hook up a good or crappy antenna to a receiver and of course the "S" reading will reflect the antenna's ability to extract the signal and get it to the radio. Two antennas, two readings, same location. The difference between the actual field strength (as measured by calibrated equipment) and what the radio reports is called the antenna factor. Most of the time you won't know the antenna factor (AF) of your antenna so the readings your radio is reporting are relative readings only. To use a radio to make actual field strength readings at a location requires an antenna with a known AF and usually a calibrated receiver as well.

Finally, I'm sure it is possible for the manufacturer of a portable radio with a fixed antenna to include AF values in a look up table so that it will display a corrected field strength reading. I would probably want to verify the accuracy before relying on the measurements as absolute values.

That's how I see it. Please correct me if I'm wrong!
Rick








kevin asato <kc6pob@...>
 

Poor performance on the BCB. That's pretty relative. I had a high school FM radio station transmitter that was supposedly well filtered. Still, it was very possible for all the attached wiring and everything else associated with the transmitter to actually allow the retransmission the BCB signal out the FM antenna. Signal was so strong that it affected all the telephones in the area, tool. BCB transmitter was a 50KW site and my high school was about 1/2 mitle away.Not a happy situaltion. Bottom line is that rf has a habit of sneaking in on you, even when you do plan for it.
Not meant as a comment of derision but, Best of Luck in your endeavor.
73,
kevin
kc6pob


What I am
thinking of trying is to find a receiver with good
sensitivity at the low end of the short wave bands to give
me an indication of the active performance of the receiver
but which has a poor performance on the BCB.  Then I can
interface in my own antenna.    I am thinking that I need
to look for a receiver with a short ferrite but with great
2MHz performance.


Vimal
 

If you need a way to minimise the ferrite signal,you may try Tecsun PL360 with plug in ferrite antenna; or CC-EP RADIO which has a switchable internal ferrite option,just my 2 cents .

On 06-Sep-2014, at 4:47 am, "Phillips phillicom@... [ultralightdx]" <ultralightdx@...> wrote:

need a way to minimise the ferrite signal.


Richard Jones
 

You're going to need something that gives you the ability to put a band stop filter in between your antenna and your radio. Clifton Labs sells them or you can certainly build your own. That still may not protect you from harmonics coming from the transmitter(s). Good luck!