Field strength meters : How ?

[Robert Lacoste]

Hi,

I'm wondering if someone could explain how the low cost RF field level
meters actually works. I mean devices that display field level in V/m or
mW/cm2, over a very wide band (MHz to GHz).

For me it is easy to build a wideband "field level" receiver, giving the
level of a received signal from a dipole antenna expressed in µV or dBm, but
the relationship between received level and field strenght is frequency
dependant, right ? Something like "field level=received level-antenna
gain-20log(F)+constant "? So how to they get the field level without knowing
the frequency ? Or do they measure it ? Is there something I'm missing ?

Any link to wideband antenna design will also be welcome...

Many thanks for your help...

Bob.

 

[Rene Tschaggelar]

Hi,

I'm wondering if someone could explain how the low cost RF field level
meters actually works. I mean devices that display field level in V/m or
mW/cm2, over a very wide band (MHz to GHz).

For me it is easy to build a wideband "field level" receiver, giving the
level of a received signal from a dipole antenna expressed in µV or dBm, but
the relationship between received level and field strenght is frequency
dependant, right ? Something like "field level=received level-antenna
gain-20log(F)+constant "? So how to they get the field level without knowing
the frequency ? Or do they measure it ? Is there something I'm missing ?

No idea, sorry

Any link to wideband antenna design will also be welcome...

Wideband antennas, meaning those exceeding 10% of the centerfrequency
are called log-antennas. Their design inherently contains several
decades of frequencies. They are to a dipole what a trumpet is to a pipe.
A trumpet is an exponential horn, basically. So one should wonder why the
antennas are called log, intead of exponential.

Rene

 

[Joe Legris]

Hi,

I'm wondering if someone could explain how the low cost RF field level
meters actually works. I mean devices that display field level in V/m or
mW/cm2, over a very wide band (MHz to GHz).

For me it is easy to build a wideband "field level" receiver, giving the
level of a received signal from a dipole antenna expressed in µV or dBm, but
the relationship between received level and field strenght is frequency
dependant, right ? Something like "field level=received level-antenna
gain-20log(F)+constant "? So how to they get the field level without knowing
the frequency ? Or do they measure it ? Is there something I'm missing ?

Any link to wideband antenna design will also be welcome...

Many thanks for your help...

Bob.

It you can get your hands on a fixed-geometry broadband antenna, this is
potentially do-able, but an inexpensive "wideband" device with a
retractable whip antenna cannot possibly provide accurate results - it
will only give rough readings. This is because the effective area of the
antenna, which determines its ability to capture radiated energy, is a
function of wavelength and of the size of the antenna.

If the measurements were resticted to wavelengths where the antenna is
"electrically short" (i.e. much shorter than the shortest wavelength to
be measured), then its performance would be more or less independent of
the antenna length.

 

[Baphomet]

Hi,

I'm wondering if someone could explain how the low cost RF field level
meters actually works. I mean devices that display field level in V/m or
mW/cm2, over a very wide band (MHz to GHz).

For me it is easy to build a wideband "field level" receiver, giving the
level of a received signal from a dipole antenna expressed in µV or dBm, but
the relationship between received level and field strenght is frequency
dependant, right ? Something like "field level=received level-antenna
gain-20log(F)+constant "? So how to they get the field level without knowing
the frequency ? Or do they measure it ? Is there something I'm missing ?

Any link to wideband antenna design will also be welcome...

Many thanks for your help...

Bob.

The field strenth meter I have Jerrold A.I.M. 718 (they call it an antenna
installer meter) has a tuned R.F. front end so it does, in effect, measure
frequency.

 

[Robert Lacoste]

Thanks guys for your answers. Searching through the web I've found a
reference stating that a very short dipole (far smaller than the
wavelength/4) can be modelled as an ideal voltage source of E/2 (where E is
the field strength in V/m) in series with a capacitor, and that a constant
antenna factor could be acheived by using such short dipole with a
capacitive load in parallel. May be is-it the way they work. Anyone have
comment on this solution ? Calculation tables ?

Friendly yours,
Robert Lacoste
http://www.alciom.com

 

[Roger Lascelles]

Thanks guys for your answers. Searching through the web I've found a
reference stating that a very short dipole (far smaller than the
wavelength/4) can be modelled as an ideal voltage source of E/2 (where E is
the field strength in V/m) in series with a capacitor, and that a constant
antenna factor could be acheived by using such short dipole with a
capacitive load in parallel. May be is-it the way they work. Anyone have
comment on this solution ? Calculation tables ?

Friendly yours,
Robert Lacoste
http://www.alciom.com

Interesting. I think you have found the answer. I don't recall seing a
capacitive load in commercial meters, but someone may have a schematic handy
and can tell us.

15 years ago I asked a cranky uni lecturer how a field strength meter was
calibrated. He said "volts output equals volts per meter multiplied by
antenna length in meters". This guy would make it up if he didn't know, but
its worth mentioning.

Roger

 

[Paul Burridge]

The field strenth meter I have Jerrold A.I.M. 718 (they call it an antenna
installer meter) has a tuned R.F. front end so it does, in effect, measure
frequency.

Though not to any great accuracy, one assumes.