Field Strength
 

Hi all,

I wanted to build an RF relative field strength meter, so set about
searching on the Web for any existing designs. Those I turned up
weren't particularly impressive, so I decided to start from scratch
and design my own. I've just completed that this afternoon. I've
allowed for 0.25mV input to give rise to FSD on the microammeter.
Question being, however, is that going to be sensitive enough? Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away? I
guess I should have posed this question *before* designing it, but who
among us can honestly say they haven't designed something without
knowing what the spec is? :-)
Anyway, ballpark figures gentlemen, please.

p.
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

I wanted to build an RF relative field strength meter, I've allowed for 0.25mV
input to give rise to FSD on the microammeter.
Question being, however, is that going to be sensitive enough?
Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away?

Figuring the field strenth of a .5 W xmtr at 6 feet is difficult. Depends on
the antennas on the xmtr and the FSM and their polarization. Also, depends on
the sensisitivty of your FSM (field strength doesn't depend on sensisitivty but
your reading will).
Since this is a *relative FSM* I would get something working, use that as a
reference, and go from there.


73 Gary N4AST

I wanted to build an RF relative field strength meter, I've allowed for 0.25mV
input to give rise to FSD on the microammeter.
Question being, however, is that going to be sensitive enough?
Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away?

Figuring the field strenth of a .5 W xmtr at 6 feet is difficult. Depends on
the antennas on the xmtr and the FSM and their polarization. Also, depends on
the sensisitivty of your FSM (field strength doesn't depend on sensisitivty but
your reading will).
Since this is a *relative FSM* I would get something working, use that as a
reference, and go from there.


73 Gary N4AST

Paul Burridge wrote:

Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away?

I'll SWAG & hope I'll be corrected if I tell ya wrong. Assuming a 1/2
wave antenna on the xmit & rcv, take the volts/meter you're applying on
the antenna, divide by 4 PI / (distance)^2, with distance being in
wavelengths.

Then there is the famous propogation equations which involves 32,
recieve and transmit antenna gains, and the log of the distance and
frequency. You can then go from power to voltage according to the
antenna Z. I'll look up the equation for you if you don't get a better
answer.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

Paul Burridge wrote:

Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away?

I'll SWAG & hope I'll be corrected if I tell ya wrong. Assuming a 1/2
wave antenna on the xmit & rcv, take the volts/meter you're applying on
the antenna, divide by 4 PI / (distance)^2, with distance being in
wavelengths.

Then there is the famous propogation equations which involves 32,
recieve and transmit antenna gains, and the log of the distance and
frequency. You can then go from power to voltage according to the
antenna Z. I'll look up the equation for you if you don't get a better
answer.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

On Fri, 12 Dec 2003 16:41:06 +0000,
said...
Hi all,

I wanted to build an RF relative field strength meter, so set about
searching on the Web for any existing designs. Those I turned up
weren't particularly impressive, so I decided to start from scratch
and design my own. I've just completed that this afternoon. I've
allowed for 0.25mV input to give rise to FSD on the microammeter.
Question being, however, is that going to be sensitive enough? Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away? I
guess I should have posed this question *before* designing it, but who
among us can honestly say they haven't designed something without
knowing what the spec is? :-)
Anyway, ballpark figures gentlemen, please.

p.



E = sqrt(30PG)/d Volts/meter - Field Strength

P = Tx power
G = Tx antenna gain - and don't forget the gain of the meter's
dipole.

S = PG / (4.pi.R^2) Watts/meter^2 - Power density

R = distance

S = E^2/377 (E field^2)/(Z of free space in the far field)

Where the far-field starts is antenna dependant, but it is
acceptaed the the boundary is where the inverse-square-law for
power density above becomes invalid, that is, as you approach the
antenna, the rate of change of S decreases and S is no longer
inverse-square-law dependant.

For large aperature antennas (dishes, dipoles, etc.) this seems to
work out to

R = 2L^2/lambda

lambda is wavelength and L is length of antenna

HTH,
Mike

On Fri, 12 Dec 2003 16:41:06 +0000,
said...
Hi all,

I wanted to build an RF relative field strength meter, so set about
searching on the Web for any existing designs. Those I turned up
weren't particularly impressive, so I decided to start from scratch
and design my own. I've just completed that this afternoon. I've
allowed for 0.25mV input to give rise to FSD on the microammeter.
Question being, however, is that going to be sensitive enough? Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away? I
guess I should have posed this question *before* designing it, but who
among us can honestly say they haven't designed something without
knowing what the spec is? :-)
Anyway, ballpark figures gentlemen, please.

p.



E = sqrt(30PG)/d Volts/meter - Field Strength

P = Tx power
G = Tx antenna gain - and don't forget the gain of the meter's
dipole.

S = PG / (4.pi.R^2) Watts/meter^2 - Power density

R = distance

S = E^2/377 (E field^2)/(Z of free space in the far field)

Where the far-field starts is antenna dependant, but it is
acceptaed the the boundary is where the inverse-square-law for
power density above becomes invalid, that is, as you approach the
antenna, the rate of change of S decreases and S is no longer
inverse-square-law dependant.

For large aperature antennas (dishes, dipoles, etc.) this seems to
work out to

R = 2L^2/lambda

lambda is wavelength and L is length of antenna

HTH,
Mike

Scott Stephens wrote:

Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

Your web site is very hard to read with the dark blue background and
black text. A lot of people have vision problems, and can not read this
color combination.
--
13 days!


Michael A. Terrell
Central Florida

Scott Stephens wrote:

Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

Your web site is very hard to read with the dark blue background and
black text. A lot of people have vision problems, and can not read this
color combination.
--
13 days!


Michael A. Terrell
Central Florida

What you are descrbing is a "signal sniffer", not a signal strength meter.