On Sat, 13 Dec 2003 23:59:21 GMT, Active8
,invalid wrote:

clues. so you're still on that project.

Well, it's *related* to "that" project, yes.

tweak the sensitivity and get a peak at S9 on the CB radio type signal
meter I'm using for this purpose.

! Adjust the meter to get the answer you want. Ok.

Yes! I'm only interested in *relative* field strength. That's why I
asked for a "ballpark figure" to be plucked from the air. I think you
may be thinking of some fancy type of instrument like some guy
mentioned he had that cost 1500 bux. These ham-type jobs I'm
interested in cost just pennies to make as they don't need any
absolute standard of accuracy; only a relative indication. You tweak
the meter's sensitivity control to show antenna A of the TX giving
rise to say S5 on the meter. You then change to antenna B and see if
the reading is any higher or lower. It's really as simple as that.

I'll use a distance of 6m.

Noted, thanks.



--

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

On Sun, 14 Dec 2003 12:53:20 +0000,
said...
On Sat, 13 Dec 2003 23:59:21 GMT, Active8
,invalid wrote:

clues. so you're still on that project.

Well, it's *related* to "that" project, yes.

tweak the sensitivity and get a peak at S9 on the CB radio type signal
meter I'm using for this purpose.

! Adjust the meter to get the answer you want. Ok.

Yes! I'm only interested in *relative* field strength. That's why I
asked for a "ballpark figure" to be plucked from the air. I think you
may be thinking of some fancy type of instrument like some guy
mentioned he had that cost 1500 bux.

'twas me talking to someone else.

These ham-type jobs I'm
interested in cost just pennies to make as they don't need any
absolute standard of accuracy; only a relative indication. You tweak
the meter's sensitivity control to show antenna A of the TX giving
rise to say S5 on the meter. You then change to antenna B and see if
the reading is any higher or lower. It's really as simple as that.

Yup. gave you the eqs to figure out your ballpark figure. I figured
a ballpark figure might work if you gould figure out how much
voltage appeared at your RFSM antenna. had I known you had a
reference transmitter i might have suggested checking your RFSM
reading at one location with the ref and another closer location
with the test Tx, identical readings indicating sucess or close to
it.

I'll use a distance of 6m.

Noted, thanks.

On Sun, 14 Dec 2003 12:53:20 +0000,
said...
On Sat, 13 Dec 2003 23:59:21 GMT, Active8
,invalid wrote:

clues. so you're still on that project.

Well, it's *related* to "that" project, yes.

tweak the sensitivity and get a peak at S9 on the CB radio type signal
meter I'm using for this purpose.

! Adjust the meter to get the answer you want. Ok.

Yes! I'm only interested in *relative* field strength. That's why I
asked for a "ballpark figure" to be plucked from the air. I think you
may be thinking of some fancy type of instrument like some guy
mentioned he had that cost 1500 bux.

'twas me talking to someone else.

These ham-type jobs I'm
interested in cost just pennies to make as they don't need any
absolute standard of accuracy; only a relative indication. You tweak
the meter's sensitivity control to show antenna A of the TX giving
rise to say S5 on the meter. You then change to antenna B and see if
the reading is any higher or lower. It's really as simple as that.

Yup. gave you the eqs to figure out your ballpark figure. I figured
a ballpark figure might work if you gould figure out how much
voltage appeared at your RFSM antenna. had I known you had a
reference transmitter i might have suggested checking your RFSM
reading at one location with the ref and another closer location
with the test Tx, identical readings indicating sucess or close to
it.

I'll use a distance of 6m.

Noted, thanks.

On Sun, 14 Dec 2003 13:59:38 GMT, Active8
,invalid wrote:

Yup. gave you the eqs to figure out your ballpark figure. I figured
a ballpark figure might work if you gould figure out how much
voltage appeared at your RFSM antenna. had I known you had a
reference transmitter i might have suggested checking your RFSM
reading at one location with the ref and another closer location
with the test Tx, identical readings indicating sucess or close to
it.

I'll use a distance of 6m.

Noted, thanks.

Just curious, but would carrying out comparative measurements at say
only 6 to 10 feet give rise to invalid readings??
--

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

On Sun, 14 Dec 2003 13:59:38 GMT, Active8
,invalid wrote:

Yup. gave you the eqs to figure out your ballpark figure. I figured
a ballpark figure might work if you gould figure out how much
voltage appeared at your RFSM antenna. had I known you had a
reference transmitter i might have suggested checking your RFSM
reading at one location with the ref and another closer location
with the test Tx, identical readings indicating sucess or close to
it.

I'll use a distance of 6m.

Noted, thanks.

Just curious, but would carrying out comparative measurements at say
only 6 to 10 feet give rise to invalid readings??
--

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

On Sun, 14 Dec 2003 15:18:34 +0000,
said...
On Sun, 14 Dec 2003 13:59:38 GMT, Active8
,invalid wrote:

Yup. gave you the eqs to figure out your ballpark figure. I figured
a ballpark figure might work if you gould figure out how much
voltage appeared at your RFSM antenna. had I known you had a
reference transmitter i might have suggested checking your RFSM
reading at one location with the ref and another closer location
with the test Tx, identical readings indicating sucess or close to
it.

I'll use a distance of 6m.

Noted, thanks.

Just curious, but would carrying out comparative measurements at say
only 6 to 10 feet give rise to invalid readings??


Lots of things can foul you up.

I think I estimated the near field/far-field at 3.7m so I wouldn't
do it. IIRC the change in field strengh with respect to distance in
one of the zones of the near field varies inversely with the cube
of the distance, as opposed to the square as it does in the far-
field.

BTW, I've seen other far-field eqs where the antenna diameter (not
aperature size) is used and I haven't bothered looking into the
origin, but I'm just trying to impress upon you the fact that
nothing's written in stone.

The 6m I gave as an example might not be all that great. These
short distances you mention have me thinking you're locked in a
dungeon somewhere so I tried to keep it reasonable. A sniffer might
be ok at close range and an active RFSM would be good at greater
distances.

Personally, since you've got a reference Tx, I'd take the whole
deal outside (they do let you out, don't they) and see what happens
with greater distances. Even a gym or auditorium would work if
you're concerned with how well it works indoors.

I remember the 40MHz part, but I'm not sure what yer up to. Just
curious.

-mike

On Sun, 14 Dec 2003 15:18:34 +0000,
said...
On Sun, 14 Dec 2003 13:59:38 GMT, Active8
,invalid wrote:

Yup. gave you the eqs to figure out your ballpark figure. I figured
a ballpark figure might work if you gould figure out how much
voltage appeared at your RFSM antenna. had I known you had a
reference transmitter i might have suggested checking your RFSM
reading at one location with the ref and another closer location
with the test Tx, identical readings indicating sucess or close to
it.

I'll use a distance of 6m.

Noted, thanks.

Just curious, but would carrying out comparative measurements at say
only 6 to 10 feet give rise to invalid readings??


Lots of things can foul you up.

I think I estimated the near field/far-field at 3.7m so I wouldn't
do it. IIRC the change in field strengh with respect to distance in
one of the zones of the near field varies inversely with the cube
of the distance, as opposed to the square as it does in the far-
field.

BTW, I've seen other far-field eqs where the antenna diameter (not
aperature size) is used and I haven't bothered looking into the
origin, but I'm just trying to impress upon you the fact that
nothing's written in stone.

The 6m I gave as an example might not be all that great. These
short distances you mention have me thinking you're locked in a
dungeon somewhere so I tried to keep it reasonable. A sniffer might
be ok at close range and an active RFSM would be good at greater
distances.

Personally, since you've got a reference Tx, I'd take the whole
deal outside (they do let you out, don't they) and see what happens
with greater distances. Even a gym or auditorium would work if
you're concerned with how well it works indoors.

I remember the 40MHz part, but I'm not sure what yer up to. Just
curious.

-mike

On Sun, 14 Dec 2003 16:47:09 GMT, Active8
,invalid wrote:


I remember the 40MHz part, but I'm not sure what yer up to. Just
curious

You once referred to me as the "battlebots guy" - although I post on
many other aspects of electronic design as well, so that's not
entirely accurate. I trust this jogs your memory. :-)
--

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

On Sun, 14 Dec 2003 16:47:09 GMT, Active8
,invalid wrote:


I remember the 40MHz part, but I'm not sure what yer up to. Just
curious

You once referred to me as the "battlebots guy" - although I post on
many other aspects of electronic design as well, so that's not
entirely accurate. I trust this jogs your memory. :-)
--

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

Paul Burridge wrote:

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?

Yes. If it's not, either move closer, or add an op amp with
a gain control after the diode(s) & cap. Mine uses 2 gain
controls - 1 on the input to the op amp, and one that sets
the gain of the op amp. A further improvement is the use
of a cheap DPM - no parallax and a wider "full scale" range
with no loss of sensitivity. The downside of the cheap DPM
was the need for 2 9 volt batteries. (I have since built a
small DC-DC converter that occupies the volume of a single
9V battery. That may allow using a single rechargeable
9V to power the converter - but I haven't tried it yet
to be sure that the converter doesn't produce noise that
would be detected by the instrument. The converter does
produce dual 9V fully isolated outputs at at least 10 ma
per output, way more than the instrument needs.)


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