On Fri, 12 Dec 2003 16:41:06 +0000, 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? 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


Hello Paul,
have a look here,
http://www.alphalink.com.au/~parkerp/noapr97.htm

Here is a crystal set circuit.
http://circuitos.tripod.cl/schem/r85.gif
Convert it to a field strength meter by
Replacing the headphones with a large sensitive meter,
something big enough that you can still see the pointer,
when viewed from across the room. Bigger the better.
Use a germanium diode.
Use a metal box.
Use a short telescopic aerial.
Coil and variable capacitor to cover, 40Mhz,. 35Mhz and 27Mhz,
I am guessing those are the frequencies of interest, use
a switch if necessary to add/remove some turns or
add/remove a capacitor.

I know you have a grid dip oscillator so fiddling the
coil and capacitor values to get the frequency ranges
will be a snack for you.

Using your field strength meter only six feet away is too
close. Keep it as far away as possible from your radio
control transmitter but still being able to see it, that is
the reason for the big meter movement. I am thinking
of big cheap plastic 6 inch square types. Anything will
do so long as you can see it from across the room and
the movement is microamps full scale and not milliamps.

I am sure I have explained this to you yonks ago, well,
if I have, never mind. maybe you forgot :-)

Set up your known good commercial radio control set
at one end of the room, and field strength meter at the
other side of the room. Note the meter reading. Now
compare readings with your experimental transmitter.
Is it more or less? Make adjustments to your experimental
transmitter. This is the fun part.

I found that sitting my field strength meter (even though it
had rubber feet) on my wife's metal serving tray reduced
hand capacitance and made it nicer to adjust. So experiment
with and without a sheet metal base. Maybe the first circuit
without a parallel tuned circuit would be less fuss to use.
Maybe you can knock up both types and tell us which
was better. Heh heh heh....

Regards,
John Crighton
Sydney