In message , JB
writes
"Ian Jackson" wrote in message
...
In message , JB
writes
"Ian Jackson" wrote in message
...
In message , JB
writes
"Roy Lewallen" wrote in message
m...
ve2pid wrote:
Hi to all,

Hope that my question has some sense...: If we double the power
radiated by an antenna (+3 dB), how does it translate on the
S-meter
at a receiver 'far' away.. i.e. is the difference in dB on that
meter
has some mathematical relation to the 3 dB change at the
transmitter?

73 de Pierre

No, none at all. S-meters almost always just measure the AGC
voltage,
which is only approximately logarithmic. And manufacturers adjust
the
sensitivity of the meter to please the customers, who don't like
"dead"
meters. So the sensitivity of S-meters varies greatly from rig type
to
type and from one end of the scale to the other. I measured the
sensitivity of the S-meter on my Icom 730 on one band. Here's how
big
an
S-unit is on my rig:

S1 - S2 1.4 dB
S2 - S3 1.3 dB
S3 - S4 1.6 dB
S4 - S5 2.3 dB
S5 - S6 1.8 dB
S6 - S7 3.2 dB
S7 - S8 3.1 dB
S8 - S9 4.0 dB
S9 - "S9 + 10 dB" 5.6 dB
"S9 + 10 dB" - "S9 + 20 dB" 7.3 dB
"S9 + 20 dB" - "S9 + 30 dB" 6.6 dB
"S9 + 30 dB" - "S9 + 40 dB" 10.5 dB
"S9 + 40 dB" - "S9 + 50 dB" 11.3 dB
"S9 + 50 dB" - "S9 + 60 dB" 13.5 dB

Now let's suppose that you built a new 5 element Yagi antenna and I
honestly reported that your signal went from S2 to S6 when you
switched
to it from your dipole. The gain is really 7 dB, about par for the
beam,
but you read the postings on the Web and decide that an S-unit is
"defined" as 6 dB, so the gain improvement must be 24 dB. Wow! Your
modest beam has the same gain as a beam with a 25 WAVELENGTH boom
and
more than 50 elements! By making that incorrect assumption about the
sensitivity of my S-meter, you've overestimated the gain by a factor
of
63.

S-meter sensitivities vary all over the map, so any assumption you
make
about how many dB in an S-unit is very likely wrong, and often
grossly
wrong.

Roy Lewallen, W7EL

Nail on the head. A step attenunator in line will be the fastest way
to
determine the linearity of your scale if you can't afford a calibrated
generator. FM rigs are all over the map on this because some radios
have
better limiting than others, and it might be actual limiter reading or
some
other derived reading.

You'll need an adequately-screened signal generator to do this test. If
it isn't, the signals which leak out can bypass the attenuator and
enter
a not-too-well-screened receiver. If this happens, the results you get
will be false.
--
Ian

Good point. If the signal is off the air it is less likely to be an
issue.
You need to seriously work on your setup to get better than 60 db of
resolution. I homebrewed the 3, 6, 10, 20 step attenuator from one of
the
ARRL publications and find it quite useful. Output cable is double
shielded.

But, with homebrew attenuators, beware of the RF 'jumping over' the
switches at the higher frequencies. This can limit the maximum
attenuation attainable to something which is a lot less than the
switches indicate. [Well, mine did, anyway!!]
--
Ian

http://www.arrl.org/notes/hbk-templates/stepatt.pdf
This is the one I based mine on. Fewer sections. The Slide switches are
superior because of lower inductance and the bulkheads help with isolation.
I found there was another version in the TIS section:
http://www.arrl.org/tis/info/pdf/9506033.pdf

As the pictures show, there is screening between the switches (even
though it's a bit skimpy on the second unit). Even in the first, for the
ultimate in isolation, it would be even better if there was an RF gasket
on the inside of the top cover which, when screwed down, makes contact
with the tops of all the screens, and makes each switch compartment
virtually watertight.

I looked again at mine and it is actually 10, 10 and 20 built on G10 scraps.
The blue foam Color TV tuner cleaner helped with insertion loss performance
(if you can find it anymore). It seems to work well and in testing with
several FM service monitors, seems to be within 1db to UHF. I had built it
with T-hunting in mind. For more attenuation I use fixed BNC attenuators.

--
Ian