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Re: R: S units



Hi all,

This discussion got my curiosity up since I do two-way radio maint. at work
and commonly deal in dBm/uV signals.  I was firstly surprised to see a
different standard for HF vs VHF?  Looking at the table Tony provided seems
reasonable in light of normal VHF radio sensitivities I see (granted mostly
FM).  A threshold of -121 dBm corresponds to 0.19 uV which is close to what
many 2m FM radios exhibit (for 12 dB Sinad).  Commercial FM radio specs are
more often 0.25 uV.

So the S-9 = -73 dBm makes more sense.  Here is a table for VHF based on
S-9 = -93 dBm:
S-9  -93
S-8  -99
S-7  -105
S-6  -111
S-5  -117
S-4  -121
S-3  -127
S-2  -131
S-1  -137
S-0  -141

-141 dBm* is pretty hard to achieve at normal voice bandwidths (i.e. 2.5
KHz).  My FT-847 is spec at -122 dBm on VHF!  OK I do not have the manual
in front of me (here at work) so maybe the ssb sensitivity is better (I
seriously doubt that).  So if you take into consideration going from 15 KHz
bw (FM) to 2.5 KHz (SSB), that is a change of 6 or 7.8 dB.  Sure won't gain
you 20 dB.

Of course this is all theoretical as no manufacture makes a radio whose
s-meter matches the standard.  

My guess is my FT-847  shows about 30 dB range from S-0 to S-9.  I use an
average of 4 dB/s-unit when making comments about it.  But actual
measurements on 2m with a calib signal source shows highly non-linear
response by the s-meter.  I get as much as 7 dB and as little as 2 dB
between meter readings.  Which all proves that s-meter signal reports are
"crap"...no surprise to most of us, I am sure!

73, Ed - AL7EB
*Note:  By using a good low-noise preamp, system sensitivity (MDS) of -145
dBm at bw=2.5 kHz is achievable ( just not to be found in stock VHF radios).


At 10:54 AM 10/6/03 -0400, Anthony Monteiro wrote:
>Hi Guys,
>
>Here is a copy of the standard:
>
>http://www.y06.de/y06/tools/S_Meter.pdf
>
>It shows S9 = -73 dbm0 (i.e ~50 microvolts) for
>30 MHz and lower and S9 = -93 dBm0 (i.e. ~5 microvolts)
>for over 30 MHz.
>
>As I recall, it was James Millen of the National
>Radio Company who made the original proposal
>of a standard 50 uV = S9 and 6 dB per S-unit.
>
>However, I can't think of a single manufacturer of
>Ham radio gear that gets even close to following
>this. I recently checked my FT-817 and it is less
>than 1 dB between S0 and S2!
>
>Cheers,
>Tony AA2TX
>
>---
>
>
>At 03:45 PM 10/6/2003 +0200, i8cvs wrote:
>
>>Hello Gerry K6TXD,
>>
>> From the "VHF MANAGERS' HANDBOOK " of IARU Region-1:
>>
>>Standardization of S-meter Readings
>>
>>At the IARU Region-1 Conference in Miskolk-Tapolka (1978)
>>following recommendation was adopted:
>>
>>IARU Region-1 adopts as a provisional standard the value of 6 dB
>>for the "S-point", and this value will be recommended to manufacturers
>>of equipments.
>>
>>At the meeting of the IARU Region-1 VHF/UHF/SHF Committee in
>>Dusseldorf (April 1989) the following recommendation was adopted,
>>ad later approved by the IARU Region-1 Executive Committee:
>>
>>The S 9 reference level for frequency bands above 144 MHz is -93 dBm
>>available signal power at the receiver input.
>>                                            --------------------------------
>>
>>Please note that -93 dBm is the power applied to the receiver input and not
>>the power indicated by your signal generator  with open output.
>>
>>73" de
>>
>>i8CVS Domenico
>>
>>
>>----- Original Message -----
>>From: Gerald Horn <ghorn666@earthlink.net>
>>To: <amsat-bb@AMSAT.Org>
>>Sent: Monday, October 06, 2003 5:55 AM
>>Subject: [amsat-bb] S units
>>
>>
>> > Hello
>> >
>> > Is there a definition of what an S unit is. I am building a home brew
>> > receiver and I don't know what to calibrate the meter to.
>> >
>> > gerry k6txd


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