[TowerTalk] Speaking of S units

[Jim Reid]

At 02:18 PM 8/7/97 -0700, Dick Flanagan wrote:
>>[...] Let's save all the newbies out there from
>>engaging in a lot of wasted effort by thinking for one second that an S unit
>>can be equated with 6 decibels.
>
>The newbies are already being confused because the General Class question
>pool contains the One-S-Unit-Equals-Four-Times rule in lots of places.
>
>There are lots of inconsistencies and ambiguities in this business.

Well,  here are the results I just got from my 950SDX and 1000D.
Used WWVH as a constant carrier source:  about 25 miles due West
from my QTH,  with a slight ridge between.  WWVH uses dipole
"curtains" to direct the main portion of their energy (10kW
transmitters on both 5 and 10 mHz) to the West. So I am behind
the main "beams".

Used two antenna set ups:  with my 20 meter beam,  could only get
about an S=7 reading on 10 mHz,  but on 5mHz,  using my 160 meter
full size 1/2 wave long dipole,  could get an S=9 signal via
the Johnson Matchbox tuner. Same initial s-meter readings on
both rigs,  S=5 at 10 gathered by the 20 m beam; S=9 at 5 using
the 160 dipole.

With the 1000D,  you must first turn off the front end preamp,
the Nor to IPO switch click.  On 10 mHz, the preamp off within the D,
the signal was too weak for any reading on the s-meter as gathered in
by my 20 meter beam.

With the 950SDX,  the reading at 10 on the beam was also S=5;
the first attenuator setting is -6dB,  the S meter dropped to
S=4;  at -12 dB,  the S meter reads  S=2;  and at -18dB,  the
meter reads,  S=0.  The front end preamp can be left on with the
SDX as the AIP button which turns it off is separate from the
front end attenuator knob,  unlike the 1000D. Only the first 6dB
insertion resulted in a one s unit meter drop.

At 5 mHz was able to pick up more juice from the back side of
the WWVH arrays using the 160 dipole.  With both rigs was able to
get an S=9 meter reading with the front end preamps on,  and
0 attenuation from the built in attenuators.

With the 1000D,  turning off the preamp, that is switching from
NOR to IPO,  dropped the S meter reading to S=7;  going to -6dB
from there dropped the reading to S=6(note here also, one s meter
unit drop);  going down 6 more dB to
-12 dB resulted in a meter reading of S=4; and finally on down
to -18dB attenuator position,  meter reads S=2.  So on my
FT-1000D,  turning off the preamp drops the signal 2 s meter
units,  then the next stop is another 6dB insertion loss,  and
this time,  and the only time,  the drop is one s meter unit.
>From there on down,  appears to be two s meter units per 6 dB
attenuator insertion.

Using the TS-950SDX, and the identical S=9 initial signal level
at 5 mHz,  the first 6dB of attenuation drops the s meter reading
two units to S=7.  And the same two s meter unit drop occurs for
each additional 6 dB of inserted attenuation,  again using the
internal Kenwood attenuator within the rig.

So for these two rigs of mine,  each 6 dB of signal loss (or reducing
the power by 4x's) appears to drop the s meter two units,  not
one unit as seems to be the usually accepted behavior.

Now all of the above assumes the internal attenuators within each
rig are in fact 6dB of added loss per switch click;  that
the backscatter energy I was getting was reasonable constant,  I did
keep checking,  and that I read the meters correctly!

It did appear that the first 6dB of switch insertion did result a couple
of times in a one s unit meter drop,  but all other cases was two s units.

So,  for whatever its worth,  thats what I found.

73,  Jim, KH7M



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