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Re: TH-D7A(G) vs W32-A for Easy Sats?



Nate,

Good observations! ...more from me below:

At 01:18 PM 12/11/2005 -0700, Nate Duehr wrote:
>Emily Clarke wrote:
>> At 02:41 PM 12/9/2005, you wrote:
>> 
>>> I prefer the D7 over the Icom mainly because it has a slightly better 
>>> receiver
>> 
>> 
>> This is a surprising statement = the sensitivity for the W32A is 0.16uV 
>> and the sensitivity for the D7A is 0.18uV.  So I'm not sure on why you 
>> would say the D7A has a better receiver?
>
>In reference to what?  12dB SINAD?  Start of signal reception?  Some 
>other agreed-upon measurement point?  And are both the VHF and UHF 
>reception numbers the same?  I doubt that.
>
>Are these measured by you, or "as published by the manufacturer"?
>
>(Never trust the guy trying to sell you something with performance 
>numbers.  Always measure yourself or use a trusted 3rd party's 
>measurements.)
>
>Just being a pest to make sure these numbers have some basis in reality...

Absolutely!  Most professionals now use 12-dB SINAD as the reference for
measuring FM receiver performance.  Hopefully the radio manufacturer is,
too.  Twenty (or more) years ago sonething called 20-dB quieting was used
but that was an imperfect measurement of performance.  So if using
published specs read what reference they cite and also the bandwidth if for
SSB, CW or AM.  Change any of these and the measurements are not comparible.

OK so if both makers are using comparible measurement then 0.16 vs 0.18 uV
is not enough difference to matter unless you are on a test bench using
good signal generators and SINAD meters.  You can't hear the difference
that close.

>By the way, anyone who lives where a VHF or UHF receiver with that kind 
>of sensitivity isn't blasted by other local transmitters and the general 
>noise of a city -- I'm jealous.  ;-)
>
>I'd be hard-pressed (no EME guys allowed to make wisecracks here...) to 
>hear the difference between a receiver capable of 0.16uV and 0.18uV when 
>operating.

I AM an eme operator and agree.  I am also a professional radio technician
that maintains hundreds of VHF and UHF radios.  I make an annual test of
each radio for sensitivity and selectivity so have a ton of experience,
here.  My ears are so calibrated to what 12-dB SINAD sound like I do not
need the meter to get close.  Our commercial radios are only spec'd at 0.25
uV and I do not worry if they vary as much as to 0.30 uV.  No one will ever
be able to tell in the field.  Sure if a radio only tests 0.50 uV you can
tell that it is not doing so well (and it needs fixing).

>Receiver sensitivity measurements on a bench are a nice starting point, 
>but only a good USEABLE receiver sensitivity test with the antennas 
>installed, and then injecting a known weak signal to overcome the noise 
>level, is really all that useful for the real-world, after the radio's 
>installed and ready to go.

True.  But most folks have a pretty hard time making repeatable
measurements.  So if you can make a bench measurement of several radios it
will help you determine which is better and if that is particularly
significant.  You probably will not realise that level of performance out
in actual use.

>It also gives you a starting point to decide if that fancy pre-amp you 
>just bought is amplifying real signal, or just raising your noise floor 
>excessively also.
>
>Seeing at what point a weak signal overcomes your local noise floor -- 
>now THAT's useful information!
>
>The receiver technically may be able to receive lower than that, but 
>you'll never know it when operating that station!
>
>And 0.02uV?  C'mon.. that's getting into the realm of "measurable, but 
>not relevant to real-world operation".

For satellite work on VHF or UHF, you are quite correct.  In the normal
noisy urban radio environment it will not be significant.

However, it matters and can make huge differences when you go up to
microwave frequencies where there is less man-made noise (depending on
frequency, of course.  e.g. 2400-2450 MHz is impacted by lots of unlicensed
digital equipment).

I think this debate originally had to do with HT radios so probably preamps
would not be needed since one has little or no feedline.  A home-based
station with roof-top or tower-top antennas and longer coax lines will
generally (almost always) do better with a mast-mount preamp (e.g.
connected as close to antenna connector as possible).  You can prove this
to yourself on very weak signals like from repeaters more than 100-miles
away or operating FM simplex.  Of course if you operate VHF/UHF-SSB it will
make a world of difference having a good low-noise preamp (0.5 dB or so is
low enough).

For EME or copying the Mars Observer spacecraft at 8415-MHz then one gets
into the "eme-class" of station.  I will not bore you with that.  If you
are interested in analyzing the ultimate low noise receiving systems or in
eme then visit my website.
73's,
Ed - KL7UW  
=========================================
http://www.qsl.net/al7eb - BP40iq 
144-EME: FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
432-EME: FT-847, mgf-1402, 1x21-ele (18.6 dBi), 60w
=========================================
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