[Premium-Rx] WJ-8711A or WJ-8712P

[Karl-Arne Markström]

My about 30 years spent in evaluating and building systems using
"affordable" HF receivers have learned me that nothing can replace 
front-end selectivity.
Even quite "mediocre" performers became usable with a narrow-band 
preselector in front.

An "acid test" that I often used to separate "the wheat from the 
chaff" consisted of using a
TCI 548 horizontal log-periodic at the Enköping Air/Ground receiving 
site beamed due south and waiting until the signal levels were at a 
peak during the day.
then injecting a weak wanted signal at a test frequency chosen at a 
"quiet spot" in the signal path using a directional coupler.
A spectrum analyser was connected to another port of the antenna 
signal distribution network so the part of spectrum
having the highest levels could be found.

By choosing the test frequency it was possible to create "worst case 
scenarios", such as listening after
the weak signal alongside the high-powered broadcasters in the 7 MHz 
band.
Using an SRT PS300 or R&S FK101 tunable preselector it was possible to 
evaluate the effects of having switchable 
selectivity in front of the receiver input. 

In some cases they were dramatic.

Some receivers (that shall remain unidentified) drowned the wanted 
signal in a mess of 2 nd and 3 rd order products
even with quite substantial amounts of attenuation between the antenna 
and receiver, when they were used without selectivity.
On the other hand, some "mediocre" receivers showed great improvements 
with the preselection, so they approached
some of the better receivers with regard to IM suppression.

Looking back at the "bad old days" when the "radiated power arms race" 
among the HF broadcasters still prevailed,
a dynamic range limit in CW bandwidths of 90 - 100 dB was appropriate 
to design after. 
Today, it appears that a dynamic range of maybe 10 - 15 dB less is 
acceptable in the light of present spectrum usage.

I am however aware of the differences of spectrum loading in the 
military or aeronautical HF bands as compared to the amateur bands.
Contesting activities may still stress the receivers as much today as 
yesterday.

It is however pointless to design a receiving system for a close-in 
dynamic range of 100 dB or more,
where transmitters have a broadband noise floor or adjacent channel 
suppression in the order of 70 - 80 dB.

73/
Karl-Arne
SM0AOM







----Ursprungligt meddelande----
Från: Pat1McA at aol.com
Datum: 2013-01-05 20:13
Till: <michaelob666 at ntlworld.com>
Kopia: <premium-rx at mailman.qth.net>
Ärende: Re: [Premium-Rx] WJ-8711A or WJ-8712P

Greetings to all,
 
This is an interesting topic.  Basic question:
 
Has anyone actually compared the WJ-8711A and WJ-8712P receivers, side 
by  
side on the same signal under exactly the same circumstances. Same 
antenna, 
same  audio level, same line amp level or 'phones etc etc ??????
 
Is there any meaningful, noticeable, improvement ???
 
Or are we on the usual downward slope of the "Law of diminishing 
returns"  
against upward cost?
 
I am not surprised that the WJ-8711A failed the 2 and 3Ip tests in  
Sweden, 
up to now I have never managed to measure one  successfully. They just 
fold 
up and die in testing.
 
However in practice, rather like the KWM380, this is not  noticeable.
The 
recovered audio is excellent.
 
You have to be careful comparing like for like because the nominal 
0dBm  lin
e output of the WJ-8711A is seldom that and is usually a few dBs 
higher 
being  non adjustable. Thus it can sound "hotter" in direct comparison 
with 
another DSP  receiver which is actually set for 0dBm.
 
Under the same test conditions other DSP Receivers, EK896, RA3791 and 
the  
older analogue E1800/3, E1700, EK085, EK070 etc test perfectly.
 
I agree that a "score" should be assigned for "Intuitiveness and  
Operability". Some, like the 51S-1, RA1792 and the WJ-8711A are 
obvious in  operation 
but others, like the later R&S EK8XX series require a weeks  
intensive 
course to get the best out of them.
 
Years ago I was stuck as R/O on Tankers with some of the oldest 
HF/MF  
receivers ever built. One was a Siemens G11 (This may have been a 
Siemens &  
Halske?) which de-tuned as the ship pitched.
Each day it was a battle to contact GKA but the general intuitiveness 
of  
the receiver made up for the basic defects and looking back, it was 
amazing 
what  could actually be achieved in practice.
 
Best wishes for the New Year,
 
Pat G3YFK
 
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