[Elecraft] When do I need atten or preamp?

[Edward R Cole]

Interesting viewpoint.

I thought it would be interesting to take a look at my local noise 
floor.  I have the PR6/10 installed on my K3 SN-4043  (new synth not 
yet installed).  Measurements in SSB - 2.8 KHz:

10m was the band I was most interested in measuring so started 
there.  I turned DIGOUT-1 = OFF and no noise?  Oh, the preamp 
switches to ext connections so I installed a BNC jumper from IN to OUT.
S=3 on noise with no preamps.
S=5 with internal PRE on
S=7 with PR6/10=ON and PRE=OFF  (DIGIOUT-1 controls the PR6/10)
So noise is pretty high with the preamp turned on (without weak 
signals unable to determine if preamp helps actual noise foor).

12m  Identical to 10m noise.

20m:  S=4/5 with PRE=OFF

40m:  S=5 with PRE=OFF

80m:  S=5 with PRE=OFF

10-20m ANT is Hygain TH3mk4 3-element yagi at 52-foot.
80/40m is fan-style inverted-V dipole with feed at 40-foot

I generally do not use the ATT as there is not a lot of QRM/QRN to 
deal with and signals are not always strong at Alaskan 
distance.  Note that I do little HF operating so generally operate 
SSB and occasional psk-31 on 20m.  Most of the time the tuner is set 
on 20m.  All tests were with tuner set for min SWR except on 80m 
where it is bypassed.

I use the PR6/10 more for 6m SSB where is definitely helps 
(interestingly the KX3 with PRE=ON works as well on 6m 
reception).  PR6/10 also necessary for weak 10m signals.

So guess that probably supports Al's observations.  One thing I would 
take exception is working "weak signals" meaning signals that are 
weaker than noise.  Top eme CW operators can receive CW that is SNR < 
-6dB.  Digital modes like JT-65 can decode at -28 dB.  So one might 
benefit from running a preamp on 12-10-6m with a K3.  I have not run 
JT-65 on HF so leave that for those who do to comment.

Observations made at 1pm on Sunday in South-central Alaska (BP40iq).

73, Ed - KL7UW
PS:  AE6JV watch your mail for delivery in a couple days!

From: Al Lorona <alorona at sbcglobal.net>
To: Elecraft Reflector <elecraft at mailman.qth.net>
Subject: [Elecraft] When do I need atten or preamp?
Message-ID:
         <1876112178.88125.1460257728937.JavaMail.yahoo at mail.yahoo.com>
Content-Type: text/plain; charset=UTF-8

I ran several simulations of the K3 receiver using Genesys (from 
Keysight Technologies) to find out how to use the attenuator and 
preamp optimally.

An article by Jim Fisk W1HR appeared in October 1975 in ham radio 
magazine where he gave the acceptable noise figure (NF) for a 
receiver on each HF band (excluding WARC bands, which didn't exist at 
the time). For instance, on 80 meters your receiver can have a NF as 
high as 37 dB (because the external noise level is so high), on 40 
the acceptable NF is 27 dB, on 20 it's 24 dB, etc.

Going by the K3 block diagram in the Owner's Manual, I made a number 
of assumptions so my numbers are undoubtedly off by a couple of dB 
(probably in the pessimistic direction). I also assumed a CW filter 
of 500 Hz, noise blanker off, and an outdoor antenna like a dipole. I 
modeled everything up to and including the 2nd IF amp. I included 
synthesizer phase noise.

Let's talk about 80 meters. As you would expect, even with the 10 dB 
attenuator switched in the K3 has more than enough sensitivity. Since 
it's not possible to hear signals below the external noise floor, it 
doesn't buy you anything to have more sensitivity than the noise 
floor. In fact, you could have up to 21 dB of attenuation and it 
would still be totally adequate, so K3S owners can use all of their 
15 dB if they want to. And definitely DO NOT ever turn on your preamp 
on 80; the only time this would be permissible is when using a small 
receiving loop direct into the RX ANT.

On 40, having the 10 dB attenuator in (and preamp out, of course) is 
still right on the good side of sensitivity. So, kick in your 
attenuator on 40 with a clear conscience... your receiver will be 
quiet, but not deaf. Additional loss will just begin to affect you on 
40, but only if you're in the habit of listening for signals right at 
the noise floor.

On 20, the 5 dB attenuator would be okay to use. Again, this is where 
K3S owners have a little more flexibility. If you have a K3 don't use 
the attenuator, unless you want to.

The preamp really isn't needed until you get to 10 meters, according 
to my simulations. Using it below 10 meters is just asking for 
unnecessary noise!

On SSB, the margins are even better [by 7 dB] because of the wider 
2.7 kHz bandwidth. And if the HF bands are noisier nowadays than in 
1975 due to plasma TVs, wireless devices and SMPSs, the K3 would have 
even more margin than I have presented here.

Al  W6LX


73, Ed - KL7UW
http://www.kl7uw.com
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