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 <[hidden email]>
To: Elecraft Reflector <[hidden email]>
Subject: [Elecraft] When do I need atten or preamp?
Message-ID:
         <[hidden email]>
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
     "Kits made by KL7UW"
Dubus Mag business:
     [hidden email]

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[alorona]

Yes, I've heard about those amazing EME guys. I guess if you can copy signals below the noise you know who you are, and you can ignore guys like me!

I have been, of course, referring not to soundcard-decoded modes but to human ear-decoded modes, using a speaker or headphones. This is the scenario my calculations were based on.

I thank GM3SEK for clarifying that his post was an attempt at helping only the "pileup mush" guys, and point taken. The issue of setting your AGC correctly, however, applies in all situations.

Modern receivers have specs that differ by only a few dB. And yet, they sound radically different. Why? Could it be that, more than anything else, it's the AGC that makes or breaks them?

All the parameters that describe an AGC system conspire to give a receiver its characteristic "sound", and that sound ends up being something we like, something leading to listener's fatigue, or something in between.
These factors determine not only how a receiver deals with signals, but how it deals with noise. A well-behaved AGC can sound incredibly smooth. Poor (or misadjusted) AGC systems have probably caused many hams to sell otherwise good radios.

I have a few receivers here in the shack, and they all sound *totally different*. Some tire me out after a few minutes and I can listen to others all day and still enjoy what I hear.

Many modern receivers expose AGC parameters to the user and allow him or her a wide range of adjustment of those parameters. I'm starting to suspect that incorrect settings have caused many people to dismiss the K3 as "bad" or "noisy" or "fatiguing".

I happen to believe that the K3's AGC system is one of the best out there. But rather than force that opinion on anyone else, I usually encourage them to put the time into understanding and then setting their AGC parameters to try and capture that sound we all strive for. If you do this-- and use only as much gain as you need-- I really believe you'll enjoy your K3 more.


Al  W6LX
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