[Elecraft] K3 - Performance on 6M?

Sat Apr 3 06:05:35 EDT 2010

Thankyou so much for your clear explanation Edward, I really appreciate it.
I am going away for 12 days to go house / shack shopping (I am moving interstate here in Australia) and as soon as I come back, my order for a K3 with a PR6, the ATU, the second receiver AND the 2M internal transvertor will be submitted to Elecraft.
I have used Gary VK4FD's K3 #679 on occasion in the past (In fact I think I'm the only person to have used it apart from him) and I am very impressed with it's HF performance but I have not used it on 6M.
My main concern was the 6M performance and that concern and all my questions on this subject have been answered very authoritatively by you and others in this list.

Thanks to all,
Jeff Cochrane - VK4BOF
Innisfail, QLD
Australia.
(But not for much longer!)
  ----- Original Message ----- 
  From: Edward Cole 
  To: elecraft at mailman.qth.net 
  Sent: Saturday, April 03, 2010 6:03 PM
  Subject: Re: [Elecraft] K3 - Performance on 6M?

  Very good inquiry into how much improvement the PR6 preamp will provide.

  I suspect that the average ham is not very familiar with calculations 
  of noise figure, MDS, etc.
  One common error is to think that preamp gain has the sole effect on 
  minimum detectable signal (weakest signal one can hear).  In fact 
  there is a direct connection between MDS (listed as -136 dBm by 
  Elecraft for the K3) and noise figure (NF).  Gain and noise figure 
  both combine to produce receiver sensitivity.

  But not only receiver sensitivity (MDS) determines what you can 
  hear.  Sky noise, man-made interference, and antenna efficiency 
  factor into whether you can receive a particular signal level.  Hams 
  involved in satellite, weak-signal operation (typ. VHF+), and eme 
  (MoonBounce) deal with getting the MDS as low as possible.

  Most commercial HF radios are spec'd at an MDS of -122 dBm for SSB/CW 
  with a 2.7-KHz bandwidth (this is equivalent to a NF of 18 dB)

  So the K3 is 14-dB more sensitive that the bulk of ham radios made 
  (NF=5 dB).  With the PR6 it increases to 24-dB better (NF=0.76 dB).

  100-MHz is the point where noise figure begins to make significant 
  impact in MDS.  This is because sky noise is much higher as you go 
  down in frequency.  In Urban areas local man-made noise raises the 
  noise floor even higher.  So in those areas the 6m preamp may not 
  help do anything more than increase the noise in the receiver.  If 
  you can afford the 6m preamp it is probably worth trying out, 
  though.  Unless you are doing eme there will not be any advantage to 
  installing the preamp at the antenna.  But you do want to use good 
  low-loss feedline to the antenna to reduce the affect on the total 
  receiver NF.  Every dB of coax loss is directly added to receiver NF 
  (e.g. use 3-dB coax loss with a 0.5 dB NF preamp and the result is 
  3.5 dB NF).  As a rule of thumb for 6m, I would try to achieve an 
  overall receiver NF=1.5 dB.

  We usually measure noise as an equivalent temperature in units of 
  Kelvin (K).  At 50-MHz the sky temperature is about 2000K (higher in 
  urban areas).  A 0.5 dB NF preamp has a noise temperature of 
  35.4K.  Total noise temperature received is Te = Tr + Tsky + 
  Tant.  Tant for a good eme array is about 30K and probably about 70K 
  for the average 6m yagi.  So Te = 2100K if you use a good preamp like 
  the PR6.  Most ham receivers are not very low NF (the best are maybe 
  6-dB and its typ for a radio to be 12 dB NF)  The FT-847 is 12 dB NF 
  (Tr=4000K) at 144-MHz.  So adding this up Te = 4000+2000+70 
  =6070K.  You can see this is much worse in noise performance.

  But this must be converted into noise power into dBm.  This way 
  beyond what I can cover in this e-mail, but I have a spreadsheet 
  program that allows you to input your performance figures like NF, 
  coax loss, receiver bandwidth, sky noise and antenna noise to 
  determine MDS.  You can pretty quickly see how much improvement in 
  sensitivity (MDS) is produced by improving various characteristics of 
  your station
  http://www.kl7uw.com/emelink.xls

  73, Ed - KL7UW
  ------------------------------

  Message: 15
  Date: Fri, 2 Apr 2010 06:56:08 -0600
  From: "Bill VanAlstyne W5WVO" <w5wvo at cybermesa.net>
  Subject: Re: [Elecraft] K3 - Performance on 6M?
  To: <elecraft at astromag.co.uk>,  <elecraft at mailman.qth.net>
  Message-ID: <9C5E39E63A1A481EAC0D9643FD409029 at BILLHP9250>
  Content-Type: text/plain; format=flowed; charset="iso-8859-1";
           reply-type=original

  This is an excellent thread, and one that I would like to see continued. I
  use the K3 exclusively on 6 meters right now. I work both SSB and CW during
  sporadic-E conditions and WSJT digital modes on meteor scatter at other
  times when the band is "dead." Eagerly anticipating some F2 on the band in
  three or four years.

  I have used the K3 both with and without the PR6 preamplifier.

  Dave's comment brings up a crucial point:

   > "...many signals are very weak and the K3 lacks the sensitivity to deal
   > with them unless you either add a pre-amp or have a high level of local
   > noise masking those signals."

  Right. This is the salient point I want to explore further.

  It has been said by many hams over the years that if you connect an antenna
  to a receiver and you can hear the noise level increase, then you don't need
  a preamp, as it will simply amplify the noise and the signal equally. In
  general, I've found that to be true. HOWEVER...

  Let's say that the noise figure of the receiver's internal preamp is 1.5 dB,
  and the noise figure of the external preamp is 0.5 dB. Each preamp will give
  you about 10 dB of gain. It seems clear that using the external preamp
  instead of the internal one would give you another dB of signal+noise to
  noise. Is my reasoning accurate on that score? If so, then it would be
  useful to know the noise figure of the K3's internal preamp at 50 MHz. I
  don't know what it is.

  Second, in my experience, the gain of the PR6 and the gain of the internal
  preamp are NOT the same, with the PR6 having more gain -- more gain than I
  need, actually, given my ambient noise level caused by living in a large
  metro area.

  Third, since I live in the shadow of a large number of high-powered VHF
  broadcasting stations, there is a LOT of powerful out-of-band RF floating
  around. Thankfully, its effect on 6 meters was vastly decreased when our
  local Channel 2, 4, and 5 TV stations went to UHF digital. Before that
  happened, the PR6 would be badly overloaded and create some nasty IMD
  products on 6 meters unless I put my rather hefty DMI 8-pole bandpass filter
  in-line with the preamp (on receive only). This cured the overload/IMD
  problem, but at a cost of about 1 dB of insertion loss. Now that the
  low-band TV broadcasters have been evicted, I need to try the PR6 again
  without the DMI filter and see if the PR6 is still overloaded by out-of-band
  RF. I haven't yet tried that yet. But if I really don't need it, why bother?

  I just recently put the K3 back in the station setup after having it on the
  bench for several months with a transmit section problem. Replacing the
  KPAIO3 board seems to have cured that. Thanks, Elecraft. But now I'm back to
  the "preamp-or-no-preamp" decision point. Currently, it is set up without
  the PR6, and since my antenna noise level is at least 3 dB above the
  receiver MDS even in my most quiet beam direction (NNW), I'm thinking I
  don't really need to use it.

  But am I wrong in my thinking? I'd like to hear some knowledgeable comments.

  Bill W5WVO

  73, Ed - KL7UW, WD2XSH/45
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