[Elecraft] K3 - Performance on 6M?

[Bill VanAlstyne W5WVO]

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



--------------------------------------------------
From: "Dave, G4AON" <elecraft at astromag.co.uk>
Sent: Friday, April 02, 2010 4:25 AM
To: <elecraft at mailman.qth.net>
Subject: Re: [Elecraft] K3 - Performance on 6M?

> Jeff, you will probably need a pre-amp with the K3. My experience of 6m
> over the past 3 or 4 years with a 4 ele quad is that 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.
>
> My K3, serial 80, gives 10 dB SINAD at -126 dBm in a 400 Hz bandwidth
> using the internal pre-amp. Adding an external Advanced Receiver
> Research P50VDG pre-amp via the KXV3 RX ANT IN/OUT sockets improved the
> sensitivity by 10 dBm which is a huge improvement for weak signal work.
> The Elecraft pre-amp is a similar specification and should give similar
> results. For those who compare figures using MDS the "with and without"
> figures in a 400 Hz bandwidth are -136 dBm and -146 dBm. Image rejection
> is marginal at 62 dB.
>
> Power output on 6m is fine and the worse case harmonic is -60 dB.
>
> 73 Dave, G4AON
> ---------------------------------
>
> Hi everyone,
> Sorry to trouble you all again but I'd like to find out more on the K3's
> 6M performance as very little seems to be mentioned on the Elecraft 
> website.
> <SNIP>
>
> Jeff Cochrane - VK4BOF
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