[Elecraft] K3 - Performance on 6M?

[Wes Stewart]

I'm going to violate the "Top posting is the rule" rule for this one.

--- On Fri, 4/2/10, Bill VanAlstyne W5WVO <w5wvo at cybermesa.net> wrote:


> 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.

There is no correlation between the NF of an amplifier and its gain.  So your "each preamp will give you about 10 dB gain" is true only by accident. I'm not going to try to express the math in the forum*, but the noise figure of a cascaded pair is well-known and shows that the overall noise figure is dependent upon the individual noise figures of the two amplifiers and the gain of the first.

After you study this you will know that it *isn't* clear that using the external preamp will give you another 1 dB S/S+N, it could be anything.

*Google "cascaded noise figure" and you will get more info than you want to know.

> 
> 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.

It is no surprise that the two preamps differ in gain. It would be more surprising if they did.  Again, considering the cascaded NF equation will show the necessity for higher gain in the first amplifier when the NF of the second is high.  It's a pity that your local situation is what it is, but that doesn't negate the need for a higher gain external preamp ahead of the poor NF of the K3's internal preamp in lower noise environments.  

BTW, a noise factor >1 (NF>0 dB) *always* degrades the received SNR.  It may not be significant, but it is a fact.

I don't know what the K3's NF actually is, but it is stone deaf on six meters and isn't much better on ten.  I can hear signals that are Q5 on my TS870 that are Q2 on the K3.

> 
> 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.

I'm going to bite my tongue and leave U.S. politics out of this but if a TV station pops up on channel 2 again don't be surprised.

http://www.broadband.gov/download-plan/