[Elecraft] K3 and Preamp

[Don Wilhelm]

There is also a converse on preamp use that needs to be stated IMHO - 
and mostly pertains to bands below 15 meters.

If there is a significant (3 to 6 dB) increase in noise heard in the 
receiver when the antenna is connected (tuned where there are no signals 
present) then the receiver gain is sufficient and the addition of a 
preamp (or turning the internal preamp on) will only reduce the dynamic 
range of the receiver.
For those who are not familiar with dynamic range, it is the difference 
between the weakest signal that can be heard and the strongest signal 
that will not cause overload.

What this means to normal operating is that when the band consists of 
very strong signals as well as the weak ones you would like to copy, you 
will be better off turning the preamp off and perhaps even inserting 
some attenuation or turning down the RF Gain.

Now, if all stations within the bandpass of the receiver front end are 
weak, adding preamplification will not create receiver front end 
overload, and can be safely added.

So whether or not a preamp will help depends on the band, the antenna, 
propagation conditions and many other factors.  No blanket statements 
can be made, particularly on the lower HF bands.
For 10 meters and above, band noise drops drastically with frequency and 
what Ed has stated is certainly true.

73,
Don W3FPR



On 9/6/2014 4:36 PM, Edward R Cole wrote:
> Maybe most of you already know this, but...
>
> It has been accepted that the K3 receiver needs a little help of a 
> preamp on 10m and 6m (especially 6m).  That is why Elecraft came out 
> with the PR6 and later the PR6-10 preamps.
>
> At lower frequencies the level of sky noise is much higher which 
> overwhelms any noise generated internally by the receiver, thus a 
> preamp would only contribute gain making both noise and signal 
> louder.  But around 30-MHz sky noise  starts dropping so that it is 
> necessary for the first stage of a receiver to be "quieter" 
> (technically that is called a low noise figure).  If the noise 
> generated in the receiver is significantly equal to sky noise the 
> receiver sensitivity is not optimum (meaning you can't copy weak 
> signals).  This trend continues downward with increase of frequency up 
> to 1.2 GHz where sky noise is minimum (on earth). One cannot receive 
> "weaker" signals without a low-noise preamp close to the antenna at mw.
>
> So, use of the internal preamp is generally not needed below 20m as 
> all it will do is increase noise volume and not improve receiver 
> sensitivity.  But at 15m it often helps.  At 10m the internal preamp 
> is inadequate to obtain maximum sensitivity and an external low-noise 
> (figure) preamp can really help.
>
> Like I said at the beginning, maybe you already knew this...
>
> 73, Ed - KL7UW
> http://www.kl7uw.com
>     "Kits made by KL7UW"
> Dubus Mag business:
>     dubususa at gmail.com
>
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