[Elecraft] K3 receiver noise figure with KRX3 option

[Joe Subich, W4TV]

> The "obvious" way to couple the two is with a 3dB hybrid but that
> will cost me 3dB degradation in noise figure on both receivers.

Yes, that's correct.  However, in general one does not care so much
about the noise figure of the receivers since the "antenna noise" is
significantly higher than the receiver noise floor.  The only time
it might be an issue is on a very quiet band with an inefficient
antenna (3 element yagi/vertical on 6M in a rural area ... dipole
or vertical on 10 M in a rural area, etc.).

> Is there a super low noise amplifier that I can't find on the
> diagrams to make up for the hybrid when the KXR3 is fitted?

No.  There is no amplifier to make up for the -3dB loss when the
hybrid is activated (when the Sub RX is enabled and uses the main
antenna).  The loss is certainly noticeable with signals at the
receiver noise floor - e.g. weak signals on 6 or 10 meters.

> Where I'm struggling is in finding where the signal goes after
> leaving the hybrid.

One output of the hybrid goes to the sub RX input via P2 Pin 3 of
the SUB IN module.  the other output returns to the main RX input
via K1A and P1, Pin 9 (connects to J1 on the main board).

73,

   ... Joe, W4TV


On 2015-02-18 4:11 PM, Alan Ibbetson wrote:
> I want to parallel the receivers on my recently-acquired K2 with my K3.
> I have the KXV3A RX ANT option on my K3 and K160RX option on my K2. The
> "obvious" way to couple the two is with a 3dB hybrid but that will cost
> me 3dB degradation in noise figure on both receivers.
>
> The KRX3 option in the K3 has to address the same problem and I'm
> wondering if there are any clever tricks I can copy. I am finding the
> schematics hard to follow but I can see a 3dB hybrid splitting the
> signal path in two. Where I'm struggling is in finding where the signal
> goes after leaving the hybrid. Is there a super low noise amplifier that
> I can't find on the diagrams to make up for the hybrid when the KXR3 is
> fitted? Or maybe perhaps the hybrid is there all the time whether the
> sub receiver option is installed or not and the design absorbs the loss
> into the overall noise figure?
>
> Or is it all a lot cleverer than that?
>
> Thanks,
>
> Alan G3XAQ
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