[Elecraft] Re: K3 receiver sensitivity on 50MHz

[Ian White GM3SEK]

Wayne
wrote:
>Ian White GM3SEK wrote:
>
>> On the subject of 50MHz, Wayne, please could you say more about the 
>>receiver sensitivity specification?
>> At present the sensitivity spec is the same on all bands, namely: 
>>"-136 dBm (typical), preamp on, 500Hz b/w"....
>
>Hi Ian,
>
>The K3 is optimized for high dynamic range, which requires a 
>high-intercept preamp and mixer as well as the narrowest possible 
>band-pass filters (consistent with good return loss). We also used PIN 
>diode T-R switching to eliminate relays during QSK. There are small 
>front-end losses associated with each of these design choices.
>
>However, there is a way that one could have the best of both worlds. 
>Those who would like an extremely low NF on 6 meters (or other bands) 
>could patch in an external low-noise preamp between the RX ANT IN and 
>OUT jacks. I've asked one our staff engineers to look into such a 
>design, although there may be third-party products already available.
>
>You'd need the KXV3 option. The KXV3's RX ANT IN/OUT jacks are fully 
>isolated from the transmit path, so high-sensitivity devices could be 
>used in the preamp. The RX ANT IN/OUT jacks are also pre-BPF and can be 
>turned on/off per-band. A 6-meter-only implementation with its own 
>optimized BPFs could provide a world-class NF.
>
>Further, you could power the external preamp from the K3's 12-V DC 
>accessory output (up to 0.5 A, switched).
>
>I personally prefer this means of achieving a very low 6-meter NF, 
>because it doesn't result in compromises to the rest of the design. 
>There are other rigs on the market that incorporate a second, 
>switchable preamp specifically for this purpose, but none of these can 
>match the K3's close-in dynamic range on HF.
>
>73,
>Wayne
>N6KR

Thank you for the prompt and thoughtful reply.

Nobody should join a mindless stampede for the lowest possible NF on 6 
metres. For more than 20 years, I have been urging VHF/UHF DXers to 
optimize their dynamic range in exactly the same way that we do on HF.

The receiver noise floor at VHF needs to be lower than at HF, to take 
advantage of the lower antenna noise level; but it's a very delicate 
tradeoff. Above all, it is vital to use no more preamp gain than 
absolutely necessary, because unnecessary gain will have a dB-for-dB 
impact on the strong-signal handling.

The optimum solution is always to use a preamp at the antenna; but that 
isn't always practical, which is why most transceivers offer the option 
of a built-in preamp. The band-definable RX IN/OUT jacks on the KXV3 
will allow us to use a dedicated 6m preamp - if not actually inside the 
rig, at least right behind it.

Should Elecraft supply a 6m preamp as an option for the K3? Yes, I think 
you should, because third-party preamps are notorious for having far too 
much gain. To preserve the K3's reputation for high dynamic range on 
*all* bands, it would be best to keep the gain/NF tradeoffs for 6m under 
factory control. Obviously this option is not a high priority; but it 
certainly would be worthwhile.



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek