[Elecraft] When do I need atten or preamp?

[Al Lorona]

I ran several simulations of the K3 receiver using Genesys (from Keysight Technologies) to find out how to use the attenuator and preamp optimally.

An article by Jim Fisk W1HR appeared in October 1975 in ham radio magazine where he gave the acceptable noise figure (NF) for a receiver on each HF band (excluding WARC bands, which didn't exist at the time). For instance, on 80 meters your receiver can have a NF as high as 37 dB (because the external noise level is so high), on 40 the acceptable NF is 27 dB, on 20 it's 24 dB, etc.

Going by the K3 block diagram in the Owner's Manual, I made a number of assumptions so my numbers are undoubtedly off by a couple of dB (probably in the pessimistic direction). I also assumed a CW filter of 500 Hz, noise blanker off, and an outdoor antenna like a dipole. I modeled everything up to and including the 2nd IF amp. I included synthesizer phase noise.

Let's talk about 80 meters. As you would expect, even with the 10 dB attenuator switched in the K3 has more than enough sensitivity. Since it's not possible to hear signals below the external noise floor, it doesn't buy you anything to have more sensitivity than the noise floor. In fact, you could have up to 21 dB of attenuation and it would still be totally adequate, so K3S owners can use all of their 15 dB if they want to. And definitely DO NOT ever turn on your preamp on 80; the only time this would be permissible is when using a small receiving loop direct into the RX ANT.

On 40, having the 10 dB attenuator in (and preamp out, of course) is still right on the good side of sensitivity. So, kick in your attenuator on 40 with a clear conscience... your receiver will be quiet, but not deaf. Additional loss will just begin to affect you on 40, but only if you're in the habit of listening for signals right at the noise floor.

On 20, the 5 dB attenuator would be okay to use. Again, this is where K3S owners have a little more flexibility. If you have a K3 don't use the attenuator, unless you want to.

The preamp really isn't needed until you get to 10 meters, according to my simulations. Using it below 10 meters is just asking for unnecessary noise!

On SSB, the margins are even better [by 7 dB] because of the wider 2.7 kHz bandwidth. And if the HF bands are noisier nowadays than in 1975 due to plasma TVs, wireless devices and SMPSs, the K3 would have even more margin than I have presented here.



Al  W6LX