Jim, Your comment is a little confusing. First you state that you believe the External preamp definitely improves sensitivity (when external noise level is low). I agree. I said the same in my original e-mail.
Then you state that you are not suggesting the ARR is better. Not better than what? It is definitely better than the internal preamp, but it is probably close to the same performance as the PR6 (regarding sensitivity). Elecraft claims exceptional dynamic range for the PR6 which the ARR probably does not have (which is important in urban high noise environs or in high density QRM that happen in openings/contests). I did some checking of the K3 specs: Sensitivity with (internal) preamp turned-on is spec at -136 to -138 dBm at 500-Hz bw. -138 dBm is equivalent to noise figure of 9.5 dB. With the PR6, the K3 is spec at -143 to -144 dBm at 500-Hz bw. That is equivalent to a noise figure of 5-dB. Since the PR6 spec is 0.5 DBNF with 18 dBG this implies that the K3 receiver has a NF of 20-dB (all preamps off). The ARR preamp is spec at 0.5 dBNF with 22-dBG so the resulting sensitivity should be -148 dBm. So doing a little more reverse analysis, this implies the internal preamp NF is something like 5-dB if you assume the gain is 10-dB (I could not find what the gain of the internal preamp is). How does this translate for the 6m operator? In high noise environs the internal preamp is probably adequate since external noise predominates. In low noise areas the PR6 or ARR definitely will improve sensitivity by 7 to 10 dB. If you are considering doing eme or meteor scatter on 6m the low-noise external preamp is going to help quite a bit. Antennas elevated above the horizon do not see the thermal noise of the ground or as much man-made noise. Sky noise at 6m is still quite high compared to higher VHF and UHF frequencies so placing the preamp at the K3 is acceptable if your coax is not lossy. Remember my statement that I though running both internal and external preamps might not result in better sensitivity? I took the numbers I developed for both the PR6 and internal preamp and input to my spreadsheet. The result is total sensitivity of -152 dBm. So maybe running both internal and external preamps is better. (As jim point out) you have to evaluate this in real conditions as results will differ depending on your local noise environment. One caveat is turning on both preamps may lead to overdriving the receiver resulting in reduced dynamic range or even distortion. 73, Ed - KL7UW Final note: SSB sensitivity numbers at bw of 2.8 KHz are 7.5 dB lower than at bw of 500-Hz. ------------------------------ Message: 31 Date: Thu, 24 Jun 2010 13:15:30 -0700 From: "Jim Brown" <[email protected]> Subject: Re: [Elecraft] K3 PR6 preamp To: "[email protected]" <[email protected]> Message-ID: <[email protected]> Content-Type: text/plain; charset="us-ascii" On Thu, 24 Jun 2010 11:52:46 -0800, Edward R Cole wrote: >Running both preamps probably only raises the noise level and >does not improve receiver sensitivity in any significant >manner. I suggest that you defer judgment on this until you're trying to copy very weak signals during a band opening. The additional preamp at the patch point (in my case an ARR) definitely DOES improve RX sensitivity at times when the external noise level is low. I'm not suggesting that the ARR is better -- indeed, I doubt that it is. But it was bought and paid for when the K3 showed up, and it works fine. :) 73, Jim K9YC 73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 144-600w, 432-100w, 1296-60w, 3400-fall 2010 DUBUS Magazine USA Rep [email protected] ====================================== ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
