Very good inquiry into how much improvement the PR6 preamp will provide. I suspect that the average ham is not very familiar with calculations of noise figure, MDS, etc. One common error is to think that preamp gain has the sole effect on minimum detectable signal (weakest signal one can hear). In fact there is a direct connection between MDS (listed as -136 dBm by Elecraft for the K3) and noise figure (NF). Gain and noise figure both combine to produce receiver sensitivity.
But not only receiver sensitivity (MDS) determines what you can hear. Sky noise, man-made interference, and antenna efficiency factor into whether you can receive a particular signal level. Hams involved in satellite, weak-signal operation (typ. VHF+), and eme (MoonBounce) deal with getting the MDS as low as possible. Most commercial HF radios are spec'd at an MDS of -122 dBm for SSB/CW with a 2.7-KHz bandwidth (this is equivalent to a NF of 18 dB) So the K3 is 14-dB more sensitive that the bulk of ham radios made (NF=5 dB). With the PR6 it increases to 24-dB better (NF=0.76 dB). 100-MHz is the point where noise figure begins to make significant impact in MDS. This is because sky noise is much higher as you go down in frequency. In Urban areas local man-made noise raises the noise floor even higher. So in those areas the 6m preamp may not help do anything more than increase the noise in the receiver. If you can afford the 6m preamp it is probably worth trying out, though. Unless you are doing eme there will not be any advantage to installing the preamp at the antenna. But you do want to use good low-loss feedline to the antenna to reduce the affect on the total receiver NF. Every dB of coax loss is directly added to receiver NF (e.g. use 3-dB coax loss with a 0.5 dB NF preamp and the result is 3.5 dB NF). As a rule of thumb for 6m, I would try to achieve an overall receiver NF=1.5 dB. We usually measure noise as an equivalent temperature in units of Kelvin (K). At 50-MHz the sky temperature is about 2000K (higher in urban areas). A 0.5 dB NF preamp has a noise temperature of 35.4K. Total noise temperature received is Te = Tr + Tsky + Tant. Tant for a good eme array is about 30K and probably about 70K for the average 6m yagi. So Te = 2100K if you use a good preamp like the PR6. Most ham receivers are not very low NF (the best are maybe 6-dB and its typ for a radio to be 12 dB NF) The FT-847 is 12 dB NF (Tr=4000K) at 144-MHz. So adding this up Te = 4000+2000+70 =6070K. You can see this is much worse in noise performance. But this must be converted into noise power into dBm. This way beyond what I can cover in this e-mail, but I have a spreadsheet program that allows you to input your performance figures like NF, coax loss, receiver bandwidth, sky noise and antenna noise to determine MDS. You can pretty quickly see how much improvement in sensitivity (MDS) is produced by improving various characteristics of your station http://www.kl7uw.com/emelink.xls 73, Ed - KL7UW ------------------------------ Message: 15 Date: Fri, 2 Apr 2010 06:56:08 -0600 From: "Bill VanAlstyne W5WVO" <[email protected]> Subject: Re: [Elecraft] K3 - Performance on 6M? To: <[email protected]>, <[email protected]> Message-ID: <9c5e39e63a1a481eac0d9643fd409...@billhp9250> Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original This is an excellent thread, and one that I would like to see continued. I use the K3 exclusively on 6 meters right now. I work both SSB and CW during sporadic-E conditions and WSJT digital modes on meteor scatter at other times when the band is "dead." Eagerly anticipating some F2 on the band in three or four years. I have used the K3 both with and without the PR6 preamplifier. Dave's comment brings up a crucial point: > "...many signals are very weak and the K3 lacks the sensitivity to deal > with them unless you either add a pre-amp or have a high level of local > noise masking those signals." Right. This is the salient point I want to explore further. It has been said by many hams over the years that if you connect an antenna to a receiver and you can hear the noise level increase, then you don't need a preamp, as it will simply amplify the noise and the signal equally. In general, I've found that to be true. HOWEVER... Let's say that the noise figure of the receiver's internal preamp is 1.5 dB, and the noise figure of the external preamp is 0.5 dB. Each preamp will give you about 10 dB of gain. It seems clear that using the external preamp instead of the internal one would give you another dB of signal+noise to noise. Is my reasoning accurate on that score? If so, then it would be useful to know the noise figure of the K3's internal preamp at 50 MHz. I don't know what it is. Second, in my experience, the gain of the PR6 and the gain of the internal preamp are NOT the same, with the PR6 having more gain -- more gain than I need, actually, given my ambient noise level caused by living in a large metro area. Third, since I live in the shadow of a large number of high-powered VHF broadcasting stations, there is a LOT of powerful out-of-band RF floating around. Thankfully, its effect on 6 meters was vastly decreased when our local Channel 2, 4, and 5 TV stations went to UHF digital. Before that happened, the PR6 would be badly overloaded and create some nasty IMD products on 6 meters unless I put my rather hefty DMI 8-pole bandpass filter in-line with the preamp (on receive only). This cured the overload/IMD problem, but at a cost of about 1 dB of insertion loss. Now that the low-band TV broadcasters have been evicted, I need to try the PR6 again without the DMI filter and see if the PR6 is still overloaded by out-of-band RF. I haven't yet tried that yet. But if I really don't need it, why bother? I just recently put the K3 back in the station setup after having it on the bench for several months with a transmit section problem. Replacing the KPAIO3 board seems to have cured that. Thanks, Elecraft. But now I'm back to the "preamp-or-no-preamp" decision point. Currently, it is set up without the PR6, and since my antenna noise level is at least 3 dB above the receiver MDS even in my most quiet beam direction (NNW), I'm thinking I don't really need to use it. But am I wrong in my thinking? I'd like to hear some knowledgeable comments. Bill W5WVO 73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com 500-KHz/CW, 144-MHz EME, 1296-MHz EME 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
