Home |
Search |
Today's Posts |
#9
|
|||
|
|||
RRS - The Topic is Shortwave and the Goal is to Talk About (Write About) Shortwave related things . . .
Telamon wrote: Very good Terry I don't see any reason for a flames. You post good information and real world experiences. What you posted above is a good example of what people reading the news group need to see in order to make educated decisions about antenna systems. I just add that the natural noise floor is not flat and Terry's examples above are at some particular SW frequency. If I see a good natural noise figure chart link on the web I'll post it here. Well, I'll just add more comment. What the information above alludes too and that is as you decrease the bandwidth of your receiver the noise floor should go down. Narrowing the bandwidth decreases the amount of spectrum that goes through the receiver and that lowers the total background noise at the receiver output. You will notice that Terry references the filter bandwidth in his examples. -- Telamon Ventura, California --------------------------------- The last time I ventered an opinion on splitters dxAce and others took exception to my logic. So I decided to do some actual research. I expected violent disagrement. At first glance it does seem that adding 3.2dB of attidional attenuation in the signal path would have a clear negative effect on the reception of weak signals. When I first noticed this by direct experimentation, I was dubious. But after several tests with the weakest signal I could receive I decided it wasn't a fluke. After getting flamed, I wondered if I was misreading or misunderstanding what was really happening. So I decided that while the math showed the logic was correct, I wanted real world data. At about this same time I became convinced that the standard audio chain in the typical SW was at best "crap". I had doubted the observations at Kiwa about capacitors improving intelligibility of weak signals. I found that by pulling the audio straight off the detector diode and feeding it to a clean MSOFET amp, stations that were un-intelligible "mush" were clearer. I played with better AM detectors wanting to avoid the hassle of a synchronous detector, but in the end bowed to reality. I built an outboard synch detector, and with each improvement I found I could dig out weaker, or at least "mushier", stations. The remote ISM beacon started out as a SSB unit, but due to some drift after 3 months of operation I converted it to a very linear AM unit. Nothing fancy, I was given a Rio PMP300 with a 64Meg memory card. The whole project was based on the remote ISM test set a Radio Astronomer friend had used. But I wanted real voices signals, not "random" noise. After all my experiments I came back around to the observation that in most cases, the local man made noise floor sets the limit on reception. Better detectors allow you to understand slightly weaker signals. But the improvement, while very worth while, is at best maybe 6dB. And that 6dB is really optimistic under ideal situations. I had a AOR7030+ in my care for a 5 weeks while our friend arranged shipping to NZ. The AOR7030+ is a much better receiver then my R2000. And I would give a left whatever to own one. But inspite of the better selection of IF filters, a quiter RF/IF section and greatly improved detection and audio chain, there were very few signals that I could understand on the AOR and not on the R2000 with the added InRad 2.1KHz SSB filter. It has been an odd autum. We have had 3 power outages since the 1st of Oct and I have been home to listen through all three of them. The last one lasted from 8:00PM until 5AM. Needless to say I missed work the next day. It never ceases to amaze me how much the low level AC grunge covers, and how many very weak signals you can receive when the lights are out. I really enjoyed the last, long lights out event as I could compare my R2000s to the AOR. The AOR is a much better receiver, if you have a low enough local noise floor. Under these conditions the superority of the AOR stood out. It wasn't that I couldn't dig out most of the signals on the R2000, it was just a LOT harder. I am not complaining mind you, it was educational to see what a first class receiver can do. It also helped me decide to take a serious look at selling our place rolling the excess back into TIAA and moving to the sticks. The main advantage a synchronous detector offers is the reduced ear fatigue. With a standard diode detector I found that it felt like my head clogged up after listening to weak, nasty stations. The other advantage is the music we recover is mush cleaner. My wife plays flute and after listening to some flute music on a Peruvean station she admited that was the "cleanest SW I have heard. It sounded like the flutest was in the room with us." She had doubted the practicality of my summer long experiments. She had enjoyed SW but thought that distant stations had to sound "bad". But since it was the hottest summer we have had, we weren't going outside that much.s she thought it made more sense for me to play detector then antenna builder. She uses "my" gear in the day when I am not at home and I had to give her the 3rd R2000 I traded for so she can listen to what she wants when I am at home. The last R2000 has the AD607 synch detector mounted inside with a homebrew 5W mosfet AF amp. It sounds pretty darn sweet. Terry |