On Fri, 25 Feb 2005 11:53:52 -0500, "xpyttl"
wrote:

"Spajky" wrote in message

IMHO not, (practically maximum about 10x less probably with good

The current record on 80 meters is 546.8 miles with 0.0406 mW.

with just probably 40mW IMHO (not 40 microW)

Incredible distance, must had ben hell of a reciever & antennas &
probably a reflected wave ... wow if this is true .. even for very
narrowBand non-modulated carrier CW

BTW, that my simplified formula I calculated for Phono work, with CW
the distances achieved normally I know are few times greater ...
--
Regards , SPAJKY ®
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
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Unless you have an incredibly poor antenna or an incredibly poor
receiver, you'll never hear the receiver noise on 80 meters because the
atmospheric noise is much greater. There are a number of sources of this
noise, man-made and natural, and its level varies a great deal. It's not
the same as loss, and although it's a "receive end" problem as you say,
it's nothing you can improve by altering the receiver. There's just a
little about it in the ARRL Handbook, but I'm sure a web search will
reveal a great deal of additional information.

Apparently you assumed that the atmospheric noise is zero. That's not a
valid assumption at HF.

I still wouldn't be surprised at a 40 mile contact with 7 mW however.

Roy Lewallen, W7EL

Peter Barbella wrote:
Hi Roy,

I made no assumptions about the atmosphere, neither loss, nor noise.

I did assume that my antenna is about 0 dB gain and I further assumed about
10 dB loss for the temporary wire running through the wall of my house to
the antenna (just a simple piece of wire running up a maple tree). I
thought the 10 dB would be generous.

I assumed that the antenna of the person receiving was also 0 dB and I
assumed that his receiver would have a sensitivity of about -80 dBm. I
think that's pretty conservative. I would guess that modern receivers go
well under -100 dBm for sensitivity; especially for CW operation.

As I said, I assumed nothing about atmospheric loss. I'm not quite sure
what atmospheric "noise" is. Where does that come from? Is it what you
call atmospheric loss, or is it something else? In any event, I would
consider that to be a "receive end" problem.

I got a not from someone who worked a 500 mile contact with another QRP
crazy who was using 50 micro (not milli, micro) watts. That's amazing. It
gives me hope that I'll get my first contact soon.

Thanks for the reply.

Peter Barbella
KB1LZH


"Roy Lewallen" wrote in message
...

I don't have experience with a power level that low, but I'm curious about
what assumptions you made about atmospheric noise and antenna gain to
arrive at the 40 mile figure.

Roy Lewallen, W7EL

Peter Barbella wrote:

I've built a little 80 meter transmitter that seems to be putting out
about 7 milliwatts. By my caculation, that should be good for about 40
miles. Anyone have any experience with low a power output?

Pete
KB1LZH

"Spajky" wrote in message
...

with just probably 40mW IMHO (not 40 microW)

Yeah, 40 microwatts, really.

I probably should hasten to add that this was not a random QSO. The
transmitting station has been sending a beacon at various times,
frequencies, and power levels for several weeks. There have probably been
several hundred stations listening for that beacon. While this particular
one was the best on 80 meters, it really isn't all that much of an outlier.
Plenty of other stations were in the same general neighborhood on other
nights.

It did surprise me to see this on 80, though. As Roy points out, the
absence of atmospheric noise is not a valid assumption on HF, and noise does
increase with decreasing frequency. This is exactly the opposite to your
radar experience where it is all about receivernoise. Even at 10 MHz, the
most basic receiver will be sufficiently sensitive that it is limited by
atmospheric noise. This seems to be a particularly bad winter for noise on
80, although there have been occasional evenings when the conditions have
been pretty astonishing.

Also, the noise causes in the lower HF region tend to be somewhat different
than those in the upper HF. Paul Harden made a couple of nice postings to
QRP-L explaining some of that after that quasar whacked us back in December
I think it was. Interesting to note that the news media only picked up on
that event last week. Pretty astonishing, though, that something millions
of light years away could be so energetic as to shut down 80 meters for a
couple of days!

By the way, I see that Paul will be speaking at Dayton (FDIM) this year. I
was planning to go anyway, but it would be worth the trip just to hear him.
This guy *really* understands what space weather does to our propagation.

...

On Sat, 26 Feb 2005 10:29:29 -0500, "xpyttl"
wrote:

It did surprise me to see this on 80, though. As Roy points out, the
absence of atmospheric noise is not a valid assumption on HF, and noise does
increase with decreasing frequency.

However, the noise power is proportional to bandwidth, so if the
throughput rate is not defined, slow down the bit rate to reduce the
required receiver bandwidth, until the noise is weaker than the
signal. While the lower HF noise is not pure white noise, this
principle can still be applied.

For instance on the 135 kHz LF band, it is quite common to use QRSS
with a dot time of several seconds or even a minute and the reception
is done visually on the "Waterfall" (spectrogram) display. Although
the LF transmitter power might be several hundred watts, the
transmitter antenna efficiency is usually much worse than -30 dB, so
the radiated power is well below 1 Werp and the noise level is much
higher than on HF.

Paul OH3LWR

On Sat, 26 Feb 2005 10:29:29 -0500, "xpyttl"
wrote:

Yeah, 40 microwatts, really.

I probably should hasten to add that this was not a random QSO. The
transmitting station has been sending a beacon at various times,
frequencies, and power levels for several weeks. There have probably been
several hundred stations listening for that beacon.

oh, that was the case ...

.... well, I recalculated reached distance for usuable everydays
connection in real normal conditions with average equipment
/forgetting the extremes!/ (after some additional thinking reading
these posts) to even simplier formula than before & more realistic (a
bit larger distances reached) as follows:

to reach the distance of ONE full wavelenght (ex.80m for 3,75MHz) you
approx. need Rf RMS voltage on TX out for antenna (50ohm) :

1mV - for narrowBand unmod.carr.CW
3,16mV - for ordinary CW & SSB (+10dB)
10mV !! - for ordinary FM & AM or other (another +10dB = +20dB)
30mV - for PacketRadio - for no data loss (+ another 10dB)

.... so original poster (Peter) could get out of his set normally 5Km
for sure (more than 3 miles with his 7mW-atts).

IMHO this is some kind of reasonable expectations about recieving
range ... bye .. :-)
--
Regards , SPAJKY ®
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
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