On Thu, 24 Feb 2005 13:21:02 GMT, "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.

IMHO not, (practically maximum about 10x less probably with good
simple antennas & RX-ver & not too much noise & disturbances & w/o big
obstacles & using narrowBand CW modulation for solid reception of
direct surface wave ...

http://www.radioinnovation.com/Howto/how_far.htm

if your Qrp TX puts to antenna 7mW /0.6Vrms-50ohm/ & an average RX has
sensitivity around 0.6microVolts/50ohm, there is difference in
voltages about 1 million times (120db) !

for a 40 mile distance you would need approx. a 1/2W TX or even more
IMHO @ your 3,75MHz TX! you can gain something with both side very
good antennas ...

I am not really an expert in the matter, but I was also searching for
years for some kind of "idiot proof" but simple (not complicated math)
formula for receiving range but for higher frequencies like 40MHz or
so .. Since the recieving of a signal is proportional with voltages on
antenna (& reverse proportional to rise of frequencies) I time ago
derived/simplified a lot the math formula (confirmed myself by some
experiments I made).

you need on TX antenna /Vrms=Eff@50ohm/ minimum:

50mV/1km/1MHz [simpliest as possible formula!] for 1/4 wavelenght
or such/similar gain antennas.

that means in your case 0,19V/1km (@3,75MHz) & since you have 0,6Vrms
on TX output means that can you reach normally 3-4km (2-3 miles!)

But I could be also wrong ... :-)

To enhance reception double the TX output voltage/power or make more
efficient antennas. Thats easier with higher frequencies to make high
gain ones & not to be really monster big on big towers ...

PS.: another simplified answer derived from above simplified formula:
your 7mw TX has a range (@80m wavelenght) for about 40x the wavelenght
receipt with an ordinary common RX .. :-) ....

hope it was enough simple explanation ...
--
Regards , SPAJKY ®
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
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"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. By
comparison, your transmitter is QRO. Of course, that was with good
antennas. The transmitter was an off-center fed dipole, the receiver a 1000
ft. beverage.

80 meters is a pretty QRP-unfriendly band. But on a cold winter night when
the sun is behaving, it can be pretty good.

You might want to rummage around on the archives of QRP-L
http://mailman.qth.net/pipermail/qrp-l/
There are a fair number of folks fooling with this kind of setup.

...

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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"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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