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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