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

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

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

On Thu, 24 Feb 2005 20:40:27 GMT, Peter Barbella wrote:

I got a note 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.

There's a Real Cult of QRPp'ers - extra, extra low power ops - that try
to achieve ever higher and higher "Millions of Miles per Watt" numbers.
(Of course, BPL will kill them if it rolls out.)
Just imagine _accurately_ measuring power out at the microWatt levels!

This ol' hobby has a lot of interesting nooks and crannies.

gl with your project.
73
Jonesy

Hello Pete .....I worked a fellow on my SMT with 360 mw who was using 16 mw
from his car using a hamstick. He was from the Annapolis MD area and I am
from nr Pittsburgh. The thing that got me was that he was using a 40 meter
Hamstick. Therefore the antenna efficiency at this freq. would be, to say
the least, ..marginal. I don't remember the call or report as it is in my
logs in storage somewhere. I will be retrieving them when the new radio
room goes into operation hopefully in a few months. Give me an e-mail Pete
if you want to try a sked one night ...nothing ventured nothing gained ....

God Bless 73 Tom Popovic KI3R Belle Vernon Pa.

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

On Thu, 24 Feb 2005 07:46:53 -0800, Roy Lewallen
wrote:

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


You don't have an 80 meter parabolic dish in your back yard? (aimed
horizontally . . . )

Maybe with a power line running the 40 miles and some filters . . .

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?

================================

I once worked 10 miles on CW, with 10 milliwatts on 160 meters, in daylight
with my 20 feet long antenna lying on the ground, and a local noise level of
S6. The other fellow had a half-wave dipole running 40 watts. He copied me
OK.

Propagation must have been by groundwave (on account of time of day) partly
over densely populated industrial areas and partly farm land.

But to translate from this information the possibility of acheiving 40 miles
on 80 meters with 7 miliwatts requires some extra information from your end.

It's best to begin calculation with some sort of standard - some facts
which you already know by experiment or hearsay and can reasonably be
depended upon.

Suppose it is already known that with 100 watts on 80 meters, between two
not necessarily identical antennas, of unequal efficiencies, with a radio
path length of 100 miles, gives an S-meter reading of S-9 + 12 dB at one end
of the path.

At 6dB per S-unit, this reading is 2 S-units greater than S-9. If you like,
call it S-11.

Suppose the power of the transmitter at the other end goes QRP and is turned
down to 10 milliwatts. This is 1/10,000 of the original power, or 40dB down,
or 40/6 = 6.7 S-units smaller in received signal strength.

S-11 minus S-6.7 = S-4.3

So provided the noise level is less than about S-4, the 10 milliwatt
transmission will be fairly easily copied at a distance of 100 miles. Which
is now not so ridiculous as it may first sound without some thought being
given to it.
----
Reg, G4FGQ

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!"
E-mail AntiSpam: remove ##

"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.

...

"Peter Barbella" wrote in message
news:2xkTd.30756$ya6.20070@trndny01...
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


Guys,

With this post, I have received several useful suggestions and comments.
Thank you very much.

Many of you are curious about the assumptions I made when I made my
calculation of 40 miles. I simply went with something I know: the radar
equation. I made a simple adaptation to the radar equation.

It starts with a simple premise that the power in my transmitter will be
radiated equally in all directions. That is equivalent to a zero DB gain
antenna. I assumed that the receiver will have a sensitivity of -80 dBm. I
then, simply applied the basic fact that power diminishes as the square of
the range. That leaves a power density at any given range of P divided by 4
pie R*2. (Forgive the lack of an equation editor)

The next assumption involved the capture area of the receiving antenna. In
radar, the capture area of an antenna is proportional to the wavelength
squared. With a zero DB receiving antenna, the capture area would be simply
lambda squared divided by four pie.

With those simple assumptions you can determine the amount of power
available for a receiver. Since my little transmitter is a simple
breadboard with three transistors and a wire that runs out of my house and
up a maple tree, I made a further assumption that I have about 10 DB of loss
delivering my transmitter power to the antenna.

That 10 DB loss along with the assumption of only minus 80 dBm sensitivity
in the receiver seemed pretty conservative to me. Most receivers can
probably do much better, especially with CW operation.

Of course, as RegEdwards has pointed out, what is really needed is a good
benchmark. Several of you provided me with some interesting benchmarks.
Thank you very much for your replies.

Regards,
Peter Barbella
KB1LZH




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