QRP vs a flashlight
I work with a lot
of science folks in a federal government laboratory. Some of them are aware of
my radio hobby and occasionally ask questions about recent activities. I am never sure if they are just being
polite, but I am always willing to give an enthusiastic response. I participate
in the QRP fox hunts that go on Tuesday and Thursday nights. In the summer,
it’s Tuesday nights and the activity is on 20M only. So..
this week, the answer to the question involved an
explanation of the fox hunts and my satisfaction on finding both foxes in
One of the science
folks asked what the big deal was… a very short radio CW contact in the South West US ? I thought about that a moment and said” Look…
it’s 5 watts and very difficult conditions. Power wise, it’s about the same as using a
small flashlight , from the ground in
Here it is:
Let’s start with
the power. QRP, to the fox hunting crowd means 5 watts or less as
measured at the output of you transmitter. That’s not what is coming from the
antenna, of course !
I use a 2 element Yagi-Uda antenna made by Stepir that I can point in the direction of
Now, lets move on
to the radio path to
OK, what do we do with this stuff ? Well, first off, we load up a program that can predict the ionospheric propagation for these conditions between these two points. There are all kinds of programs for this but an easy to use and free one is “W6EL Prop” from Sheldon, W6EL. You input the sun and geomagnetic data, the date and the “Terminal” locations and it calculates a bunch of stuff based on a complex model used by governments and broadcasters to predict exactly these sorts of things.
The first thing it tells us, is the maximum MUF (maximum useable frequency) for the entire day is 12.6 MHz, and that corresponds to 0100 UTC coincidentally. So, how come we are working on 14 MHZ. ? Well, that is the band the QRP fox hunts work on, and that’s that. The MUF being below us doesn’t mean we can’t communicate, it just means it’s going to be hard, or might not happen at all.
Next, we look at
the physics of the hops required for the trip, and it turns out we need two
F layer hops to get our puny signal to
Now we can add some other stuff. I know I am only getting 2.5 watts to my antenna. This is -16 db from 100 watts. So, we know the POWER is reduced 16 dB from the signal referenced to 100 watts. Since the predicted signal level was 32 dB referenced to .5 uV with the 100 watt signal and a common receiver input impedance, our new predicted level is 32-16=16 dB referenced to .5 uV. ( 1 uV is one millionth of a volt… a pretty small signal) This is a reasonable assumption based on the "System" input and output.
If we assume .5 uV is the same as a S meter reading of 0 (
another reasonable assumption), then our signal from
Some statistics: We made a contact with 2.5 watts over 3100 kM. Therefore, the energy per KM is .00008 Watts. For a bright flashlight, one might be able to see it 10 KM away under good conditions, which is .25 watts per KM. The radio signal is 312 times more efficient !
Next time someone asks you what the big deal is in making a low power contact like this, tell them about the flashlight alternative ! It’s something most people will understand right away. Another interesting fact is that this contact was made with out any expensive telecom infrastructure like fiber optics, copper wire, or microwaves being used. So the economic comparison is pretty lopsided as well.
This puts a new “Light” on low power radio communications !
Jeff Smith, VE1ZAC
Note: the above is not a "Rigorous" analysis. It's more of a "Back of envelope doodle" analysis.