QRP vs a flashlight

 

Jeff Smith   VE1ZAC

 

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 Texas and Colorado and working them to score points. I described how difficult it was, and finding them both was an accomplishment.

 

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 Halifax, to communicate with a similar flashlight owner in Texas”.  Well, that generated interest and I promised to provide a quick analysis of my comment.

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 Texas. It has about 12 dB of forward gain, which means most of the energy (not all) heads in the direction I want and broadly illuminates an area that includes the intended fox location. I have about 100 feet of RG8X going to this antenna through a series of switches and protection gadgets, all of which rob some the signal from getting to the antenna. It’s also safe to assume the antenna is well matched to the coax since the Stepir is continuously adjustable for just this purpose, among other things. Using a few calculators I estimate the transmission line loss is about .7 dB. I have lots of PL259 style connections to deal with, and estimate a further 1.5 dB loss because of them. Further, the antenna, as a transducer is not 100% efficient, and I estimate .7 db loss in the antenna. ( I will ignore the further loss of signal leaking out of the back of the antenna… but there is signal there, of course) The total accumulated loss is approximately 3 dB. That means I have (maybe) 2.5 watts of energy to radiate towards Texas and Colorado.  The flashlight analogy ?  Well a bright flashlight with a modern bulb (non LED type) can be in the 1 to 2 watt range. So lets say we take a flashlight and try and signal somebody in Texas with it.  There are no terrestrial conditions where that is going to happen !  ( Maybe in the vacuum of space you could see the flashlight from that distance ?)

 

Now, lets move on to the radio path to Texas. We need a little more info. The fox hunts always start at 0100 UTC which means darkness in Nova Scotia and dusk in the South West. We are in a sunspot minimum and the solar flux conditions right now are about 66. This number is about as low as it goes. The geomagnetic field for this week was quiet, so that doesn’t come into play very much. Our frequency of choice is 14.055 MHz. That is the upper end of the CW portion of the 20M amateur band. The distance to Texas, as the crow flies, is about 3100 KM.

 

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 Texas. That means we have to reflect off an ionosphere F layer, and then to ground, and back up and down again to make the trip. Each reflection weakens the signal further. Note that the ground can be bad to awful for reflecting and the F layer isn’t going to be a great help because of the low sunspot count and the low MUF. Anyhow, the software gives us a predicted receive signal strength from a typical 100 watt signal  aimed at the receiving location from the specified transmitter location. In our case, it’s “32 D”. This means, upon investigation, 32 dB greater than .5 uV at the receiver antenna, and D means it is likely to be true only 1 to 25% of the time. In other words, the odds aren’t good. (propagation predicting is not an exact science)  This connection works both ways of course.

 

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 Halifax is expected to be about S2 to 3 on the receiver. ( Assume 1 S unit is 6 dB… not always so) And, there is only a 25% chance, at best, of the signal being that good at the time we wish to operate. I can’t speak for Texas last Tuesday night, but we had at least S2 of noise on the band and it was very very hard to pick out the signal from the other end. However, enough information was exchanged correctly to count as a completed contact. ( this isn’t supposed to be easy !) And in fact, during the 1  1/2 hour operating period, I could only hear both fox stations at an intelligible level for, maybe, a third of the time. W6EL's propagation program was pretty accurate, it turns out.

 

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 !

 

Enjoy !

 

Jeff Smith, VE1ZAC

 

Note: the above is not a "Rigorous" analysis. It's more of a "Back of envelope doodle" analysis.