March 21, 2008
Choosing A Band
I wanted to build a simple transmitter-receiver for portable milliwatting and a "return to basics" project. To keep it simple I decided to use crystal control, and operate near a QRP calling frequency. (The Northern California QRP Club -- NorCal -- sells inexpensive crystals). I had a choice of 40, 30 or 20 M. To help decide I consulted W6ELPRop, an older but still useful propagation-forecasting program.
The rule of thumb for QRP operation is to use the highest-frequency band that's open. These days, most of the time that's 20 M. I wanted to operate portable, meaning my antennas would be relatively low wires with relatively high angles of radiation (takeoff angles). With 1 W or less output, it would be impractical to look for long-haul DX. Yes, it can be done with 1 W and wire antennas, but let's look for a band that will provide many contacts -- and less frustration.
Here is the W6ELProp 20-M prediction for the path from my South Florida QTH to Chicago, for late March 2008, Solar Flux 68.
Stuart, FL to Chicago, March 23, 2008, Solar Flux 68 (20 Meters)
Compare the signal levels and signal-to-noise ratios from 1700 UTC to 2300 UTC to the predictions for 40 M, at the same times and for the same Solar Flux:
Stuart, FL to Chicago, March 23, 2008, Solar Flux 68 (40 Meters)
From those figures, 20 M looks like the band -- or does it? The second-to-last columns in the tables show the takeoff angles at which those signal levels and S/N ratios were calculated. For 20 M, the angle was 13 degrees; for 40 M, 29 degrees. Having already done some antenna modeling, I determined that a dipole or inverted vee at about 50 feet (15 m) would radiate at a takeoff angle considerably higher than 13 degrees. So I went back to W6ELProp and recalculated, assuming a minimum takeoff angle of 32 degrees on 20 M. For a power output of 1 W, W6ELProp could not find a usable signal on 20 M over this path! The Solar Flux must be almost twice as high before the high-angle 20-M antenna becomes useful on this path. Under best-case conditions, 20 M would be the better band; under the conditions available to me, 40 offers the best chances for domestic QSOs as far away as Chicago, Maine or Texas. (Forty Meters works so well during daylight hours now, because solar activity is low. During periods of high solar activity, ionospheric D-region absorption increases dramatically, greatly limiting daytime 40-M propagation. With a solar flux of 200, the daytime Florida-Chicago path is nearly impossible on 40 M. For higher takeoff angles, the path is still poor on 30 and 20 M. Low solar activity is not entirely negative!)
If instead of a dipole or inverted vee, I switched to a quarter-wavelength vertical with elevated radials, I could probably do well into Chicago, Maine and Texas on 20 M. But stringing up a ground plane by yourself usually results in radials fouled on the feedline and much foul language from the operator. A ground-mounted vertical in the South Florida woods, over sandy soil, would be a poor performer.
I've made many stateside and DX QSOs on 20 M from the same location, but using my KX1, which produces 3.5 - 4 W output. My planned 1-W transmitter will be a full 6 dB weaker.
What's best for me is not necessarily best for you. My purpose was to demonstrate the steps to take when planning to operate QRP. If you know which combination of band, antenna and operating times is most likely to produce contacts, you'll get more fun out of QRP.
I hope this information was helpful to you. See you on the calling frequencies!