VK3YE amateur radio pages

What's possible with QRP

 

All material on this site (c) Peter Parker VK3YE
1997 - 2014.

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pedestrian mobile pedestrian mobile pedestrian mobile

What's possible with QRP and what's regular with QRP are two different things.

You might read about some amazing contact done with 100 milliwatts to a wet noodle. Sure it might have happened but the frequency, location and path were probably just right. And the other guy was probably using the best equipment to a massive directional receive antenna in the best location. Leaving out this 'fine print' can cause some to give up if their experiences don't align with what others achieved. And it isn't beyond the realms of possibility that QRPers are a bit like gamblers; they only talk about their wins, not their losses (or the hours of fruitless calling).

What's possible

The following, some of my best contacts with QRP, illustrates what's possible.

* VK6 to Japan. 500mW crystal controlled CW with a G5RV antenna on 50 MHz. This was at the top of one of the best solar cycles (1991-92). My contact was also likely using an excellent antenna.

* VK3 to W8JI. 5w on 1.8 MHz. I was located at a bayside location. W8JI is known for his top-notch 160 metre station, particularly on receive.

* VK3 to Europe (several stations). 5w to a 40cm magnetic loop on 28 MHz whilst pedestrian mobile. I was beside a salt water bay. All stations would have been using significant power to substantial antennas.

* VK3 to VK1 (400km+). 5w to a 4el yagi on 144 MHz SSB. An excellent station at the other end, some local height and aircraft flying overhead at the right time are all required here.

These contacts are not routine. Although some might be with particuarly well equipped stations. And, in the aircraft enhancement case, good timing was required.

What's regular

But even without these advantages 'full-time' QRP can be quite satisfying and provide many comfortable contacts. The following could be considered contacts routinely achieveable with QRP.

* 1.8 MHz daytime contacts up to about 50km with 5 or 10 watts. Requires a reasonable antenna. The same distances may be possible with an inferior antenna but only to stations with low noise on receive.

* 3.5 MHz contacts out to about 300 or 400km during the day and somewhat greater at night. 2500km contacts are not rare but require efficient antennas.

* 7 MHz contacts out to about 800km during the day and 2500km at night. The longer distances are much easier to work than 3.5 MHz. Portable stations in low noise locations may do better. Daytime distances are somewhat shorter during summer and longer during winter.

* 10 MHz contacts out to about 1000km during the day. There are a lot of times on the Melbourne - Sydney path when noon signals are marginal on 7 MHz but solid on 10 MHz. Longer distances should be somewhat easier on 10 than 7 MHz (including 15000 km contacts around the greyline times) but the band can be volatile. Very short distances are much less workable on 10 (and higher) than 7 MHz during the day.

* 14 - 28 MHz contacts in the 1000 - 3000km range should be fairly common (in theory). However it depends on where they have their antenna turned towards and whether they are concentrating on DX contacts. A good test of a QRP station (in Australia) is whether it can make regular contacts into Europe or North America. If so then it is probably reasonable. An even better test is whether the stations worked are using modest antennas including simple verticals and dipoles.

* 50 MHz contacts up to several tens of kilometres. 500 - 1500 km sporadic E contacts should be readily possible during midsummer.

* 144 - 432 MHz contacts on SSB should be common out to about 100 - 150 km with very basic antennas. Add some height and antenna gain and the workable distance quickly goes out to 200 - 300km dependent on the other station's location and station. For example aircraft enhancement contacts with well equipped stations may be possible at around 400 - 500km distance provided timing is right.

The above applies to a range of modes such as SSB, SSTV, PSK31 and CW. Even better results should be possible with weak signal digital modes such as WSPR.

Demonstrations of QRP

The power of 3dB

Important to successful QRP is understanding the effect of changing power at various signal to noise ratios encountered on the bands.

If the signal is strong 3dB is hardly noticeable. If it's buried in noise it could be decisive.

Here's my reply to someone who was comparing a 3dB difference in radio signals with the effect of turning down a stereo to put out 10w instead of 20w.

The 10 - 20w audio difference is akin to comparing a signal that's s9+40 with one that's s9+37 dB with little ambient noise. There is little difference to the signal to noise ratio as the power is dropped so it's hardly noticeable. The FCC rule that requires US amateurs to use the minimum power practical for comfortable communication is bound in this logic.

But what if you're near the margins of readability which is where most of us work (especially if QRP)?

While it's less obvious than for FM transmissions, there is a threshold effect on HF where changes in power (either up or down) has a disproportional effect to your communications effectiveness. It's more significant at certain power levels than others. For HF SSB up to about 3000km, most contacts possible with 100w will also be possible with 10w (which is why people rave about the performance of the KX3). But go down to 1w and your success rate drops enormously unless your antenna/location is well above average.

If your signal is buried in band noise or amongst adjacent QRM then a 3dB increase is the difference between your signal being buried in the noise (and barely readable) and twice as strong as the noise (and readable). Or:

* Between not being heard and having perceptible signals to elicit a 'QRZ?' from the other guy

* Between hearing the occasional word and getting enough through for most words to be copied.

* Between difficult/painful copy and quite good copy that makes the contact a pleasure for the other operator.

In noisy 'real world' operating conditions with QRP a 3dB difference is significant especially if competing with noise.

To conclude, I wouldn't go throwing 3dB away in a hurry unless you are getting substantial gains in low cost, simplicity, small size and antenna lightness in return.