WSPR with 10 milliwatt

QRSS needs low power

I regularly visit the WSPR Old Database to see, what is going on with very low power. It is striking every time, that almost all stations use much power. But then I’m not talking about 10 watts, for which many people pull up their eyebrows and  shake their head. But his is rather innocent, because luckily there are not that many operators, that use 10 watts. No, many stations use 5 watts or 200 milliwatts. Many operators believe, probably due to unfamiliarity with QRSS, that 5 watts is always QRP. But of course, that is not the case.

QRP with WSPR is 12 milliwatt or less of course

10 milliwatt is QRP with QRSS such as WSPR

WSPR is a QRSS system, which uses a 400 times narrower bandwidth than SSB, due to the low bit rate. Therefore, the power must be reduced by the same factor to transmit a signal with a comparable strength. A WSPR signal with a power of 10 milliwatts has the same power density as an SSB signal of 4 watts. So 10 mW is a good power for a WSPR propagation beacon.

Very good propagation

From my experience with low and very low power (in the BQC marathon), I speak of very good propagation, if CW QSOs with 200 mW or less are possible. A CW Station with 50 W then continues with S9. CW with 200 mW can be compared with a WSPR signal of 10 mW. A WSPR station with a power of 10 mW is thus only spotted if the propagation is very good.

To derive the propagation from WSPR spots, which are made with different power, I calculate the Lowest Possible Power (LPP) of the spots.

A spot with an SNR of -29 dB is received without errors. If a station with a power of 10 mW is spotted with an SNR of -19 dB, that signal is 10 dB stronger than is necessary for a “solid copy” with an SNR of -29 dB. The power could thus be 10 dB lower. The (calculated) lowest possible power is then 1 mW. Does WSPR with 10 milliwatts seem too low? Then just keep reading, to see what the LPP diagram shows.

Lowest Possible Power diagram

In order to determine the calculated Lowest Possible Power, but without having to calculate for myself, I developed the Lowest Possible Power diagram (LPP diagram). The diagram allows you to determine the lowest possible power from the SNR of the spot via the diagonal line of the used power.
The lower the Lowest Possible Power, the better the propagation.

From the value of the SNR you go to the right until the diagonal line, indicating the power. The spot is plotted on a diagonal line, corresponding to the power used, to the height of the SNR. And then you read the lowest possible power on the horizontal axis. The values on the horizontal axis are indicated in milliwatts.

Lowest Possible Power diagram

Lowest Possible Power diagram

The green dot shows a spot of a station with 10 mW, that is received with an SNR of -22 dB. The signal is 7 dB (5x) stronger than necessary to be spotted. The lowest possible power for this spot is 10/5 = 2 mW. A WSPR signal of 2 mW can be compared with a CW signal of 40 mW, which is received with 559. Hi.

The yellow dot shows a spot of a station with 50 mW, which is received with an SNR of -15 dB. The propagation of both stations is the same because the lowest possible power is the same.

Attentive readers have already noticed, that for all the spots the propagation is the same. The LPP diagram mercilessly shows the difference between the subtle signal of a WSPR station with 10 mW and the (unnecessarily) 27 dB stronger station with 5 watts.

WSPR with 10 milliwatts

To show that even 200 milliwatts is too much for very good propagation, I show two LPP diagrams of two different time slots. A station with 10 mW from 9A, indicated with a yellow dot, is active in both time slots. The signals in both diagrams show spots received by a station in DL at 1100 km from the station with 10 mW. I have chosen these two time slots, because the diagrams show that with very good propagation, with 10 mW it is possible to work well, while there are also much stronger stations in the 200 Hz wide WSPR band. The band is 7 MHz.

Lowest Possible Power diagram

Lowest Possible Power diagram

In the first time slot a large number of stations is active (8). The spot with a power of 5 W and an SNR of +11 dB has the strongest signal. This spot also exhibits the best propagation, with the Lowest Possible Power of 0.5 milliwatts. The spot with 200 mW and an SNR of -6 dB and the spot of 10 mW, both have a Lowest Possible Power of 1 mW. The propagation is the same for these two spots. The 7 spots on the right of the 10 mW line reveal that the propagation is very good. After all, they are in the light gray area with the designation: Very good propagation. The LPP diagram also shows, at a glance, that for the 6 spots, from 200 mW to 5 W, a power of 10 mW had been sufficient. Nice diagram!

Lowest Possible Power diagram

Lowest Possible Power diagram

Only four signals are received in the second time slot. The spot with 2 W and an SNR of +4 dB and the spot of 10 mW and an SNR of -19 dB, both have the lowest possible power of 1 mW. Both spots have the same propagation. If the station with 2 W reduces the power to 10 mW, the signal is received with an SNR of -19 dB.

Clutter spots

I consider spots with 5 W as “Clutter spots”. A WSPR signal with 5 W can be compared to 2 kW with SSB. There are already spots, if the propagation is “next to nothing”. And when finally the propagation becomes interesting, the stations that use 10 mW can be over shouted.
If a WSPR station is spotted with 5 W with a SNR of -29 dB, then this station must use at least 2 kW to make a QSO. Spots with 5 W only show very good propagation, with an SNR of 0 dB or more. But they will leave little room in the dynamic range of the receiver for spots with 10 mW.

Look in a different way

But let’s be not too dramatically, because with the LPP diagram you have a fine means to compare the propagation of spots made with different power. Just when the power is actually too actually too high. Hi. The Lowest Possible Power diagram allows you to compare spots without any calculation.

As I scroll through the table in the WSPR Database, I look for stations that send with 10 milliwatts. There are not that many.

But I also watch stations with 1 W or more, that are received with an SNR of 0 dB or higher. These stations also reveal very good propagation. Using CTRL F and searching for +, all plus signs in the column of the SNR light up. So you can quickly recognize the strong signals.
A spot with an SNR of +1 dB has the lowest possible power, which is 1000x lower than the power used. So with a power of 5 W the lowest possible power is 5 milliwatts. That calculates very easily.

Have fun during your next visit to the WSPR Database.

Propagation beacons

Propagation beacons use a power, that is much lower, than the power that is used in QSO’s. This is a good practice. By doing so, the beacon is only received, when the propagation is good or very good. A power of 10 milliwatt is an excellent choice for WSPR.

All WSPR stations decode several WSPR signals simultaneously. So a station of 10 milliwatt can easily be OVER SHOUTED by a station with 5 watt.

Of course, it takes patience, effort and there for courage, to use low power. But you will be rewarded with interesting spots

WSPR with 10 milliwatt is an excellent choice. Please notice that even 100 milliwatt is too strong. WSPR with 100 milliwatt is like using QRO in the beacon band.

WSPR with more than 100 milliwatt actually causing QRM.

Please notice that a WSPR signal with 5 W is 27 dB stronger than a signal of 10 mW.

This can reduce the dynamic range of a distant receiver unnecessary (with 27 dB). So there will be little room for signals of 10 milliwatt in the distant receivers.

Using 5 watt can have the same effect on distant receivers as a high level of local noise.

WSPR 10 mW

WSPR 10 mW shows interesting propagation

CW Propagation beacons vs WSPR

When the propagation is very good a signal of 50 W will be received with 599 and a CW beacon with 200 mW will also be received, but with 559. From the perspective of a propagation beacon, the signal with the lower power is the most interesting, because this signal is only received when the propagation is very good.

From 50 W to 200 mW

Reducing from 50 W to 200 mW gives a difference of 4 S-points of 6 dB

WSPR with 10 mW

A CW beacon with a power of 200 mW is very interesting and can be compared with a WSPR beacon with 10 mW. Because WSPR is about 20 times as slow as CW, the power of your WSPR station needs also be 20 times lower. This is 13 dB lower.  (See table)

A power of 10 mW is very interesting for a WSPR beacon.

So please use the power in the green fields of the table for WSPR beacons.

You will not always be spotted with very low power, but for another reason then than lack of propagation. hi.

But when you are spotted, it is at least a very interesting spot.

CW beacon vs WSPR

Useful power of a WSPR beacon


This diagram can be used to compare the power with WSPR, CW and SSB.

WSPR Beacon power

Power with WSPR as a beacon

WSPR Beacon

The power of 10 milliwatt is an excellent power for a WSPR beacon. WSPR with  10 mW will reveal good and excellent propagation.

Used with 10 milliwatt, the WSPR beacon will not over shout other stations, when the propagation is reasonable to poor.

With 10 mW you will not be spotted, when the propagation is very poor, so you will not make clutter spots. Hi.

Notice that only very low power signals, can reveal excellent propagation.

Propagation beacons

For propagation beacons that use CW to give their call-sign, a power level of 5 watt is most frequently used and provide perfectly acceptable signals. Even milliwatt transmissions can be useful, though these are easily overlooked.


The beacons of the NCDXF/IARU International Beacon Project  offer the possibility to judge the signal strength. A transmission consists of the call-sign of the beacon sent at 22 words per minute followed by four one-second dashes.

The call-sign and the first dash are sent at 100 watts. The remaining dashes are sent at 10 watts, 1 watt and 100 milliwatts.

Listen and experience

Experience the dynamic range of the propagation.

Listen to the recordings of the beacons to HEAR the difference in propagation for yourself.

The recording in which you can hear all four dashes, shows a dynamic range of 30 dB. 3 Changes of 10 dB. hi.

Please notice, that the 4th dash is the most interesting dash.

Play the recordings again to notice, what the back ground noise does.

It’s remarkable to hear the propagation in just 4 dashes.


See this interesting analysis of WSPR spots with 200 mW down to 1 milliwatt, that are made in 2014, by a station that reduced the power, in intervals of 24 hours, from day to day.

WSPR 200 mW to 1 mW

Power reduced from day to day

640 Spots were made with 200 mW. Only in 32 spots the full power of 200 mW was needed. Many (111) and  (98) spots that were made with 200 mW, could have been made with 5 mW and 10 mW.

This analysis shows the dynamic of the propagation. If you look at the figures in red, it is obvious that many spots that were made with 200 mW, 100 mW and 50 mW could have been made with 10 mW or even 5 mW.

A number of spots that were made with 5 mW, could have been made with 1 mW or even 0.5 mW.


The changes in propagation can be enormous. The SNR gives a good indication of  the strength of the WSPR spot.

A WSPR spot with a SNR of -29 dB is a solid copy.

The stronger the signal, the higher the SNR will be.

With a SNR of -19 dB, the signal is 10 dB stronger.
With a SNR of  -9 dB, the received signal is even 20 dB stronger, than necessary for a solid copy.

The line of 200 mW in the table shows, that when the SNR is -19 dB, you can reduce your power from 200 mW down to 20 mW and still be spotted. The spot will have a SNR of -29 dB. So the “Calculated lowest possible power” of this spot is 20 mW.

When the SNR is -9 dB, you can even be spotted with 2 mW. So the calculated lowest possible power of this spot is 2 mW. WOW

WSPR Lowest possible power

Just use 10 mW or even 1 milliwatt.

Consider to reduce your power, when you notice that in half of the spots the SNR gets better than -14 dB.

Is WSPR weak signal?

Not with 5 W!

Please notice, that a signal with a very low transmission speed like a WSPR signal, is actually a very  STRONG signal, because of it’s very small bandwidth.

Because of the long duration of each bit, the energy in each bit is huge.

A WSPR signal with a power of 5 watt can be compared with a SSB signal of 2 kilowatt.
(No offence. Please read further)

A WSPR signal of 5 watt and a SSB signal of 2 kilowatt, both have the same Power Spectral Density.

Power (Spectral) Density

WSPR with 10 mW is QRP

WSPR with 10 milliwatt or less
can be compared with SSB with QRP

200mW WSPR vs 80 W SSB

WSPR with 200 milliwatt can be compared with SSB with 80 W

Power density

P = Pd x B

P = Power in W
B = Bandwidth in Hz
Pd = Power density in W/Hz

Small bandwidth

WSPR is very slow. It takes minutes to transmit the characters, that in phone would take you just a few seconds.

Because WSPR is very slow, it uses a very small bandwidth. The bandwidth of WSPR is much smaller, than the bandwidth of a SSB signal. The difference in bandwidth is hugh. (400x)

A signal with a smaller bandwidth, needs less power. The smaller the bandwidth, the lower the power must be.

WSPR with low power

WSPR uses a bandwidth of about 6 Hz, which is 400x smaller than the bandwidth of about 2400 Hz, which is used for a SSB signal.

So the power that is needed to make WSPR spots, must also be 400x lower, than the power that you use with phone.

WSPR spots with 200 mW, will show where your SSB signal with 80 W will be heard.

WSPR spots with 10 mW, will show where your SSB signal with 4 W will be heard.

See the WSPR Power Table below.

Power density

The reduction of power is proportional to the reduction in bandwidth.

The power density in the SSB signal is 80 W / 2400 Hz = 33 mW/Hz.
And the power density in the WSPR is 200 mW / 6 Hz = 33 mW/Hz.

The signals have the same power density.

The power density in the SSB signal is 4 W / 2400 Hz = 1.7 mW/Hz.
So the power of the WSPR signal with the same power density is 1.7 mW/ Hz  x 6 = 10 mW

The power density of a WSPR signal of 5 W is 5/6=0.833 W/Hz. So the power of the SSB signal that can be compared with the WSPR signal is 2400 Hz x 5/6 W/Hz = 2000 Watt.

Tip 1

WSPR over a long, long time on the same band.
Take the time, to see the absorbing layer, below the reflective layer, disappear.
Even if the M in MEPT would mean, to put your set under your pillow. Hi.
If you stay long enough on one band, you have the chance to notice unexpected propagation.

Tip 2

When you see, that half of your spots are very strong with a SNR of -14 dB or more, then reduce your power.

Tip 3

A 10 dB attenuator will not affect reception, just about any commercial radio is sensitive enough. (According G4ZFQ)

Low power CW contest QSO’s

From my own experience

I made many contest QSO’s in CW, using 500 mW with stations all over Europe and up to 3000 miles away in Canada and US.

Further I make QSO’s with stations in many European countries using 50 mW or even less, using my FT-817 and a 10 dB and 20 dB  attenuator.

So what do you think about WSPR spots with just 10 mW  (10 dBm)  or even less.

The WSPR spot database

I regularly visit the WSPR Spot Database, to check out the “amazing” results of stations that use very low power.

If the data is sorted by Miles per Watt, then the stations are shown, that cover great distances with just 1 milliwatt (hi). Be creative in discovering your own interesting queries.

Do you want more than 50 spots. You can fill in 5000 or more in the field “Number of spots”.



Visit the WSPR database

Visit the WSPR database

Please be critical towards spot with less than 1 milliwatt. These spots often contain stations with invented calls and unlikely locators. Most of the time there is something wrong with each individual spot.

I ignore all spots with less than 1 mW, when I make an analysis.