WSPR with an attenuator

WSPR spots with very low power

Reduce your power to 10 mW to make WSPR-spots, that can be compared with a SSB signal of 4 W. WSPR with 10 milliwatt will give interesting spots when the propagation is excellent, but will not de-sensify the receivers of other WSPR users in the same band. With WSPR the all stations, world wide, transmit in a 200 Hz wide (actually small) band.

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

Use a simple Power Attenuator combined with a reduction in audio level, to use WSPR with 10 milliwatt.

The attenuator must be designed for a nominal input power of  your signal.

Reduce audio to reduce WSPR power

Reduce your power with WSPR by reducing the audio.

A Very Simple Attenuator for WSPR

Very Simple Attenuator for WSPR

A Very Simple Attenuator for WSPR 2 x 10 dB
Nominal power 1 W

A simple attenuator

I designed this very simple attenuator for WSPR, with as little resistors as possible.

The first section of the attenuator has a nominal input power of 1 watt, when you choose to use 4 resistors of 1 watt, as I show in the schematic. The resistor of 100 ohm can be  a resistor of 1/4 W, because this resistor has to dissipate far less power than the resistors at the input of the first section. With an input power of 1 W, there will be 100 mW at the output.

The second section is designed with resistors of  1/4 W. Therefor this section will have a nominal power of 250 mW.

The attenuation of each section is actually 9.9 dB instead of 10 dB. This is absolutely no problem with WSPR. hi.

An attenuator of 10 dB reduces the power to 1/10. When you switch on both 10 dB sections, the attenuation is 20 dB and the power is reduced to 1/100.

An attenuator of 10 dB can be used to reduce the power of the WSPR signal from 1 watt to 100 milliwatt (20 dBm) or to reduce the power from 500 mW to 50 mW (17 dBm).

An attenuator of 20 dB can be used to reduce the power of the WSPR signal from 1 watt to 10 milliwatt or to reduce your power from 500 mW to 5 mW.

The attenuator can get warm, during the 2 minutes in which your WSPR signal is transmitted. An attenuator of 10 dB has to dissipate 90% of the input power.

Output power of the attenuator

Output power of the attenuator

Chose your power

The first step is rather easy. Chose the lowest power of your set or chose the set with the lowest power. (hi) For example, the lowest power of the FT-817 is 500 mW.

First tune the antenna with the attenuator set to 0 dB. No attenuation.
Measure or adjust the output power on the lowest power setting or on a setting that you can measure.

Reduce the audio level

If the lowest power of  your set is 10 watt or 5 watt, you can reduce the output power by reducing the audio level. The audio level can be reduced in the WSPR program. So you can reduce to 500 mW, 200 mW or 100 mW.

If you reduce the audio level by 10 dB, then the output power will also reduce by 10 dB. A reduction of 10 dB means the the power is divided by 10. So the power of the WSPR signal will be reduced  from 5 watt to 500 mW.

To go from 5 W to 100 mW, the audio level must be reduced with 17 dB. This will reduce the power to 1/50.

To see how to reduce the power of your WSPR signal, please use the link to the interesting page Setting WSPR power, of Alan G4ZFQ.
Scroll down to: WSPR itself has a digital output control to see how to reduce the “Transmit digital gain”.

Very low power with an Attenuator

The power from the set e.g. 1 watt (1000 mW), 500 mW or 100 mW, can be reduced further to milliwatt level by a (power) attenuator.

An attenuator of 10 dB reduces a power of 100 mW to 10 mW or 50 mW to 5 mW. An attenuator of 20 dB reduces a power of 1 W to 10 mW. (You will still make spots.) A attenuator of 20 dB reduces a power of 500 mW to 5 mW.

The (power) attenuator must be designed to dissipate the power of the signal from the set (e.g. 1 W) and can get warm.

Be sure that the power of your WSPR signal at the input of the attenuator, is lower than the Nominal power of your power attenuator. If not, then reduce the power of your WSPR signal, by reducing the audio level.

SNR

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

Calculated Lowest Possible Power

The calculated lowest possible power is calculated from the power of the transmitting WSPR station and the SNR of the received spot.

The lower the calculated lowest possible power, the better the propagation.

Use the SNR to follow the breathing of the ionosphere

10 mW —> 1 mW

Start with 10 mW and reduce further when the SNR goes up.
WSPR Lowest possible power

Start with 200 mW or even 10 milliwatt.
Reduce when half of your spots show a SNR of -14 dB or better.

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

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.

No T/R switch needed

The attenuator reduces the power during the transmission, but, since there is no T/R switch, the received signal will also be attenuated.

A 10 dB attenuator will not affect reception, just about any commercial radio is sensitive enough. According G4ZFQ on his interesting page: Setting WSPR power.

Setting WSPR power
By Alan G4ZFQ

Visit the interesting page, to read how to reduce the audio level to reduce  the power of your WSPR signal.

The resistors will get warm

Tjeerd “Gose” PA3GNZ was inspired by this page about attenuators and some additional info about power attenuators, that I sent him. So he designed a 10 dB attenuator, for an input power of 5 watts, using 2 watt metal film resistors. Tjeerd uses the attenuator, to reduce the power of  5 watts, that is coming from the set, to reduce his WSPR signal to 500 mW. He noticed that during the 2 minute transmission period the attenuator gets warm.

You can find the drawing of the symmetrical 10 dB attenuator, for an input power of 5 watts, on the weblog of Tjeerd >>.

I have until now (2018), not yet transmitted with WSPR.

The focus of my activity, is the use of low and very low power in CW contest QSO’s.

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”.

wsprlogo

WSPR

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.