WSPR with very low power

Dare to let your WSPR beacon whisper.

Power with WSPR compared to SSB

WSPR uses a very small bandwidth, compared to SSB. So the power that is needed to make WSPR spots, that can be compared with your  SSB signal, is much lower than the power that you use with phone.

WSPR vs SSB power 0.25%

WSPR with 0.25% of your power in phone

  • Reduce your power to 0.25% (1/400) of what you use with SSB. (-26 dB)

E.g. 80 W in SSB v.s. 200 mW with WSPR
(200 mW = 23 dBm)

Dare to let your WSPR beacon whisper.

Power with WSPR compared to CW

WSPR uses a smaller bandwidth than CW. So the power that is needed to make WSPR spots, that can be compared with CW, is 20 times lower than the power that you use with CW.

WSPR vs CW

WSPR with 5% of your power in CW

  • Reduce your power in WSPR to 5% (1/20) of what you use with CW. (-13 dB)

E.g. 4 W in CW v.s. 200 mW in WSPR.
(200 mW = 23 dBm)

Dare to let your WSPR beacon whisper.

Reducing power with WSPR with an 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. The lowest power of my FT-817 is 500 mW.

First then tune the antenna.

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. So you can reduce to 500 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.

Attenuator

The power from the set e.g. 500 mW can be reduced further to milliwatt levels by an power attenuator.

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

The power attenuator must be designed to dissipate the power of the signal from the set (e.g. 500 mW) and will get warm.

Be sure that the power of your WSPR signal, 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.

Attenuator for WSPR

Attenuator for WSPR 10 dB 20 dB

!0 dB and 20 dB Attenuator for WSPR
Nominal power 1.1 W

Output power of the attenuator

Output power of the attenuator

This schematic shows an attenuator for a nominal input power of a 1.1 watts.

The attenuator is designed with carbon resistors of 1 W. The resistors with a star* can be 1/4 W, because they dissipate far less power than the other resistors.

The 10 dB attenuator reduces the power to 1/10 and the 20 dB attenuator reduces the power to 1/100

The 10 dB attenuator can be used to reduce the power of the WSPR signal from 1 watt to 100 milliwatt or to reduce the power from 500 mW to 50 mW.

The 20 dB attenuator 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.

In the attenuator of 10 dB 90% of the input power will be dissipated in the attenuator it self and 99% in thee 20 dB attenuator.

The attenuator will get warm, during the 2 minutes in which your WSPR signal is transmitted.

Tip 1

When you use an attenuator for WSPR, please use 10 dB or 20 dB. With a step of 3 dB, you will not notice any difference at all with WSPR.

Tip 2

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

Tip 3

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.

Low power CW contest QSO’s

From my own experience

I made many contest QSO’s in CW, using 500 mW with stations 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 5 mW  (7 dBm)  or even less.

Disclaimer

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.

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

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”.

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.

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 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 >>.