## 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.

### A Very Simple Attenuator for WSPR

### 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.

##### 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.

Consider to reduce your power, when you notice that in

half of the spotsthe 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

#### 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”.

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.