The use of attenuators

A simple way to reduce the power of your QRP set to milliwatt level, without modifying the set, is the use of attenuators. Homebrew 50 ohm attenuators are easy to build and can be placed between the set and the antenna or the tuner. I developed very accurate Power Attenuators built with resistors from the E12-series.

fxdpwrtx

The antenna is connected to the transmatch. (tuner)  I match the antenna with the transmatch, before I use an attenuator, with the attenuator set to 0 dB. After adjusting the SWR, the output of the attenuator sees 50 ohms.

Disclaimer. When you use the attenuator, you can NOT use the auto tuner in the transceiver.

On the air

I always use search and pounce, when I use the lowest possible power. When I use an attenuator, I transmit, but also listen trough the attenuator. I rapidly switch to the wanted (low) power, before I answer a CQ. Only when I am certain that my signal is not heard, I will increase my power. When I have to increase my power, I usually make a step of 3 dB or 4 dB.
The S-meter is very important, when I use the lowest possible power. When a received signal is S9 + 10 dB, instead of 599, I reduce my power with 10 dB.

Lowest possible power

When the S-meter goes up, my power goes down.

Tip for CW:
For CW QSO’s with the lowest possible power, use steps of 3 dB or 4 dB. A step of 3 or 4 dB is the optimal step to increase power when you work with the lowest possible power.

Tip for WSPR:
For WSPR, just use a 10 dB or/and a 20 dB attenuator, to reduce your power with 10 dB, 20 dB or 30 dB. When you reduce your power with a step of 3 dB with WSPR, you will not notice any difference at all.

Experience in CW contest QSO’s:
A contest station will work any station that they hear. I noticed in many CW QSO’s, that when my signal was heard, but my call was not copied, even after repeating 10 times, I only had to double my power, for my call to be copied immediately correct. So in low power QSO’s in CW, a step of 3 dB or 4 dB is the optimal step to increase the power. The step of 10 dB is too large and 1 dB is too small.

FT-817 with attenuators

 

ft817swr

The power of the FT-817 is adjustable in a menu’s, so adding attenuators of 10 dB and 20 dB gives the possibility to operate with a few milliwatts

Simple

When I started with the use of very low power, I placed an attenuator of 10 dB and an attenuator of 20 dB behind the FT-817. To adjust the power, I use the FT-817 in combination with the attenuators. With an attenuator of 10 dB and 20 dB, it is simple to calculate the power that goes to the antenna. An attenuator of 10 dB will divide the power by 10. This is merely moving the decimal point one place to the left. An attenuator of 20 dB will divide the power by 100. This is moving the decimal point two places to the left. When the FT-817 is set  to 500 mW and the attenuator is set to 10 dB, the attenuator will reduce the power to 1/10th, so the power will be 50 mW. The table to the right, is run through from bottom to top.

The FT-817’s QRP sequence

ft817p-k

The power of the FT-817 can be adjusted in one of the menu’s. I always chose for this menu, to be accessible at any moment. The power is reduced with every push of the button. The sequence is:  5 W,  2.5 W,  1 W,  500 mW and then 5 W again. However when I use the power adjustment, it is to increase the power, because my power is not enough to be heard. To increase the power the button A must be pressed 3 times to increase the power with one step. For example form 500 mW to 1 W.

50 ohm attenuators

The inverted V is connected with a 300 ohm ribbon to the transmatch. (tuner)  I match the antenna with the transmatch, before I use an attenuator, with the attenuator set to 0 dB. After adjusting the SWR, the input of the transmatch is 50 ohms, so the output of the attenuator sees 50 ohms and is properly terminated.

Disclaimer. When you use the attenuator, you can NOT use the auto tuner in the transceiver.

Attenuators for a set with fixed power

Fast in QSO’s
The sections of this attenuator are 3 dB, 7 dB, 10 dB and 20 dB. So the this attenuator will be switched in steps of 3 dB or 4 dB.  In stead of switching the power of the set, the attenuator will be switched. Because of the sections of  3 dB and 7 dB, the switching is instantly, thus very fast.

QRP set with 5 watts and an attenuator set to 17 dB

If your set has the possibility to chose the power, you can chose the lowest power, for example 500 milliwatt.

Or you can chose to use more power, for example 2 watt, so you have always 2 W “under the hood”, which you can chose instantly.

The table below, is run through from the bottom to the top

tab40dbk

 

(4x6r-att-3,7,10en20dB)