PA1B 20 - 10 - 7 - 3 dB power attenuator measured

We measured the new built 6 watt PA1B  Power Attenuator, in the shack of Mischa PA1OKZ.
The attenuator is built with metal film resistors of 2 watts, 600 mW and 400 mW, from the E12-series.
I was curious how well the metal film would do HF.       Click for schematic

Conclusions
* The 20 - 10 - 7 - 3 dB  sections are very accurate
* The 20 and 10 dB  sections show an accuracy of 0.1 dB to 0.2 dB at 30 MHz
* When all sections are switched OFF, the attenautor has a attenuation of
    0.13 dB at 1 MHz  to 0.61 dB at 30 MHz.
    This "extra" attenuation is frequency dependent.

Relative attenuation
The purpose of an attenuator is to attenuate the signal with the value 
of the sections, that are switched "ON".
In this table the attenuation with all section switched OFF is 0 dB.
The table shows the "extra" attenuation of this section, when a section is switched "ON ".

Relative attenuation of the PA1B  20 - 10 - 7 - 3 dB power attenuator    

At 10 MHz all sections are very accurate.
The 10 dB and 20 dB sections are more frequency dependent.
The section of 3 dB is the most accurate section. The section of 7 dB has an accuracy of 0.1 dB.
The section of 10 dB and 20 dB show an accuracy of 0.15 dB at 30 MHz.

I am very content with these values, considered that the resistors are resistors of 5% from
the E12-series. The measured values of all resistors in this attenuator, show that the accuracy of the resistors is about 2 %.

Absolute attenuation
This "extra" attenuation is caused by the fact, that the circuit formed by switches is not 50 ohm.
The value of 0.13 to 0.61 dB is typical for the switches that are used and the distance between the switches.
We measured about the same values in another attenuator with two section of 20 dB and one of 10 dB, that is built in the same way.

Absolute attenuation of the PA1B  20 - 10 - 7 - 3 dB power attenuator     

Inside of the box of the attenuator with simplified schematic

Attenuators and tuning

50 Ohm attenuators
The attenuators shown on the PA1B Attenuator page are all very accurate 50 ohm attenuators. The accuracy is obtained by placing two, or more,  resistors in parallel, to form the required ohmic values very accurately, with resistors from the E12-series.
But the accuracy of the attenuators is also dependent on the termination of the attenuators. 
When the attenuator is terminated with 50 ohms, the attenuator will show also an input impedance of 50 ohms and most important, will give the right attenuation.

Setup

The attenuator will be placed behind the set. The SWR-meter is not shown in the drawing, but is placed between the attenuator and the tuner (transmatch). I work with this setup, as long as I work with the attenuators and by doing so, I made many fine QRPpp QSO's, using an inverted Vee as antenna.

The right setup.        Tune, as accurate as possible, for a SWR of 1:1,

Tuning

Tuning is done with the attenuator on bypass (0 dB). Tune carefully to get a SWR of exactly 1 : 1. When the SWR is 1:1, the input of the tuner will show an impedance of 50 ohms, so later with the attenuator switched on, the attenuator is terminated with 50 ohms and the attenuator will give the right attenuation. It's very important to tune for 1:1 as accurate as you can, to get a termination with 50 ohms for the attenuator.

SWR

I got an e-mail from a follow Blogger, Mike VE3WDM. He was using the autotuner of the Elecraft K3. The K3 is connected to  the SWR-meter, which is connected to the attenuator, which is connected to coax to the antenna. Mike wrote that he had a SWR of 1.6 : 1, with the attenuator on by-pass. Since the autotuner is in the set and the SWR-meter is behind the set, this means that the coax to Mike's attic antenna shows an impedance of 31.25 ohms or 80 ohms. (50/1.6 or 50x1.6)

The e-mail of Mike made me realize, it is not possible to use the autotuner in the set, together with the attenuators. The tuner has to be behind the attenuator. However, it is always possible to use another (homebrew) tuner behind the attenuators. Mike later said that he will place a tuner behind the attenuators and will turn off the tuner in the set . 

So place the tuner behind the attenuator and tune as accurate as you can to get a SWR of 1 : 1, to get the right attenuation. 

Accuracy of homebrew attenuators

To go below the lowest power of my FT-817, I use attenuators of 10 dB and 20 dB. The attenuators are built with resistors of 5%, but the accuracy is far better than 5%.

A few years ago the homebrew attenuators were measured, during a visit to Mischa PA1OKZ. The 10 dB attenuator turned out to be 9.95 dB. The 20 dB attenuators were 19.92 dB and 19.95 dB.
An excellent result for homebrew attenuators. So I have no doubt about the accuracy.
The accuarcy is much better than I expected.

Because I built the attenuators for a power of more than 1 watt, I had to use more than 6 resistors.
I calculated the values of the resistors with a pocket calculator and built the attenuators with resistors that were at hand, using the table from a informative article of Douwe PA0DKO.