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.
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.
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.
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.