Preamp Noise Figure Tables

You can improve the sensitivity of an FM tuner by adding an RF preamplifier. The amount of improvement depends on preamp gain, preamp noise figure, and tuner noise figure.

The following tables show system noise figure improvement in dB for preamp gains of 5 to 25 dB, preamp noise figures of 0 to 6 dB, and tuner noise figures of 1 to 10 dB. System noise figure degrades for blank cells.


                                  T u n e r   N F                        G = 5 dB
      1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.
  0.0 0.7 1.0 1.3 1.5 1.8 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.4 3.6 3.7 3.8 4.0 4.1 4.1
  0.5 0.2 0.5 0.8 1.1 1.4 1.7 2.0 2.2 2.4 2.6 2.9 3.0 3.2 3.4 3.5 3.7 3.8 3.9 4.0
P 1.0     0.1 0.4 0.7 1.0 1.3 1.6 1.9 2.1 2.4 2.6 2.8 3.0 3.2 3.3 3.5 3.6 3.7 3.9
r 1.5             0.3 0.6 0.9 1.2 1.5 1.8 2.0 2.3 2.5 2.7 2.9 3.1 3.3 3.4 3.6 3.7
e 2.0                 0.2 0.5 0.9 1.2 1.4 1.7 2.0 2.2 2.4 2.7 2.9 3.1 3.2 3.4 3.5
a 2.5                     0.1 0.5 0.8 1.1 1.4 1.7 1.9 2.2 2.4 2.6 2.8 3.0 3.2 3.3
m 3.0                         0.1 0.4 0.7 1.0 1.3 1.6 1.9 2.1 2.3 2.6 2.8 3.0 3.2
p 3.5                             0.0 0.3 0.7 1.0 1.3 1.6 1.8 2.1 2.3 2.5 2.7 2.9
  4.0                                     0.3 0.6 0.9 1.2 1.5 1.8 2.0 2.3 2.5 2.7
N 4.5                                         0.2 0.6 0.9 1.2 1.5 1.7 2.0 2.2 2.5
F 5.0                                             0.2 0.5 0.8 1.2 1.4 1.7 2.0 2.2
  5.5                                                 0.2 0.5 0.8 1.1 1.4 1.7 1.9
  6.0                                                     0.1 0.5 0.8 1.1 1.4 1.7

          
                                  T u n e r   N F                       G = 10 dB
      1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.
  0.0 0.9 1.3 1.8 2.2 2.6 3.0 3.4 3.8 4.1 4.5 4.9 5.2 5.5 5.8 6.2 6.4 6.7 7.0 7.2
  0.5 0.4 0.8 1.3 1.7 2.1 2.5 3.0 3.3 3.7 4.1 4.5 4.8 5.2 5.5 5.8 6.1 6.4 6.7 6.9
P 1.0     0.4 0.8 1.2 1.7 2.1 2.5 2.9 3.3 3.7 4.1 4.4 4.8 5.1 5.5 5.8 6.1 6.4 6.7
r 1.5         0.3 0.8 1.2 1.6 2.1 2.5 2.9 3.3 3.7 4.0 4.4 4.8 5.1 5.4 5.8 6.1 6.4
e 2.0             0.3 0.7 1.2 1.6 2.0 2.4 2.9 3.3 3.6 4.0 4.4 4.7 5.1 5.4 5.7 6.0
a 2.5                 0.3 0.7 1.1 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.7 5.1 5.4 5.7
m 3.0                     0.2 0.7 1.1 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.3 4.7 5.0 5.4
p 3.5                         0.2 0.7 1.1 1.5 2.0 2.4 2.8 3.2 3.6 4.0 4.3 4.7 5.0
  4.0                             0.2 0.6 1.1 1.5 1.9 2.4 2.8 3.2 3.6 3.9 4.3 4.7
N 4.5                                 0.2 0.6 1.1 1.5 1.9 2.3 2.8 3.2 3.5 3.9 4.3
F 5.0                                     0.2 0.6 1.0 1.5 1.9 2.3 2.7 3.1 3.5 3.9
  5.5                                         0.1 0.6 1.0 1.5 1.9 2.3 2.7 3.1 3.5
  6.0                                             0.1 0.6 1.0 1.5 1.9 2.3 2.7 3.1


                                  T u n e r   N F                       G = 15 dB
      1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.
  0.0 1.0 1.4 1.9 2.4 2.9 3.3 3.8 4.3 4.7 5.2 5.6 6.0 6.5 6.9 7.3 7.7 8.1 8.5 8.9
  0.5 0.5 0.9 1.4 1.9 2.4 2.9 3.3 3.8 4.2 4.7 5.1 5.6 6.0 6.5 6.9 7.3 7.7 8.1 8.5
P 1.0     0.5 0.9 1.4 1.9 2.4 2.8 3.3 3.8 4.2 4.7 5.1 5.6 6.0 6.5 6.9 7.3 7.7 8.1
r 1.5         0.4 0.9 1.4 1.9 2.4 2.8 3.3 3.8 4.2 4.7 5.1 5.6 6.0 6.4 6.9 7.3 7.7
e 2.0             0.4 0.9 1.4 1.9 2.3 2.8 3.3 3.7 4.2 4.7 5.1 5.6 6.0 6.4 6.9 7.3
a 2.5                 0.4 0.9 1.4 1.9 2.3 2.8 3.3 3.7 4.2 4.7 5.1 5.6 6.0 6.4 6.9
m 3.0                     0.4 0.9 1.4 1.9 2.3 2.8 3.3 3.7 4.2 4.6 5.1 5.5 6.0 6.4
p 3.5                         0.4 0.9 1.4 1.8 2.3 2.8 3.3 3.7 4.2 4.6 5.1 5.5 6.0
  4.0                             0.4 0.9 1.4 1.8 2.3 2.8 3.3 3.7 4.2 4.6 5.1 5.5
N 4.5                                 0.4 0.9 1.4 1.8 2.3 2.8 3.2 3.7 4.2 4.6 5.1
F 5.0                                     0.4 0.9 1.4 1.8 2.3 2.8 3.2 3.7 4.2 4.6
  5.5                                         0.4 0.9 1.3 1.8 2.3 2.8 3.2 3.7 4.2
  6.0                                             0.4 0.9 1.3 1.8 2.3 2.8 3.2 3.7


                                  T u n e r   N F                       G = 20 dB
      1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.
  0.0 1.0 1.5 2.0 2.5 3.0 3.4 3.9 4.4 4.9 5.4 5.9 6.4 6.8 7.3 7.8 8.2 8.7 9.2 9.6
  0.5 0.5 1.0 1.5 2.0 2.5 3.0 3.4 3.9 4.4 4.9 5.4 5.9 6.3 6.8 7.3 7.8 8.2 8.7 9.2
P 1.0     0.5 1.0 1.5 2.0 2.5 2.9 3.4 3.9 4.4 4.9 5.4 5.9 6.3 6.8 7.3 7.8 8.2 8.7
r 1.5         0.5 1.0 1.5 2.0 2.5 2.9 3.4 3.9 4.4 4.9 5.4 5.9 6.3 6.8 7.3 7.8 8.2
e 2.0             0.5 1.0 1.5 2.0 2.5 2.9 3.4 3.9 4.4 4.9 5.4 5.9 6.3 6.8 7.3 7.8
a 2.5                 0.5 1.0 1.5 2.0 2.4 2.9 3.4 3.9 4.4 4.9 5.4 5.9 6.3 6.8 7.3
m 3.0                     0.5 1.0 1.5 2.0 2.4 2.9 3.4 3.9 4.4 4.9 5.4 5.9 6.3 6.8
p 3.5                         0.5 1.0 1.5 2.0 2.4 2.9 3.4 3.9 4.4 4.9 5.4 5.8 6.3
  4.0                             0.5 1.0 1.5 1.9 2.4 2.9 3.4 3.9 4.4 4.9 5.4 5.8
N 4.5                                 0.5 1.0 1.5 1.9 2.4 2.9 3.4 3.9 4.4 4.9 5.4
F 5.0                                     0.5 1.0 1.5 1.9 2.4 2.9 3.4 3.9 4.4 4.9
  5.5                                         0.5 1.0 1.5 1.9 2.4 2.9 3.4 3.9 4.4
  6.0                                             0.5 1.0 1.4 1.9 2.4 2.9 3.4 3.9


                                  T u n e r   N F                       G = 25 dB
      1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.
  0.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 6.9 7.4 7.9 8.4 8.9 9.4 9.9
  0.5 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 6.9 7.4 7.9 8.4 8.9 9.4
P 1.0     0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.4 6.9 7.4 7.9 8.4 8.9
r 1.5         0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.4 6.9 7.4 7.9 8.4
e 2.0             0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.4 6.9 7.4 7.9
a 2.5                 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.4 6.9 7.4
m 3.0                     0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.4 6.9
p 3.5                         0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.4
  4.0                             0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
N 4.5                                 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
F 5.0                                     0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
  5.5                                         0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
  6.0                                             0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

The first table shows that improvement is possible even for a preamp gain of just 5 dB, which has low risk of tuner overload. The last two tables illustrate the diminishing returns for high preamp gain. Gain values range from a few dB for distribution amplifiers to 25+ dB for mast-mount TV/FM preamps.

Preamp noise figures range from about 0.5 to 6 dB, with low values for optimized, open-loop designs, often narrowband, and high values for high-linearity wideband amplifiers with feedback. A noise figure of 0 dB is impossible, but it shows the limiting case.

Tuner noise figures range from perhaps 3 to 10 dB. As a rough approximation, noise figure equals monophonic 50-dB quieting sensitivity minus 10.5 dBf. This estimate is for full IF and audio bandwidth. I derived it by measuring six tuners. They yielded differences of 10.0, 10.4, 10.4, 10.5, 10.5, and 11.3 dB.

The tables evaluate the following equation:

S = T - 10log[10P/10 + (10T/10 - 1) / 10G/10]

where S is system NF improvement, T is tuner NF, P is preamp NF, and G is preamp gain, all in dB.

A utility for calculating any variable given the other three is here.

Output Attenuation

Adding variable attenuation after a high-gain preamp provides a flexible system. You can dial back the gain until any tuner overload ceases. Output attenuation degrades the noise figure surprisingly little.

The following tables show noise figure degradation in dB for preamp gains of 10 to 25 dB, preamp noise figures of 0 to 6 dB, and output attenuations of 1 dB to the preamp gain.


           A t t e n u a t i o n    G = 10 dB
       1   2   3   4   5   6   7   8   9   10 
  0.0 0.1 0.2 0.4 0.6 0.9 1.1 1.5 1.8 2.3 2.8 
  0.5 0.1 0.2 0.4 0.5 0.8 1.0 1.3 1.7 2.1 2.6 
P 1.0 0.1 0.2 0.3 0.5 0.7 0.9 1.2 1.5 1.9 2.3 
r 1.5 0.1 0.2 0.3 0.4 0.6 0.8 1.1 1.4 1.7 2.1 
e 2.0 0.1 0.2 0.3 0.4 0.6 0.7 1.0 1.3 1.6 2.0 
a 2.5 0.1 0.1 0.2 0.4 0.5 0.7 0.9 1.1 1.4 1.8 
m 3.0 0.1 0.1 0.2 0.3 0.4 0.6 0.8 1.0 1.3 1.6 
p 3.5 0.0 0.1 0.2 0.3 0.4 0.5 0.7 0.9 1.2 1.5 
  4.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.1 1.3 
N 4.5 0.0 0.1 0.2 0.2 0.3 0.4 0.6 0.7 1.0 1.2 
F 5.0 0.0 0.1 0.1 0.2 0.3 0.4 0.5 0.7 0.9 1.1 
  5.5 0.0 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0 
  6.0 0.0 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.7 0.9 


                       A t t e n u a t i o n            G = 15 dB
       1   2   3   4   5   6   7   8   9   10  11  12  13  14  15 
  0.0 0.0 0.1 0.1 0.2 0.3 0.4 0.5 0.7 0.9 1.1 1.4 1.7 2.0 2.5 2.9 
  0.5 0.0 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.5 1.9 2.3 2.7 
P 1.0 0.0 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.7 0.9 1.1 1.4 1.7 2.1 2.5 
r 1.5 0.0 0.1 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.5 1.9 2.3 
e 2.0 0.0 0.1 0.1 0.1 0.2 0.3 0.3 0.4 0.6 0.7 0.9 1.1 1.4 1.7 2.1 
a 2.5 0.0 0.0 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.3 1.6 1.9 
m 3.0 0.0 0.0 0.1 0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1.1 1.4 1.7 
p 3.5 0.0 0.0 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.7 0.8 1.0 1.3 1.6 
  4.0 0.0 0.0 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1.2 1.4 
N 4.5 0.0 0.0 0.0 0.1 0.1 0.1 0.2 0.3 0.3 0.4 0.5 0.7 0.8 1.0 1.3 
F 5.0 0.0 0.0 0.0 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.8 0.9 1.2 
  5.5 0.0 0.0 0.0 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.7 0.8 1.0 
  6.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.8 0.9 


                                A t t e n u a t i o n                       G = 20 dB
       1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16  17  18  19  20 
  0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.8 0.9 1.2 1.4 1.7 2.1 2.5 3.0 
  0.5 0.0 0.0 0.0 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.7 0.8 1.0 1.3 1.6 1.9 2.3 2.7 
P 1.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.8 0.9 1.2 1.4 1.7 2.1 2.5 
r 1.5 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.7 0.9 1.1 1.3 1.6 1.9 2.3 
e 2.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.7 2.1 
a 2.5 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.6 0.7 0.9 1.1 1.3 1.6 1.9 
m 3.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.7 
p 3.5 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.6 0.7 0.9 1.1 1.3 1.6 
  4.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 
N 4.5 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.4 0.6 0.7 0.9 1.1 1.3 
F 5.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.6 0.8 1.0 1.2 
  5.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1.1 
  6.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.6 0.8 1.0 


                                             A t t e n u a t i o n                              G = 25 dB
       1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25 
  0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.8 2.1 2.5 3.0 
  0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1.1 1.3 1.6 1.9 2.3 2.8 
P 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.8 2.1 2.5 
r 1.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1.1 1.3 1.6 1.9 2.3 
e 2.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.8 2.1 
a 2.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1.1 1.3 1.6 1.9 
m 3.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.5 1.8 
p 3.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1.1 1.3 1.6 
  4.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.5 
N 4.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1.1 1.3 
F 5.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.6 0.8 1.0 1.2 
  5.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.4 0.5 0.6 0.7 0.9 1.1 
  6.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.3 0.4 0.5 0.6 0.8 1.0 
 

For example, if you dial back the gain of a 25-dB preamp with a 2-dB noise figure by 15 dB, the noise figure increases 0.3 dB. The preamp plus attenuator is equivalent to a preamp with 10-dB gain and a 2.3-dB noise figure.

The attenuation tables evaluate the following equation:

D = 10log[10P/10 + (10A/10 - 1) / 10G/10] - P

where D is NF degradation, P is preamp NF, A is attenuation, and G is preamp gain, all in dB.

You can locate the preamp at the antenna and the attenuator at the tuner. A simple 100Ω potentiometer should work fine. Minimum attenuation is 2.8 dB. Although its output impedance isn't constant, it doesn't vary wildly from 75Ω, which minimizes misloading of the tuner input circuit. Output return loss is 11.3 dB minimum. Input return loss is 11.6 dB maximum and drops toward 0 as attenuation increases. Depending on its length, the feedline may transform a low impedance to a high value at the preamp. Nearly all preamps are unconditionally stable, but a high load impedance might cause a marginally stable preamp to oscillate. Interchanging the attenuator ports will prevent this, although the impedance presented to the tuner will be far from 75Ω near maximum attenuation.

For good return losses at both ports, add fixed resistors. Minimum return loss is 10.6 dB input and 13.9 dB output. Attenuation varies from 4.2 to 19.8 dB.

A dual 100Ω pot provides minimum input and output return losses of 11.3 dB and minimum attenuation of 2.8 dB.

Wiper       Single Pot         Single Pot + Fixed          Dual Pot     
  %     Atten   RLin  RLout    Atten   RLin  RLout    Atten   RLin  RLout
 100      2.8   11.3   11.3      4.2   35.1   15.8      2.8   11.3   11.3
  90      3.7   11.6   14.4      5.1   43.2   20.3      4.4   15.6   15.6
  80      4.7   11.5   18.3      6.0   49.5   27.9      6.1   21.6   21.6
  70      5.8   11.0   24.1      6.9   43.2   40.9      7.8   33.8   33.8
  60      6.9   10.1   39.0      7.9   35.1   25.7      9.6   30.8   30.8
  50      8.2    9.0   29.8      9.0   28.8   21.0     11.5   23.5   23.5
  40      9.8    7.6   23.2     10.2   24.0   18.3     13.7   20.4   20.4
  30     11.8    6.0   20.1     11.7   20.0   16.5     16.4   18.6   18.6
  20     14.7    4.2   18.3     13.5   16.5   15.3     20.1   17.6   17.6
  10     19.9    2.2   17.3     15.9   13.5   14.4     26.2   17.1   17.1
   0       ∞     0.0   16.9     19.8   10.6   13.9       ∞    16.9   16.9

This table gives attenuation, input return loss, and output return loss in dB as a function of the pot setting for the three attenuators. Preamp and tuner impedances are 75Ω and stray reactances are ignored.

A Radio Shack attenuator I found at a garage sale provided 1.5 to 20.5 dB of attenuation. I was happy with it until I measured the return losses: 1025 dB at ANT, but 117 dB at TV. Maximum DC resistance at TV was 900Ω, which will unload a tuner's input circuit. To minimize adverse effects, connect the tuner to ANT and the preamp power inserter to TV. If the preamp isn't unconditionally stable, check for oscillation at maximum attenuation.


December 18, 201188108 MHz