RF Preamps

An RF preamplifier can improve FM reception by reducing noise on weak signals. While I believe external RF noise often predominates, a low-noise preamp can ensure that tuner sensitivity is not the limiting factor. A preamp can especially benefit an older tuner that lacks an RF amplifier stage, has inadequate IF limiting, or is misaligned. Mounted at the antenna, a preamp can overcome the loss of a long feedline, which otherwise directly degrades tuner sensitivity. If you use a power divider to distribute the RF signal to multiple tuners, each split degrades tuner sensitivity at least 3 dB. Placing a preamp ahead of the power divider can prevent this.

Noise, gain, distortion, and impedance characterize a preamp. Noise figure is a measure of the noise a preamp adds. Gain is signal and external noise amplification. Third-order intercept, which relates input and spurious signal levels, describes distortion. Return loss indicates how closely the input or output impedance matches 75Ω. Lower noise figure, higher third-order intercept, and higher return loss are always better. Higher gain helps, but too much can overload a tuner front-end, which is likely to distort before the preamp does.

Homebrew Preamps

I built this RF preamp from junkbox parts. I could not find a recommended source impedance for lowest noise figure. Instead, I went for a good match since input mismatch loss directly degrades noise figure. I measured the input return loss as > 18 dB from 88 to 108 MHz. The corresponding mismatch loss is < 0.07 dB.

The untuned input inductor, 25 turns on a T-25-7 core, provides a DC ground. As long as its reactance is high, its value is noncritical. You can eliminate it entirely if the antenna is at DC ground. The output inductor is 15 turns on a T-37-10 core tapped 3 turns from the cold end. It resonates with the J309 drain/gate capacitance. I adjusted the turns spacing until the gain was about equal at the band edges. Output return loss is poor, but this lowers the midband gain peak and has almost no effect on noise figure. It does complicate measurement since the output impedance is not near 75Ω. To avoid inconsistencies, I measured everything without reconnecting the preamp.

The favorable input return loss is due to the J309 transconductance, which is about 13 mS. Invert that and you get an input impedance close to 75Ω. With additional parts you can adjust the drain current, but I didn't bother. The J309 IDSS spec is 12–30 mA. My circuit drew 17 mA.

At midband the third-order input intercept was 132 dBf. I measured the following gain and noise figure values:

 Freq    Gain     NF
  MHz      dB     dB
 88.0    12.6    3.0
 96.9    14.8    3.1
107.7    12.2    3.8

I wasn't sure I'd keep this circuit so I didn't cut the transistor leads. It's best to make them as short as possible to minimize stray inductance.

To avoid the output coil tap, reduce the output coupling capacitance. 12 turns on a T-37-10 core yielded about 15 dB gain midband and 11 dB at the band edges.

I built an even simpler preamp to mount at the antenna. The output inductor forms an L-network with the J309 drain/gate capacitance. I used 16 turns on a T-37-10 core and spread them to adjust resonance. Gain was about 16 dB midband and 12 dB at the band edges. The input coil is a Nytronics Wee-ductor. I found the Scientific Atlanta DA-PI power inserter at a garage sale. 220Ω for R1 yielded 13 V at the drain from a wall wart that delivered about 18 V with no load. Current was 20 mA.

Don't duplicate these designs. They are intended to inspire you to throw something together from whatever is in your junkbox. Unless your tuner's noise figure is unusually low, a homebrew preamp using an older transistor should improve it. To use a modern, low-noise device, follow the manufacturer's datasheet. If its gain is too high, you can attenuate the output without significantly degrading noise figure.

Garage Sale Preamps

At garage sales I find wideband cable TV amplifiers like this one. They work fine as a preamp for less sensitive FM tuners. Though it precludes mounting the amplifier at the antenna, the internal power supply with captive line cord is convenient. This particular amplifier had 13.7 dB of gain in the FM band and a third-order input intercept (IIP3) of 135 dBf. Saturated output was 136 dBf (40 mW). Return loss was 14.5 dB input and 12 dB output. The noise figure was 4.8 dB. This is not a low value, but the amplifier still improved 50 dB quieting sensitivity for a Technics ST-9030 from 18.1 to 15.4 dBf.

I traced out this circuit. The emitter voltage is −12 V and total current is 15 mA. Rated power consumption is 2 W. The feedback improves gain flatness and linearity but degrades the noise figure. Breaking the feedback loop improved the noise figure 1.4 dB and increased gain 5 dB, but it degraded return loss to 6 dB input and 9 dB output. Adding 68 pF with 1″ leads from the emitter to ground restored return loss to 13 dB input and 14 dB output. The modified preamp improved 50 dB quieting sensitivity for an Onkyo T-4150 from 17.5 to 14 dBf.

A similar amplifier with four outputs had 7.0 dB of gain. The circuit was the same as that above minus the input diodes and with tiny ferrite transformers to divide the output four ways. In this unit the transistor was a 2SC3777. IIP3 was 134 dBf. The power division degrades the noise figure just 0.2 dB.

This amplifier has adjustable gain. In the FM band I measured 18.5 to 29.5 dB. Unless you're feeding hundreds of feet of lossy coax or dividing the signal many ways, anything above 20 dB is too much gain for all but the most bulletproof of tuners in a benign signal environment. But reducing the gain to minimum degraded the noise figure so much that preamp+tuner sensitivity dropped below that of the tuner alone.

Commercial Preamps

The Kitz Technologies KT-501 preamp has a noise figure of 0.85 dB. Gain is adjustable to 17.5 dB. Jeffrey Kitz told me that the noise figure increases to about 1 dB as the gain is reduced. He measures it with an HP 8970A. He said that input and output return losses are > 10 dB and the third-order output intercept is 32 dBm. At maximum gain that should be equivalent to an IIP3 of 134.5 dBf. The unit is unconditionally stable. Kitz is a small outfit that has been making amplifiers since 2000.

Noise Figure Tables

The following tables show 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. 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 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 gain. Preamp gain ranges from several 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 designs using specialized devices and high values for older designs with lossy feedback. When given, the noise figure spec for a consumer preamp typically is 2 to 3 dB. 0 dB is impossible, but it shows the limiting case.

Tuner noise figures range from perhaps 3 to 10 dB, or even higher for older designs. As a rough approximation, noise figure equals monophonic 50 dB quieting sensitivity minus 10.5 dBf. I derived this figure by measuring six tuners using full IF and audio bandwidth. They yielded differences of 10.0, 10.4, 10.4, 10.5, 10.5, and 11.3 dB.

The tables evaluate this expression for noise figure improvement in dB:

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

where T is tuner noise figure, P is preamp noise figure, and G is preamp gain, all in dB.

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 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 lower the gain of a 25 dB preamp with 2 dB noise figure by 15 dB, the noise figure increases 0.3 dB. The preamp+attenuator is equivalent to a preamp with 10 dB gain and 2.3 dB noise figure.

The tables evaluate this expression for noise figure degradation in dB:

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

where P is preamp noise figure, A is attenuation, and G is preamp gain, all in dB.

You can locate the preamp at the antenna to overcome feedline loss and place the attenuator at the tuner for easy adjustment. 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 75Ω tuner input circuit. To minimize adverse effects, swap the connections. If the preamp isn't unconditionally stable, check for oscillation at maximum attenuation.

External Noise

System noise is likely to depend mostly on external RF noise. In the FM broadcast band, sky noise and manmade noise are the most significant external sources.

Sky noise level depends on solar activity, sun location, and galactic center location. I derived these noise figure values for 98 MHz from noise temperature curves in Thomas A. Milligan's Modern Antenna Design, 2nd ed.:

Maximum      18 dB
Average      10
Minimum       2

Manmade noise varies so much with time and location that any attempt to account for it is highly speculative. I calculated these noise figure values for 98 MHz using equations in recommendation ITU-R P.372-14:

City         22 dB
Residential  17
Rural        12
Quiet rural   0

This program calculates system noise figure. It lists system components in order. Two coax entries let you locate the preamp before or after the feedline. Preamp atten is output attenuation. System noise figure does not depend on the physical order of the attenuator, coax #2, or power divider, whose loss is typically 3.3 dB/split (e.g., 9.9 dB for eight outputs). RG-6 loss is about 2 dB/100′. Preamp benefit reference: all three preamp entries zero.

Explore how a preamp affects system noise by trying various values for sky noise and manmade noise. Both can vary over time. If you believe your external noise is generally low, you may find a preamp worthwhile. But if you expect it to be more often high than low, you may decide that a preamp offers little benefit.


November 19, 202088108 MHz