RF Preamps

An RF preamplifier can reduce noise on weak FM broadcast signals. A low-noise preamp can ensure that external noise, not tuner sensitivity, limits reception. A preamp can breathe new life into an older tuner that lacks an RF stage, has inadequate IF limiting, or is misaligned. Mounted at the antenna, a preamp can overcome feedline loss, which otherwise directly degrades sensitivity. If the RF signal feeds multiple tuners, each signal split degrades sensitivity at least 3 dB. Placing a preamp ahead of the power divider can overcome this loss.

Gain, noise figure, third-order intercept, and return loss characterize a preamp. Gain is signal and external noise amplification. Noise figure quantifies the noise a preamp adds. Third-order intercept characterizes distortion by relating desired and spurious signal levels. 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.

A low-noise preamp with 5 dB gain can improve sensitivity with low risk of tuner overload. Unless large losses follow the preamp, diminishing returns set in above 15 dB and overload may occur. Preamp gain ranges from several dB for distribution amplifiers to 30+ dB for mast-mount TV preamps.

Preamp noise figures range from about 0.5 to 5 dB, with low values for optimized designs using modern devices and high values for older designs. When given, the noise figure for a consumer preamp usually is 1 to 3 dB. Tuner noise figures range from perhaps 3 to 9 dB, or even higher for older designs.

Commercial Preamps

The Kitz Technologies KT-501 has 17.5 dB gain (adjustable), 0.85 dB noise figure (increases to about 1 dB as gain is reduced), > 10 dB input and output return losses, 32 dBm third-order output intercept, no FM trap, and input electrostatic discharge protection. Kitz is a small outfit that has been making amplifiers since 2000.

The PBD CX-208 has 16 dB gain, 1 dB noise figure, 4G LTE filter, no FM trap, and can be mast-mounted.

The RCA TVPRAMP12E has 16 dB gain, < 2 dB noise figure, switchable FM trap, and can be mast-mounted.

Garage Sale Preamps

At garage sales I find older cable TV amplifiers like this one. They work fine as an indoor preamp for less sensitive FM tuners. This particular amplifier had 13.7 dB gain in the FM band. The third-order output intercept was 29 dBm and saturated output was 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 the 50 dB quieting sensitivity of 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 flattens the gain and lowers distortion, but it 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 the 50 dB quieting sensitivity of an Onkyo T-4150 from 17.5 to 14 dBf.

A similar amplifier with four outputs had 7.0 dB 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. The third-order output intercept was 21 dBm. 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. This is too much gain for all but the most bulletproof of tuners in a benign signal environment. Reducing the gain to minimum degraded the noise figure so much that sensitivity for the amplifier plus tuner dropped below that of the tuner alone.

LNA Modules

Amazon and eBay offer a variety of low-noise amplifier modules at low cost. Gain and noise figure measurements for several are here and here. Modules with an SPF5189Z require 35 volts at 6090 mA and should have 2527 dB gain and 0.50.6 dB noise figure at 100 MHz. An output attenuator can tame the gain and match 75Ω. Use SMA-to-F adapters or replace each SMA with an F. To better match 75Ω and help protect the sensitive pHEMT against electrostatic discharge, add a 340 nH inductor (6t #22, ⅜″ ID, ″ OL) across RFin. To install the LNA at the antenna, put it in a waterproof enclosure, jumper the output capacitor or add 2 H (14t #22, ″ ID, ″ OL) between VCC and RFout, and use a power inserter followed by a variable attenuator at the tuner.

 R1    R2    R3  Gain   NF
 82    75   240   18   0.6
 68   110   160   15   0.6
 62   160   120   12   0.7

This table shows the performance of an LNA plus fixed output attenuator when the LNA alone has 27 dB gain and 0.5 dB noise figure. Resistance is in ohms, gain and NF in dB. Use this calculator to design a different attenuator.

Today the performance of LNA modules is so good and their cost so low that I recommend building your own preamp from discrete parts only if you want a hands-on learning experience in RF design.

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 effective preamp noise figure surprisingly little. For example, if you lower 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 2.3 dB noise figure.

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. Output return loss is 11.3 dB minimum. The output impedance won't misload a tuner front-end by varying wildly from 75Ω. However, input return loss is 11.6 dB maximum and drops toward 0 as attenuation increases. Today virtually all preamps are unconditionally stable, but an odd load impedance might cause an older, marginally stable preamp to oscillate. Interchange the attenuator ports if this occurs.

To improve the return losses, 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.

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

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

I find variable attenuators at garage sales. This one 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. Swap the connections to avoid this.

I find power inserters at garage sales. They are also available from Amazon and eBay.

Build your own power inserter with this circuit. Component values are not at all critical. The inductor can use 14 turns of #22, ″ inside diameter and ″ outside length. The power supply should have short-circuit protection.

System Noise Figure

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 position, and galactic center position. I derived these figures for 98 MHz from noise temperature curves in Thomas A. Milligan's Modern Antenna Design, 2nd ed.:

Maximum      18 dB
Average      11
Minimum       4

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

City         22 dB
Residential  17
Rural        12
Quiet rural   0

This Windows program calculates system noise figure. Two coax entries let you locate the preamp anywhere in the feedline. Input loss includes balun loss and antenna/preamp mismatch loss. Tuner NF includes mismatch loss. Preamp benefit reference: all preamp entries 0. Stereo S/N should increase by the benefit amount.

300Ω ferrite balun loss is typically 0.75 dB. A conservative estimate for antenna/preamp mismatch loss is the sum of mismatch losses for the antenna (usually < 1 dB, see the antenna models) and preamp (typically < 0.5 dB). RG-6 loss is about 2 dB/100′. Power divider loss is typically 3.3 dB/split. See the tuner writeups for tuner noise figure and mismatch loss values. Their sum is approximately equal to monophonic 50 dB quieting sensitivity minus 10.5 dBf. I derived this figure by measuring six tuners at full IF and audio bandwidth. They yielded differences of 10.0, 10.4, 10.4, 10.5, 10.5, and 11.3 dB.

Experiment with preamp noise figure and gain. Try various values for sky noise and manmade noise, both of which 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.


August 5, 202188108 MHz