RF Preamps

The purpose of an RF preamplifier is to reduce noise. A preamp can breathe new life into an older FM tuner that uses obsolete components, lacks an RF stage, has inadequate IF limiting, or is misaligned. Mounted at the antenna, a preamp can overcome feedline loss. A preamp placed before a power divider can mitigate its 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 the preamp itself adds. Third-order intercept relates desired and spurious signal levels. Return loss indicates how closely an impedance matches 75Ω. Lower noise figure, higher third-order intercept, and higher return loss are better. Higher gain helps, but too much can overload a tuner.

A low-noise preamp with 5 dB gain has low risk of tuner overload and may noticeably reduce noise. Unless large losses follow the preamp, noise reduction levels off above 15 dB and tuner overload becomes increasingly likely. Preamp gain ranges from several dB for distribution amplifiers to 30 dB or more for mast-mount preamps.

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

A third-order output intercept of 25 dBm or more should cause troublesome spurious signals only for extremely strong input signals. Usually the tuner will overload before the preamp does. Return loss > 10 dB should be fine.

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 in Wisconsin 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 tuner in a benign signal environment. Reducing the gain to minimum degraded the noise figure so much that sensitivity for the amplifier plus test 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    56   560   20   0.5
 82    75   240   18   0.6
 68   110   160   15   0.6
 62   160   120   12   0.7
 62   220   110   10   0.9

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.

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 drop the gain of a 30 dB preamp with 1 dB noise figure by 15 dB, the noise figure increases 0.1 dB. The preamp and attenuator act like a preamp with 15 dB gain and 1.1 dB noise figure. Note that placing the attenuator before the preamp also yields 15 dB gain, but with a noise figure of 16 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. 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 950Ω, 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 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

Receive signal-to-noise ratio depends on external noise and the noise/gain/loss of each component in the signal path. In the FM broadcast band, sky noise and manmade noise are the most significant external noise sources.

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

Maximum      18 dB
Average      11
Minimum       4

Manmade noise depends on the specific location. These median 98 MHz figures derived from Recommendation ITU-R P.372-16 equations are broad generalizations. The figure is 0 dB when manmade noise is not present.

City         22 dB
Residential  17
Rural        12

This Windows program calculates system noise figure. Two coax entries accommodate a preamp anywhere in the feedline. Preamp benefit reference: all preamp entries 0. Stereo 50 dB quieting sensitivity should improve by the benefit amount. Net gain reference: antenna with balun connected directly to the tuner.

Antenna loss includes conductor loss (usually < 0.1 dB), mismatch loss (usually < 1 dB), and balun loss (typically 0.75 dB for a 300Ω ferrite balun, 0.1 dB for a halfwave coaxial balun). RG-6 loss is about 2 dB/100′. Power divider loss is typically 3.3 dB/split (8-way divider: 3 splits). Tuner noise figure 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.

The wide range of sky and manmade noise levels makes calculating a definitive system noise figure impossible. But you can change any entry to determine its significance. Both noise terms can vary with antenna bearing and time of day. 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 10, 202388108 MHz