Why do I need an LNA/Pre-Amp?
A Pre-Amp, or Low Noise Amplifier, is one of the most important components in your receive chain. But why?
Back in the '80s, when I first started on VHF and EME, the answer to that question would have been completely different. Back then the main thought when installing an LNA was to install it right at the tower, as close to the antennas as possible, in order to make up for the loss in your coax. It was all about the gain.
The days of poor quality coax are long behind us. Good quality low loss coax and hardline is extremely affordable and easily available to most Radio Hams. Surely that is an argument for not needing a Pre-Amp?
Even back in the '80's those who looked solely at the gain figure of an LNA were missing the most important reason for installing a Pre-Amp. It sets the initial Noise Figure for your entire receive system.
Many radio clubs and conferences offer LNA measuring nights. A place where your, often Homebrew, Pre-Amp can be measured for Noise Figure and Gain. The "winner" is the LNA with the lowest NF. But again, Why?
As previously mentioned the LNA sets the system Noise Figure for your entire receiver chain. The system noise figure includes the coax loss from your antenna, Pre-Amp noise figure and gain in the shack, Insertion loss of the Band Pass Filter, etc. It is most important to limit the losses in between the antenna and the LNA. Any losses preceding the LNA input degrade the System Noise Figure. For example if you have a 3dB coax loss (or SDR Switch insertion loss) before the input of a 1dB LNA that results in 4dB System Noise Figure. Obviously not desirable on VHF and another reason why you should always consider Insertion Loss when choosing your SDR Switch.
Your LNA affects the performance level of everything after it. This is why it is the first component installed after your SDR Switch. It goes before your Band Pass Filter. It goes before your power divider. It goes before your SDR or Rig. One could still make the argument that it should still go on the tower by the antennas but at some point you are going to blow an LNA up. No matter how much protection you afford it at some point it will go 'POP'. This usually happens in the middle of the coldest winter night right before the contest starts. Do you really want to climb the tower to replace it? Given the gains in the availability of good low loss coax for most of us it makes more sense to install the LNA in the shack.
Gain or Noise Figure?
The answer to this one is very easy. When looking for a Pre-Amp always pay most attention to the N.F.
As previously mentioned, losses before the LNA input must be kept to a minimum, as they directly affect the overall sensitivity of your receiver chain. On the other hand, losses after the Pre-Amp only minimally affect the System Noise figure (a.k.a. Sensitivity). The gain and noise figure of your LNA make up for nearly all of the losses after the Pre-Amp assuming that they are kept to a reasonable level.
Your aim in installing an LNA is to lower the N.F. of your receiver in order to improve your stations receive capabilities. All modern Pre-Amps will have far more gain than you require, most will have 20dB plus. What you are looking for is a stable LNA with a N.F. of less than 1dB. Indeed some will have a N.F. of less than 0.5dB.
As we mentioned most LNAs will have far more gain than your SDR receiver requires, even taking into account the 2 dB loss from the insertion of a Band Pass Filter and just over 3 dB loss from a 2 way power divider if you are using a SDRSwitch RXin RXout for dual receive. To compensate for the excess gain you can install a Step Attenuator.
A Step Attenuator should be inserted between the LNA and the BPF. It will allow you to vary how much attenuation you add between the LNA and the rest of your RX chain to give you the best possible low noise receiver without overloading your system with excess gain.
A wide band LNA and Step Attenuator.
The following outlines the use of a Wide Band Low Noise Preamp with a Step Attenuator for All Ham Bands.
To fully compensate for the excess gain of an LNA, without damaging the System Noise Figure , you can place a Step Attenuator at the output of the LNA. Do not attempt to lower the gain of the LNA itself because the gain of the LNA, taken with its low Noise Figure is what lets us take advantage of system losses after the preamp.
Wide band Pre-Amps, such as those available from Nooelec, which have a NF of only 0.7 dB and a gain of over 20 dB for less than $40.00. The Nooelec Wideband preamp specification states it covers 20 MHz to 4 GHz, but in tests it performs well down to 2 MHz. Below 2 MHz no preamp is necessary due to the high ambient noise level of lower HF Bands.
Warning: If you have strong RF stations in close proximity to your receiver, such as FM Broadcast or TV Stations, you might not be able to use a wide band LNA.
The tables below were compiled by Hasan, N0AN, in order to show an example of the optimal use of the Pre-Amp and the Step Attenuator with both an Icom IC-7300 and a SDRPlay RSPdx running on SDR Console (SDRC). I would also suggest you watch the Youtube videos by Hasan on how to maximize your SDR RX settings. These can be found on our Video Links page.
The IC-7300 internal Pre-Amp should be set to OFF whilst the RSPdx settings in SDR Console should be set to AGC OFF in the DSP Pane, Thresh: 50 dB, Therm: 58%, AGC OFF in Main Pane.
For those of you who run digital modes on 6M these are the settings Hasan is using for his SDRPlay RSPdx and IC-7300 dual RX setup.
6 Meter MSK144 Setup with SDRC v3 and SDRPlay RSPdx Hardware
IC-7300: Preamp Off, Internal Attenuator Off
Nooelec Wideband Preamp/LNA: 0.7 dB NF, 25 dB Gain
Spec'd Wideband 20 MHz - 4 GHz (works down to 2 MHz, so can be left in line with
above settings.)
Step Attenuator: Cheap one from eBay
Step Atten: 13 dB
RF Gain: 23 dB (SDRC)
IF Gain: -40 dB (SDRC)
Main AGC: Off (SDRC)
Main AGC: 75 dB (SDRC Threshold)
Visual Gain: -35 dB (SDRC)
WB DSP NB: 3.6dB (SDRC NB Threshold): Blanking 35% (depends on noise level)
Audio Level: 75 (SDRC Audio Slider)
SDRC Filter: 100 - 3600 Hz
WSJT-X Input Level: 41 dB with no signal (Thermometer Reading)
Paul, W2HRO, measured the Nooelec 20-4000 MHz Wideband LNA on his Noise Meter.
1296 MHz 17 dB gain 1.12 dB NF
