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Effective use of a Low Noise Amplifier on VHF/UHF

This article was inspired by the suggestion that modern amateur VHF/UHF transceivers lack adequate sensitivity, and require an outboard Low Noise Amplifier (LNA) or pre-amp for adequate performance.

The following is an analysis of an IC910 in typical station configurations that explore the benefit of the LNA, and the best location for the LNA.

A first and last word on S-meters for this article, be aware that S-meter deflection is not a satisfactory method of assessing receive system performance. Increasing S-meter deflection does not equate to increased sensitivity.

G/T ratio

This analysis focuses on a single figure of merit for the entire receive system, and that is the G/T ratio (the ratio of antenna gain to system equivalent noise temperature. G, Antenna gain is with reference to the antenna connector. T, system noise temperature, is the total system noise, including sky temperature (or ambient noise) referred to the antenna connector.

The G/T ratio is a meaningful indicator of system performance, unlike quoting receiver noise temperature in isolation of sky temperature and antenna gain. The ability to receive weak signals is directly related to G/T, the higher G/T, the better. Signal/Noise ratio is proportional to G/T (Signal/Noise=S*λ2/(4*π)*G/T/(kb*B) where S is power flux density, kb is Boltzmann's constant, and B is receiver effective noise bandwidth).

So, if changing the receiver Noise Figure from 5dB to 1db improves the G/T ratio by just 0.1dB/K, the real benefit of the change is just 0.1dB.

Models

Assumptions

This analysis does not consider intermodulation distortion (IMD) in the LNA. It is relatively easy to build a high gain, low noise amplifier, but more expensive to exclude unwanted signals so as to minimise IMD. An LNA with insufficient front end selectivity may degrade system performance because of intermodulation products.

Losses in connectors, change over switches etc are lumped with the adjacent major configuration element.

LNA at antenna

The baseline model is a station with IC910 transceiver (receiver), a high power amplifier located near the transceiver, sections of transmission line, an antenna of 20dBi gain, and an LNA of 0.5dB NF and 25dB gain at various locations.

Fig 1: Base model worksheet

The base model is a fairly ideal implementation, it uses the components in a way so as to achieve the best receive performance realisable with the components.

Figure 2 shows the contributions of each component to the receiver noise temperature (47.3K), as defined at the antenna connector for the downstream components. Note the significance of loss upstream of the LNA, with the 0.2m LDF2-50 jumper contributing ~7K.

The G/T ratio for this configuration is -1.8dB/K

Fig 2: Contributions to receiver Noise Figure

LNA at receiver

The second scenario is with the LNA located between the transceiver and the linear amplifier, so that the LNA does not need to switch high transmitter power. Figure 3 shows the contributions of each component to the receiver noise temperature (292K), as defined at the antenna connector for the downstream components.

Fig 3: Contributions to receiver Noise Figure

The G/T ratio for this configuration is -6.0dB/K, which is 4.2dB/K worse than the base configuration, meaning that the distant station needs about three times the transmit power to achieve the same received signal to noise ratio at this receiver. Contrasting the performance to the No LNA configuration below, there is no doubt that adding the LNA, even near the receiver, is a worthwhile improvement (subject to IMD issues).

No LNA

The third scenario is with no LNA. The receiver noise temperature is now 1341K, as defined at the antenna connector for the downstream components.

The G/T ratio for this configuration is -11.6dB/K, which is 9.8dB/K worse than the base configuration, meaning that the distant station needs about ten times the transmit power to achieve the same received signal to noise ratio at this receiver. 

Sensitivity to transceiver NF

The noise figure of the transceiver was varied from 5dB to 1dB in the above three scenarios to explore the sensitivity of G/T ratio to a better transceiver. The results are tabulated in

Table 1: Sensitivity of G/T to transceiver NF
Scenario Transceiver NF (dB) G/T (dB/K)
LNA at antenna 5 -1.8
1 -1.7
LNA at receiver 5 -6.0
1 -6.0
No LNA 5 -11.6
1 -6.7

It can be seen from Table 1, that best performance is obtained by using an LNA located as close as possible to the antenna feed point, and that if the LNA has sufficient gain, its performance dominates system performance.

Adding an LNA at the transceiver is similar to improving the NF of the transceiver, but that can not offset the performance degradation of upstream feed line losses. Hence, there is little point in designing the transceiver for lower NF when it will still require an LNA located near the antenna for good performance, and in that case, the G/T ratio is not very sensitive to transceiver NF.

Benchmark G/T

So, if G/T is a good figure of merit for receive system performance, what is a good G/T ratio?

Well, that depends a lot on frequency and application.

For high performance terrestrial communications:

yielding a G/T ratio in the region of -6dB/K. Higher G/T could be achieved dB for dB with higher antenna gain.

For EME work at 432MHz a G/T of 2 dB/K should be sufficient to hear  signals in a 100Hz wide receiver from typical stations with EIRP of 100kW or better with the moon at perigee and without assistance from ground gain or libration. Note that it may be quite feasible to make "contacts" using WSJT or similar techniques with lesser EIRP and/or G/T. At higher frequencies it is easier to achieve high antenna gain, and sky temperature is lower, so higher G/T is quite feasible.

Incomplete G/T statistics

There are tables in circulation that compare different antennas using calculated G/T ratios in a specific ambient noise environment that does not fully include the effect of feed system losses and assumes an otherwise ideal (ie noiseless) receiver system. They overestimate the G/T ratio of an entire system using any of the antennas.

It is a questionable statistic as it does not properly represent the performance of the entire system, and so defeats the usual purpose of use of G/T ratio as a single figure of merit for the entire system.

Summary

The noise scenarios presented in the models are representative of EME work on 432MHz. At lower elevations,  sky noise is higher (depending on location), and the improvement from locating the LNA at the antenna less. The effects on higher frequencies where line losses are greater and sky temperature is lower will be even more evident than described here for 432MHz.

Two key metrics that characterise the transmit and receiver performance of a station are its EIRP and G/T ratio respectively. Stations commonly advertise their transmitter output power, antenna gain and possibly LNA Noise Figure, but these quantities alone do not give an overall picture of their transmit and receiver performance. Do you know the EIRP and G/T ratio for your station?

Links

Changes

Version Date Description
1.01 16/03/2006 Initial.
1.02 17/03/2006 Reworked for 432MHz. V1.01 contained an error in loss of one cable section and should be disregarded. Benchmark G/T reworked.
1.03 15/04/2006 Updated spreadsheet.
1.04 15/07/2006 Updated spreadsheet.
1.05 20/07/2006 Comment on incomplete G/T statistics

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