Do I ‘need’ a masthead preamp to work satellites on 2m? – space noise scenario

A question asked online recently provides an interesting and common case to explore.

Base scenario is a low end satellite ground station:

  • 144MHz;
  • satellite;
  • IC-9700, assume NF=4.8dB;
  • high gain (narrow beamwidth antenna);
  • 10m of LMR-400.

G/T

A metric that may be used to express the performance of an entire receive system is the ratio of antenna gain to total equivalent noise temperature, usually expressed in deciBels as dB/K. G/T is widely used in design and specification of satellite communications systems.

G/T=AntennaGain/TotalNoiseTemperature 1/K

Example: if AntennaGain=50 and TotalNoiseTemperature=120K, then \(G/T=\frac{50}{120}=0.416 \text{ } 1/K\) or -3.8 dB/K.

The utility of G/T is that receive S/N changes dB for dB with G/T, in fact you can calculate S/N knowing G/T, wavelength, bandwidth and the field strength of the signal (Duffy 2007).

\(Signal/Noise=S \frac{\lambda^2}{4 \pi} \frac{G}{T} \frac1{k_b B}\) where:
S is power flux density;
λ is wavelength;
kb is Boltzmann’s constant; and
B is receiver equivalent noise bandwidth

Usage in this article is consistent with the industry standard meaning of G/T given at (ITU-R. 2000) (as opposed to the meaning used by some Hams who have appropriated the term for their own purpose).

Note this is not the bodgy G/T figure used widely in ham circles.

Ta

Ambient noise temperature Ta is an important factor in calculation of G/T. Ta depends on frequency, the environment, the antenna’s ability to reduce off boresight noise, and the on-boresight noise. For the purposes of this discussion let’s assume total ambient noise for the given satellite scenario at 144MHz is 250K.

Base scenario

Above is a calculation of the base scenario, G/T=-29.74dB/K.

With masthead LNA

Above is a calculation of the masthead amplifier scenario, G/T=-25.21dB/K.

With LNA at receiver

Above is a calculation of the LNA at the receiver scenario, G/T=-25.754dB/K.

Summary

Scenario G/T (dB/K)
Base -29.74
With masthead LNA Gain=20dB NF=1dB -25.21
With local LNA Gain=20dB NF=1dB -25.75

The first finding is that adding a masthead LNA with 20dB gain and 1dB NF makes a small difference to G/T and hence S/N, 4.5dB in this case.

Note that there is only a small degradation in moving the LNA from masthead to local to the transceiver. There are additional reliability / maintenance issues with masthead located amplifiers… particularly if high performance narrow band front end filtering is used. It is much more practical to house a coaxial resonator (‘can’ in repeater parlance) in the shack that at the masthead.

Practical LNAs

The foregoing analysis assumed a linear receive system, no intermodulation distortion. Now let’s talk about the real world.

Some LNAs are sold without specifications, those that have meaningful NF and Gain specifications are usually based on laboratory measurements with no interfering signals.

When attached to an antenna, the out of band signals will give rise to noise due to intermodulation distortion, so the NF in-situ might be poorer than specification NF. Indeed, the IMD noise can be so great as to deliver worse G/T with the LNA.

One way of reducing IMD noise is to limit the amplitude of interfering signals arriving at the LNA active device, and front end filtering is one possible solution.

Be aware that lots of hammy Sammy LNA designs have very little front end selectivity, relying upon the narrow band response of a high gain antenna. When these are used with low gain tuned antennas, or worse, broadband antennas like Discones, the IMD noise can be huge.

On the other hand, there are LNAs available with a very narrow front end filter… but they cost a lot more.

The benefit / necessity of front end filtering depends on your own IMD scenario.

Other scenarios

For satellite work, a low gain antenna will tend to have higher Ta by virtue of side lobe contribution, and so the improvement seen above might be diminished a little.

Terrestrial ambient noise is much higher, and the improvement would be considerably less. Likewise for an omni satellite antenna. In both cases, the improvement in G/T might be less than 1dB with the same masthead LNA… download the spreadsheet and explore.

As mentioned Ta is frequency dependent, so the case for 432MHz might be quite different than the above case. In particular, the choice of masthead mounting becomes clearer on higher frequencies.

References

Last update: 15th September, 2021, 7:04 PM