Designing high performance VHF/UHF receive systems – Part 7

Summary

This series of seven articles has:

  • explained the meaning and value of G/T as a single metric for receive system performance;
  • defined and explained the G and T terms;
  • explained the relationship between Teq and Noise Figure;
  • explained how to analyse simple cascaded stages and hence more complex networks;
  • described how to estimate transceiver Noise Figure and Teq;
  • demonstrated application of the analysis techniques to a set of practical configuration options to provided quantitative comparison of the S/N performance of the options; and
  • discussed measurement of G/T as a means of validating system performance.

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Designing high performance VHF/UHF receive systems – Part 6

Measurement of G/T

G/T can be measured using celestial noise sources provided the antenna can be pointed to them. The noise source that is most appropriate will depend on expected G/T, frequency, time etc.

Sun

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Designing high performance VHF/UHF receive systems – Part 5

Bringing it all together

This part explains how to build a model of the entire receive system to calculate G/T.

Firstly, make an inventory of all of the system elements that you intend to model.

A model needs to be no more detailed than is necessary to provide adequate accuracy for the purpose at hand.

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Designing high performance VHF/UHF receive systems – Part 4

Finding transceiver Teq

We have explained how to calculate Teq from Noise Figure, but most transceiver specifications do not give Teq or Noise Figure directly, in fact they don’t really contain sufficient information to reliably calculate Teq or Noise Figure.

Credible equipment reviews might provide an estimate of Noise Figure or Teq.

The best approach is to directly measure Noise Figure using a known noise generator and the Y Factor Method.

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Designing high performance VHF/UHF receive systems – Part 3

Relationship between Teq and Noise Figure

In the last part, the meaning of the equivalent noise temperature of an amplifier was given.

Whilst you will find that working in Teq has advantages for this analysis, amplifier specifications may not give Teq, but may give Noise Figure.

Continue reading Designing high performance VHF/UHF receive systems – Part 3

Designing high performance VHF/UHF receive systems – Part 2

G/T is defined as the ratio of antenna gain to total equivalent noise temperature.

For clarity, lets define those terms.

Gain

Gain of an antenna is defined (IEEE 1983) as the ratio of the radiation intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. (Isotropically simply means equally in all directions.)

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Designing high performance VHF/UHF receive systems – Part 1

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=50/120=0.416 1/K or -3.8 dB/K.

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VK2OMD G5RV with tuned feeder – line loss

The G5RV Inverted V antenna system at VK2OMD is fed with 9m of home made open wire transmission line using 2mm diameter copper wires spaced 50mm giving a line with characteristic impedance of 450Ω. (Varney 1958) described the tuned feeder configuration of his popular G5RV antenna system.

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Small transmitting loop review

I saw a recent ‘maker’ video describing a small transmitting loop for 40m.

The loop used a 3m length of 19mm copper pipe formed into a circle, and at the gap where the ends almost meet, a tuning capacitance is synthesised using coaxial cable.

Screenshot - 15_02_2014 , 12_06_57

Above is a screen shot from Reg Edwards loop design program. It calculates the radiation resistance at 0.005Ω, loss resistance of the loop at 0.035Ω, capacitance to resonate it of 206pF (Xc=108Ω), and a bandwidth of 3.2kHz.

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ATU voltage verification

I described a method for designing antenna systems to avoid excessive voltages in baluns and ATUs at (Duffy 2011) .

This article reports post implementation measurements of an antenna system designed using that method and using a G5RV Inverted V with tuned feeder and ATR-30 ATU with integral 1:1 current balun. The tuned feeder is a home-made line section of 2mm diameter copper conductors spaced 50mm, and 9m in length. An additional 0.5m of 135Ω line connects from the antenna entrance panel to the ATU.

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