DJ3JJ’s 432MHz EME Yagi article in Dubus 1/2010

DJ3JJ wrote up some interesting experiments with Yagis for 432 EME (Haefner 2010).

4 x ZB7013 Yagis

The first system he describes is 4 x ZB7013 Yagis with claimed gain of 22.65dB, and measured Sun/ColdSky of 4.5dB at SF=68SFU.

We can calculate the G/T from these observations, G/T=-7.15dB/K.

If we assume the gain figure is correct, then the system noise temperature is 10^(G/10-G/T/10)=10^(22.65/10–7.15/10)=955K. This is an awful system noise figure.

1 x EF7015

The next system uses a single EF7015 Yagi with claimed gain of 17dB,  and measured Sun/ColdSky of 4.0dB at SF=70SFU. (I note he states SFI=70 which questions whether he is giving solar flux at 432MHz… but lets hope it was a typo).

We can calculate the G/T from these observations, G/T=-8.08dB/K. It is not much worse than the higher gain 4 x ZB7013 system.

If we assume the gain figure is correct, then the system noise temperature is 10^(G/10-G/T/10)=10^(17.0/10–8.08/10)=322K. Though only a third of the previous case, this is still a relatively high system noise figure… perhaps double of what you might like.

4 x EF7015

The next system uses 4x EF7015 Yagi with claimed gain of 22.94dB,  and he suggests Sun/ColdSky of 10.0dB at SF=70SFU. (I note he again states SFI=70 which questions whether he is giving solar flux at 432MHz… but again lets hope it was a typo).

We can calculate the G/T from these observations, G/T=-0.33dB/K. It is quite a leap from the 1 x EF7015 system, and it is not clear that he actually measured 10dB. Lets assume that he did, that he is not just making wild estimates (and a 8dB improvement would be a wild estimate).

If we assume the gain figure is correct, then the system noise temperature is 10^(G/10-G/T/10)=10^(22.94/10–0.33/10)=212K. System noise is getting closer to what might be expected.

With G/T around 0dB/K, it is at the low end of 432MHz EME station performance, G/T up to nearly 10dB/K would represent the state of the art from 4 x Yagi systems (though more than twice as long).

The elephant in the room

It is a common practice amongst Hams to use 10.7cm flux measurements as the flux density at other frequencies. This is wrong, and if the solar flux values quoted in (Haefner 2010) are 10.7cm flux measurements, then the above results will be wrong as flux at 432 might be more like 30-50% of the 10.7cm flux. He has not given dates for the experiments, so I cannot check archives. The simple fact is that there is no reliable accurate way to preduct solar flux at 432MHz from flux at 10.7cm, examination of historical data will demonstrate that fact.

To use Sunspot number (SSI, T index) give even worse error.

Sometimes the uncertainty can be resolved by calculating a solution assuming that solar flux was given for 10.7cm and testing its feasibility. If that is done for the 1 x EF7015 scenario, and we assume solar flux was around 30SFU, then system noise temperature would be around 140K which is quite believable and it does not help to resolve the ambiguity about the author’s meaning of solar flux values.

It seems Hams boast Sun/ColdSky Y factors without understanding that they have no stand alone value, but they are very useful if the solar flux at the frequency of observations is known and the G/T figure calculated.

If I was a betting person, I would bet on Ham behaviour and that all the solar flux observations were in fact for a frequency of 2800MHz, more than 6 times that of the Sun Y factor measurements.

References

  • Duffy, O. 2007. Measuring system G/T ratio using Sun noise. VK1OD.net (offline).
  • ———. 2009. Quiet sun radio flux interpolations. http://owenduffy.net/calc/qsrf/index.htm.
  • ———. 2014. Measuring G/T. http://owenduffy.net/blog/?p=1490.
  • Haefner, A. 2010. 432MHz EME with a small Antenna In DUBUS. http://www.g0ksc.co.uk/DJ3JJ.pdf(accessed 27/04/14).
  • ITU-R. 2000. Recommendation ITU-R S.733-2 (2000)Determination of the G/T ratio for earth stations operating in the fixed-satellite service .