[Moon-net] Fw: Re: G/T question

[Leif Asbrink]

Hi Dom,

> I think that we , 2m band users , are giving G/T factor too much emphasis. 
> 2m is a noisy band (sky noise is much higher than on 70cm)  , G/T factor is 
> very usefull on 70cm , but on 2m  , I wonder?

(all the below is for 144 MHz only)

It depends on your QTH.
At my location maximum gain without any compromise is the best
for EME. (Provided an adequate noise blanker removes all the
impulse noise) There is no question whatsoever. I can measure
the noise temperature in all directions and I see no increase
above the varying cosmic noise at elevation=0 degrees in any 
azimuth. Surely some frequencies become useless due to birdies
that are received near the horizon, but even a max gain antenna
suppresses them reasonably well as soon as it is elevated.
With an elevation above 10 degrees the lost frequency range
is only a few percent. Trading 1 dB S/N on all frequencies always
for a 10 dB reduction of those birdies is not a good idea
at all at my qth. (Or rather it would not have been while I still
had the 14 element antenna and I do not expect the situation to
have changed)

Surely some 144 EMEers have difficult QTHs and may benefit
greatly from a maximum G/T design. 


> Probably , getting a reasonnable F/B ratio ( 20db) , and sidelobe rejection 
> is enougth.
NO. Not even that. A maximum gain design has strong side lobes 
and lower F/B. I was using such an antenna for about 10 years
with pretty good results in contesting and I did evaluate
it carefully. A very narrow main lobe, side lobes at about
-8dB and a back lobe at -13 dB if I remember correctly.
For its size, the gain of this array was VERY high.

Please do not take the above as a recommendation to make 
max gain designs. My next antenna (hopefully some day)
will be 4x2SA13X which is NOT an antenna designed for EME 
only. There is some loss of gain but a much larger
fraction of the total power is emitted through the main lobe.
Compared to my old 4x14 with a gain of 21.4 dBd a 4x2SA13
will give 1 dB less gain with an assoscated S/N loss of
1 dB for EME. For IONO scatter and aurora I expect it to
give about 3 dB better S/N because it will send nearly all 
power into the appropriate scatter volume:-)

I think 20.4 dBd is enough for EME with high power in a 
quiet location - but for IONO the antenna is far more 
critical.....

73

Leif  /  SM5BSZ