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Re: 5/8 wave GP antenna for sats (was Re: ...)



In a couple of places below Emily mentions that either (1) preamps may be 
marginal for omni antennas, or further on implies (2) preamps are nil for 
omni antennas.  Although I think Emily meant "lower" in the first statement 
for S/N.

Anyway, I'm not trying to pick on Emily (far from it!), but a few years 
back, and posted on this forum, I performed a measurement with my 
truck-mounted omni 70cm antenna, an M2 HO-loop about 8' off the 
ground.  The idea was to determine if the hypothesis of "high ground noise" 
at 70cm was true.

Reproduced here:

 > 70cm Mode J downlink setup:
 > 70cm M2 HO loop about 8' off ground
 > 5' 9913
 > 4x3x5 coaxial cavity filter (measured IL=0.4dB)
 > ARR 432VDG GaAsFET preamp (the hot one)
 > 5' 9913
 > FT-100
 >
 > I "know" I hear quite a bit better with the preamp than without, but today
 > a test.  I inserted a 20dB directional coupler right at the antenna, so I
 > could inject a signal generator signal (to the coupled port) in the
 > presence of "normal" antenna noise.  With a low-directivity antenna, one
 > would expect a fair amount of noise temperature.
 > Preamp off:  -130dBm injected (accounting for coupling loss) results in
 > 10.0dB SINAD, 2.4kHz filter bandwidth.
 > Preamp on:  -130dBm injected results in 20.0dB SINAD, same bandwidth.
 >
 > So, a 10dB (!) improvement in S/N!  And that seems to be a fair system
 > sensitivity too, -140dBm for 10dB (S+N)/N.
 > Moral of the story:  a 70cm preamp can make a HUGE difference in Mode
 > J:  If you don't have one, get one!

So don't let the omni antenna scare you off from a "real" preamp.  I should 
note that the rig here is an FT-100, and on 70cm you can't turn the 
internal preamp off, so this is with it on...still +10dB S/N with the external.

Really the bottom line is that rigs today have 7-10dB noise figure front 
ends, even with the internal preamps, and as such are not designed 
out-of-the-box for weak signal work, which after all satellite is all about.


At 11:47 2005-01-25, Emily Clarke wrote:
>Hi Tim,
>
>
>- A ground plane antenna is omnidirectional, so although the signal of the 
>satellite is coming from one direction, the noise is coming from all 
>directions.  So the signal-to-noise ratio is much higher.
>
>  - AO-51 transmits with 500 mw.  UO-14 was nominally 4 watts or 9db 
> greater.  So something that "heard" UO-14 at S2 or S3 will not 
> necessarily be able to hear AO-51.  Good rule of thumb - if you can hear 
> SO-50 (250mw) at S1, you will still only have S2 for AO-51.  If you can't 
> hear SO-50, you might not hear AO-51. I always recommend to people to 
> start off listening to SO-50.  If you can hear it well, you are in 
> business.  If you can't, you have your work cut out for you.
>
>  - RG-8 is lossy at 436MHz - I don't have the exact number handy but I 
> think it's about 4.9db per 100 ft.  If you are using RG-8X (popular on HF 
> and 2M ) at 436 the attenuation is 8.1db per 100/ft.  9913 on the other 
> hand is 2.6db per 100 feet.  LMR-400 is 1.9db per 100 feet.  I use 
> LMR-400 Ultra Flex for receive (436MHz) and 9913 for transmit (145MHz).
>
>- The Preamp in the 706 won't do much good because it is located in the 
>radio, not at the antenna.  So it is amplifying signals after the cable 
>losses have been factored in and won't overcome the losses in the 
>cable.  A mast mounted pre-amp will help, but I don't know how much extra 
>signal gain you will get out of an omni-directional ground plane versus 
>just amplifying ground noise.
>
>My recommendation would be to improve your antenna system by using either 
>eggbeater antennas (which will absolutely require preamps and won't help 
>the signal-to-noise ratio) or directional yagis (which would still benefit 
>from mast-mounted preamps but will improve the signal-to-noise ratio.)
>
>
>73,
>
>Emily
----
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