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Re: Preamp recommendations



Mark,
Given the beams you are buying and the coax you are using, I doubt a preamp 
is necessary at all.  The LMR-600 has less than 2 dB loss per 100' at 70 cm.  
Considering the very sensitive front end and fairly modern noise figure of 
the  FT-847 (not much worse than a high-quality preamp), unless you have 
several hundreds of feet of coax, all you will do is increase the volume at 
the receiver, not the S/N ratio.

I would recommend setting up first without one and seeing how that works for 
you.  Some ops have found a preamp helps with 2 meter de-sense, but that 
phenomenon is very site-specific.  You won't know if it's a problem until you 
set up your entire station and check it out.

BTW, I use and have used several different preamps, but all my antennas are 
low gain (unlike yours).  I find the both the ARR and Mirage units to be more 
than sensitive enough, with the ARR having the edge on gain/noise, but the 
Mirage is built like tank.  I have also built Hamtronic preamps, which work 
very well but are too tiny for my aging eyes any more :-)  There is a FAQ, I 
believe, in the KK5DO scripts on the AMSAT site.  

One further comment:  I have heard a rumor M2 is coming out with a 
polarity-switching option for their CP antennas.  That functionality may be 
worth far more to you long-term in satellite communications than a preamp 
behind high-gain beams.

73 es GL,
Jerry, K5OE

> I'm sure it has been answered before, but I have have not seen it so:
>  
>  I'm looking for a really good preamp for 70cm, the best available. That is
>  of course if I do indeed need one at all. My modus iperandi is to pretty
>  much
>  get the best available up front, so I don't care if it is expensive or if 
it
>  will
>  only give me 'a little' benefit. The preamp needs to switch between TX and
>  RX,
>  I do not mind running a PTT signal wire to the preamp, which will be 
mounted
>  on the antenna.
>  
>  I'm setting up to work 1200 and 9600 baud satellites. I'm getting some M2
>  circular beams in any day now. I'm using Time Microwave LMR  600 cable,
>  which is pretty good stuff, with very low loss. This goes to a Yaesu FT847,
>  I don't know much about the noise figure of the built in 'pre-amp'.
>  
>  Question 1: Will a preamp help at all? One book says I MUST have preamps,
>  another says I don't need them. Lots of opinions. Does anyone have yet more
>  opinions, if so I would like to hear them.
>  
>  Question 2: Is there a preamp FAQ out there?
>  
>  Question 3: If you are using preamps, what is your favorite brand/model?
>  
>  So far it looks like the SSB SP-7000 is the one to beat. It claims 10-20dB
>  with a maximum Noise figure of 0.9db. Others are publishing better noise
>  numbers
>  but do not provide curves like SSB does. Also SSB is using single gate
>  GaAsFets,
>  which I undestand are quieter than dual gate GaAsFets, and MUCH quieter 
than
>  bipolars (which realy are'nt under consideration anyway).
>  
>  - Mark West
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