[amsat-bb] Re: preamps

[Edward Cole]

At 08:59 AM 8/31/2008, you wrote:


>A recent post mentioned the need for a preamp.  This reminded me of 
>something I want to share with the group.
>
>Station here consists of an FT-817ND and an FT-857D.  The 817 is 
>pretty sensitive on UHF, but the 857 is a little lacking in comparison.
>
>I was looking for a preamp online that would help, especially with 
>the 857.  I went and ordered a Ramsey PR40.  I figured for $15 I 
>couldnt go wrong.  Although its not a GaAs device, (its a bipolar), 
>it works pretty well.  I actually didnt want a GaAs because I was 
>afraid of blowing the device anyway, being I'm running a homebrew 
>"Arrow style" antenna on a fixed elevation mast, and the 144 MHz 
>leakage getting into the preamp would probably blow the device the 
>first time I keyed.
>
>I am not using any TR switching on the preamp. (I only work the FM 
>birds at this time).
>
>After building the kit, I put it up, worked a pass on AO-27, then 
>goofed and sent 432 TX energy up the line, blew the amp.  I ordered 
>replacement transistors from MCM for $.52 each!
>
>Although I cannot measure noise figure, it specs at 1dB, 15 dB 
>gain.  I know it makes a HUGE difference on my end.  Satellites that 
>were noisy before are now full quieting and armchair copy
>
>I have found this item a very cost effective solution.  I wanted to 
>share this experience with the group.  I'm just pleased with the 
>preamp and of course, receive no personal gain by recommending this product.
>
>  Michael Heim
>ARS KD0AR
>_______________________________________________

Michael has discovered what many of us have been preaching for many 
years.  Preamps at the antenna makes a world of difference in what you hear.

But he also discovered how most preamps are blown.  Its not from 
leakage via the antenna but from transmitting on the coax going from 
the radio to the preamp.

Solutions:
    * Use coax relays to bypass around the preamp. (I am doing this 
for 222 and 432 terrestrial antennas/preamps)
    * Purchase a preamp designed for a transceiver (usually rated for 
certain maximum power levels such as 25w or 160w, etc.).
    * Modify the radio so the receiver has a separate antenna 
connector.  (I have this done on my FT-847)
    * Use of protective devices that absorb transmit power (up to 
50w, usually) and only reduce preamp gain a little bit.
So one should not be afraid to use Gasfet preamps.  They are not 
usually blown by signals entering the input side.  However, If you 
are in high lightning country you should provide for shorting the 
antenna input to the preamp to avoid HV static discharge.

Always locate the preamp close to the antenna to avoid cable losses 
in between the antenna and preamp that directly add to the noise 
figure.  Use low-loss coax in that jumper.  Coax from the preamp 
output to the radio may be low cost coax since the preamp gain will 
usually overcome these losses.

Thanks for your experience, Michael.




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73, Ed - KL7UW              BP40iq, 6m - 3cm
144-EME: FT-847, mgf-1801, 4x-xp20, 185w
http://www.kl7uw.com     AK VHF-Up Group
NA Rep. for DUBUS: dubususa at hotmail.com
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