[amsat-bb] Re: preamps

Luc Leblanc lucleblanc6 at videotron.ca
Sun Aug 31 20:24:52 PDT 2008

On 31 Aug 2008 at 11:25, Edward Cole wrote:

> 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.

> >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 

> >  Michael Heim
> >_______________________________________________
> 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.


I was a non believer too but preamp for VHF UHF is mandatory If you don't want to use the sat 3 minutes only at TCA or calling CQ in the 
void.  When someone like you is using as much power answer you it is already too late the pass band is close as all the other user also 
cranked up their power.


Who never blow his preamp please raise your hand? I blow 3 repair 1  2 to go.  But as  the famous "transistor" is always hard to get!  on 
my icom i order 6 just in case..Even if they are protected i managed to use an external voltage bias source who was always on when 
switching between bands. I ended up with the bad combination plus a mike who fell down just over my straight key and POOF not exactly it's 
insidious when you switch back to normal you don't hear the noise increased then you say !!!! (censored)

Here is below the solution i found. I know they are not the best for signal to noise ratio but as the run for the curliest girl on the 
dance floor syndrome  the next one can also dance.



Happy labor day week end.  I will try to be on the sat tomorrow if my wife gardening crisis slow down a bit...50% off on trees and plants 
over here. 6 uprooted 1 planted 5 to go BUT could be next year "finger crossed" she return back tomorrow!!!


Luc Leblanc VE2DWE
Skype VE2DWE


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