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Re[2]: Preamp bypass circuit

Hi ,

I have to agree to use a sequencer, it's working great for me with my
ft847. I also use 2 coax cables ( rx ands tx seperate ) so that when
the relay or preamp failes to work i can still use my antennas

If you need a circuit that's easy to build i can mail it ...


Thursday, February 15, 2001, 9:13:27 AM, you wrote:

>>From: "Duffin, Jon" <jduffin@dbxpro.com>
>>I have recently built up a GasFET preamp for use on the 2 meter band on
>>AO-10.  To switch the preamp out during transmit, I have built up a RF
>>sensed circuit which bypasses the preamp through a relay.  The problem is
>>that I can observe a breif RF transient that would go into the preamp (I
>>actually have not hooked up the preamp to the switch circuitry yet) before
>>the relay switches into the bypass mode.  I am fearful that this RF
>>transient would be enough to blow out the GasFET.  Does anybody have any
>>hints or ideas how to supress this transient so I won't be replacing the
>>My rig is a FT847, and so I though another idea would be to switch the
>>preamp out of the transmit path by using the 847's PTT accessory jack.  But,
>>I don't think that the results would be any different unless there is a way
>>to delay the power output stage from coming up at the same time as the
>>switching of the PTT accessory jack signal.
>>How does everyone else get this to work on their homebrew preamps?

ERC> Jon,

ERC> This is a common problem faced by hams doing eme.  Using an RF detect
ERC> circuit you will always have a finite switching time and result in a short
ERC> exposure of the preamp to Tx RF.  At 50w you may get lucky? if the Rf
ERC> circuit is quick.  At >1kW there is no luck!  

ERC> Instead I suggest you control both the preamp and FT-847 PTT with a
ERC> sequencer circuit.  This means that your mic PTT and cw key will need to be
ERC> routed thru the sequencer as well, or an accidental transmission could happen.

ERC> The sequencer disables the preamp, switches coax relays, and enables your
ERC> xmtr in that order with small finite time delays to prevent one operation
ERC> happening out of sequence.  The sequence repeats in reverse order when
ERC> returning to receive-mode.  I really think satellite operators should
ERC> consider using a sequencer for controlling their equipment.

ERC> There is a lot of literature about them in the moonbounce community.
ERC> Easiest is probably to purchase one from Downeast Microwave:
ERC> http://www.downeastmicrowave.com
ERC> check under accessories.

ERC> To be absolutely safe you should separate the Rx antenna input in the
ERC> FT-847.  But I have been successful using a sequencer on my unmodified FT-847.


ERC> ----
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Best regards,
 cahill                            mailto:cahill@dds.nl

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