[CQ-Contest] Re: Bandpass filters, front end protection

Pete Smith n4zr at contesting.com
Fri Nov 9 12:43:18 EST 2001

At 12:26 PM 11/9/01 -0500, dennis o'connor wrote:
From: "dennis o'connor" <k8do at msn.com>
To: "n4zr" <n4zr at contesting.com>
Subject: Bandpass filters, front end protection
Date: Fri, 9 Nov 2001 12:26:51 -0500

Hi Pete, read your message in the contesting digest, so if you would back
post this to the list, I will appreciate it...
 Let me propose an alternative route to the same end...   I would avoid any
mixing devices in the receive path... They ARE going generate spurious
garbage...  So, my approach is to use coaxial stub filters for each band...
They will tolerate transmit power without any t/r switching needed... They
will greatly reduce out of band signals from arriving at the front end on
RX...  They will clean up the signal you are presenting to the amp on TX
(appreciated greatly by folks like me)... Go with this first and test to
see if, and on which bands, additional receive bandpass filtering may, or
may not, be needed...  Where needed you can add the BP filters.... But, do
not put any diodes on the path... 

The diode idea which a number of people suggested for RX protection
involves a sufficient number in the string, in each polarity, so that
normal signals will not cause them to conduct.  I'd think that would work.

 Also, if one stub is not sufficient filtering, you can add a second stub
with a 1/8 wave of coax between the two, for additional filtering.... And,
filter stubs will have minimal loss of transmit power and what doesn't get
to the amp you shouldn't have been sending anyway...  Top Ten Devices,
among others, will have a good discussion on this... Use 5/8" or 3/4" catv
coax for the best performance...
Thanks, I'm going to try the stubs. 
 On the 220 needing 120 watts of drive... Wow... I have a Henry 3K with a
pair of 3-500... With the good Henry power supply putting almost 4kv on the
plates, 80 watts will drive the amp to the legal limit and 100 watts will
send it into ecstasy...  I suggest that your winter project be an external
HV supply for the SB220 as opposed to a complex bunch of bandpass boxes......

Nah, just a lousy input circuit SWR, which I need to attack one of thesde
days..  I know I could get more out at higher plate voltage, but then other
things in the design start to go.

73, Pete N4ZR

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