I'd go with more like 6 or 10 dB. The return loss of a pad is equal to
twice its insertion loss, so if you use a 6 dB pad, the worst-case return
loss would be 12 dB (i.e. if the pad were open or shorted at the far end).
This 12 dB plus whatever the return loss is of the device under test (which,
we would hope, is 14 dB or more, i.e. 1.5:1 VSWR) should keep you close
enough to 50 ohms. Not only will this extra padding help insure that all of
the ports on the duplexer are properly terminated with something close to 50
ohms, but it will also help avoid false readings that can occur when the
level of the signal generator output varies depending on the load Z. I
don't know how many times I've seen filters that were "peaked for max" that
were quite a ways out of tune because of this very reason...
--- Jeff WN3A
> -----Original Message-----
> From: Repeater-Builder@yahoogroups.com
> [mailto:[EMAIL PROTECTED] On Behalf Of JOHN MACKEY
> Sent: Monday, June 30, 2008 1:58 AM
> To: Repeater-Builder@yahoogroups.com
> Subject: Re: [Repeater-Builder] Re: Frequency Change do I
> retune duplexer?
>
> This is why it is best to put about 1-2 db 50 ohm pad between
> the duplexer
> and the receiver, to help keep the receive side at 50 ohm.
>
> ------ Original Message ------
> Received: Sun, 29 Jun 2008 10:35:15 PM PDT
> From: [EMAIL PROTECTED] <mailto:no6b%40no6b.com>
> SNIP
> > Maybe a dB or two at most. I claim that once you take that
> duplexer off
> > the spectrum analyzer/tracking gen. & install it in your
> system, the actual
>
> > isolation may very well change more than that due to notch
> pulling from the
>
> > non-50 ohm TX, RX and/or antenna. So all your efforts of
> squeezing every
> > last fraction of a dB out of the duplexer notches are for naught.
> SNIP
>
>
>
>
>
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>
>
