[1000mp] Ten-Tec Orion - Roofing filter

[Bill Tippett]

W6EIJ wrote:
 >

I've followed the discussion of the Orion - very interesting.  The current
QST has a two -page advertisement which described the filtering.  I started
thinking about the possibility of putting a selectable roofing filter, a la
Sherwood, in the Mark V.  At the expense of sounding dumb, is this possible?


         Not a dumb idea at all Tom.  In fact the new Icom IC-7800 will
have a 6 kHz roofing filter at 45 MHz in addition to the usual 15 kHz.
The basic problem is finding narrowband filters with good shape
factors at VHF frequencies.  The lowest commercial filters I've found
are 7.5 kHz at 45-70 MHz (the MP uses a 70 MHz first IF).  George W2VJN
(INRAD) has looked into narrow roofing filters for the MP but has had
no luck thus far.  It's physically much easier to make narrowband
crystal filters at HF IF frequencies which is what TenTec (Omni and
Orion), Drake and Elecraft have done for either down-conversion or
direct-conversion designs.

         Beginning with general coverage transceivers in the '80's, and
also with the need to provide FM, the JA manufacturers went to VHF
First IF's (to avoid image problems with general coverage receive)
and 12-15 kHz roofing filters to handle FM.  Unfortunately this led
to their current problems with poor strong-signal characteristics.
The IC-7800 shows that they recognize the problem, but the 6 kHz
roofing filter will be no match for Orion's standard 1 kHz or optional
500/250 Hz filters when it comes to strong-signal performance 
characteristics (IMHO).

                                                 73,  Bill  W4ZV