I'm trying to figure out in my mind how to build a box that will allow me
to select a HiZ 4 square, two beverages, an RX loop, and switch in an MFJ
1026 Noise Cancelling device using the listed RX antennas - to two radios
using their external RX antenna jack. I know that I need to be concerned
with isolation and grounding. Can I use several older ceramic rotary
switches, relays or what?
Ray,
I build one-off things for myself, but I can share some direct experience. I
always use relays now, but I have used push button and other switches in the
past.
1.) The worse unwanted coupling on 160 is going to be from shields not being
grounded to a good solid common point with very low impedance. NEVER switch
the shields, and make sure any connectors or cable entrances to the switch
ground to a very low impedance groundplane that ties all the shields
together.
2.) Many rotary and push button switches are "good enough" on 160 and 80.
They tend to lose isolation on higher bands.
3.) Long shield connections and poor shield bonding at the hub (via a wide
groundplane or enclosure) are the most problematic things on low
frequencies.
If you look at this picture:
http://www.w8ji.com/images/New%20Contest%20Room/Contest%20station%20CQWW2007/receiver-switch-matrix.jpg
at
http://www.w8ji.com/contest_station_w8ji.htm
the stuff on the upper right side of the box is the switching system. It is
"dead bug" construction with twisted pair enamel wire for receiver RF
transmission line RF bus interconnections.
It has over 80 dB isolation as it sits, with the isolation limits set by the
relays used.
The twisted wire transmission lines and open construction are not even close
to affecting isolation or RF ingress limits. If I used a plastic box or
backplane instead of metal, or if the connectors floated on the box, or
grounded through wires of just an inch or more, or shields grounded to a
small PC board ground bus, isolation (and lightning immunity) would fall
apart.
My advice is to watch the box, and watch the cable entrance grounding to the
box. It does not have to be a closed box with a lid at HF (and even low
VHF), but it has to look like a really low impedance uninterrupted
groundplane. You never want to switch a shield, or common ground through
leads or narrow foil traces of any length.
73 Tom
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