Topband: Suggestion for RX Antenna control box/switching box

[Tom W8JI]

> 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