[TowerTalk] Toroid Core Material Selection

[Keith Dutson]

>you have a layout or relay selection problem.

You are probably right.  This project originally had one pair of SO239's and
one relay.  Then I upgraded it to a second port.  Each pair and relay is on
its own small PC board with separate DC supply wire, and that may be the
root of the problem.  To make matters worse, I had a poor ground from the PC
board to the chassis, and resorted to foil tape to improve it.  Perhaps the
best thing to do now is make a single new PC board to cover all 4
connectors, and ground the board at several points with standoffs.

Keith NM5G

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Tom Rauch
Sent: Wednesday, March 23, 2005 3:36 PM
To: keith at dutson.net; TowerTalk
Subject: Re: [TowerTalk] Toroid Core Material Selection

> BTW, my homebrew project is a pair of relays to switch two
antenna
> connections in the shack.  The design bandwidth for these
two antenna
> switches is 1.8 to 450 MHz.  A friend with a spectrum
analyzer measured some
> crosstalk between the SO239 connecters and suggested the
toroids be inserted
> in the dc lines to each relay to improve isolation.
Insertion loss was also
> measured to be about 3 dB at 500MHz.  This is not
acceptable so I am looking
> for better relays.

Keith,

If you are only switching two ports to a common 500 MHz should be easy.

In all the switches I've designed, I've NEVER had to bypass or filter coil
leads. That tells me you have a layout or relay selection problem.

Capacitance across contacts limits port to port in a good design, and with a
wise selection of conventional relays the system should have less than 1/2
dB loss at the upper end and a worse case crosstalk about -50dB at the high
end.

73 Tom


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