Topband
[Top] [All Lists]

Re: Topband: TX relays

To: "Tim Shoppa" <tshoppa@gmail.com>, "Mike Waters" <mikewate@gmail.com>
Subject: Re: Topband: TX relays
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Thu, 9 Oct 2014 09:46:22 -0400
List-post: <topband@contesting.com">mailto:topband@contesting.com>
Because there are many things that go into relay selection that do not show on a data sheet, I always dissect and test relays.

I have found 30 amp power relays that overheat at 5 amps at 28 MHz, and relays that have high contact voltage ratings that make the pole inside the coil hot with full RF. They wind up with 20-30 pF capacitance from armature to coil.

Another issue is resistance and reliability at near zero contact voltage when receiving. This is probably the single biggest relay issue in our applications. A small bifurcated contact relay is better for receive reliability, and a high current hot switch design is by far the worse for receive reliability.

One particularly troublesome high power area for current are the relay internal leads, and the contact support bar materials. The things that make the wires and contact bars last a long time in repeated cycles create very high radio frequency resistances. This is why some large 30 amp power relays will discolor contacts or melt insulation at several amps on higher frequencies.

The same thing applies to contacts. Contact materials and platings that optimize hot switching create RF resistance and low level signal connection issues. A gold flash on a soft contact, for example, is excellent for receive but will instantly deteriorate if hot switched at more than a few hundred milliamperes or with an inductive load. It might handle 20 amps of closed contact RF current, but only be rated for a few amps of hot switching current. In contrast, a silver cadmium oxide contact can take tons of hot switch voltage and current, but is lousy for relay receive pass through.

Contact support bars, and the wires used in some relays, can also be very problematic. This is because the materials and any weave in wires is designed for flexibility. Alloys and construction that improves mechanical cycle life greatly reduces RF performance.

> Mike, I would be concerned about using these small relays for non-resonant
antenna switching where the impedance at the switch point may be wildly
away from 50 ohms.

This is the relay I use to switch tuning networks at non-50-ohm points:

http://www.deltrol-controls.com/products/relays/power-relays/900

Deltrol is the brand you get if you order from McMaster-Carr but all the
big relay manufacturers sell these open frame relays.

I also bend the relay contacts for wider spacing as recommended by N6RK in
QST (page 66, May 2009 QST Hints and Kinks, "Increasing Relay Voltage
Handling"). Open contact gap of 0.5 inches is readily achievable.

Tim N3QE


On Thu, Oct 9, 2014 at 6:19 AM, Mike Waters <mikewate@gmail.com> wrote:

This relay looks exactly what I was looking for to remote-switch my 160m
inverted-L to other bands, because it will withstand a lot of voltage. From
the PDF:

High insulation
Insulation distance (between coil and contacts): 10mm min.
Dielectric strength: 5KV
Surge strength: 10KV

73, Mike
www.w0btu.com

On Thu, Oct 9, 2014 at 2:23 AM, Thomas PA1M <t.b.tinge@gmail.com> wrote:

>  Fujitsu FTR-K1CK012W
>
_________________
Topband Reflector Archives - http://www.contesting.com/_topband

_________________
Topband Reflector Archives - http://www.contesting.com/_topband


-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 2014.0.4765 / Virus Database: 4037/8351 - Release Date: 10/08/14


_________________
Topband Reflector Archives - http://www.contesting.com/_topband

<Prev in Thread] Current Thread [Next in Thread>