Topband: TX relays

[Carl]

So WHAT IS the manufacturers part number of an adequate relay?

Carl
KM1H


----- Original Message ----- 
From: "Tom W8JI" <w8ji at w8ji.com>
To: "Tim Shoppa" <tshoppa at gmail.com>; "Mike Waters" <mikewate at gmail.com>
Cc: "topband" <topband at contesting.com>
Sent: Thursday, October 09, 2014 9:46 AM
Subject: Re: Topband: TX relays


> 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 at 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 at gmail.com> wrote:
>>>
>>> >  Fujitsu FTR-K1CK012W
>>> >
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