[Amps] Relais for an antenna crossover switch

[Karl-Arne Markström]

I do not think that you have to worry about the
ratings of the 30A relays, but to be sure you are better to make
an actual RF current carrying test to check for heating of the relay contacts.

The reason for this is that some relay contact alloys are not suitable
for carrying high RF currents. It appears that the contacts should be of
as pure silver as possible to avoid RF heating problems.

I have no recent experiences of high-power RF switching with ordinary 
power relays, but a 10 x 5 switching matrix that I designed in the 80's
for use with 3 - 5 kW SSB/Data commercial transmitters used a now discontinued 
ITT DPDT 15 A/250V power relay with hard silver contacts.

The matrix was built on thick double-sided G10 board (3,2 mm)
with 160 micron copper coating. Row and column traces were arranged at right angles to another,
and the matrix provided an isolation of about 30 - 35 dB up to 25 MHz. 
Power ratings were tested at 3 kW CW at a maximum  SWR of  2:1.

Looking into current designs using relay RF switching, the Siemens and the Fujitsu/Takamisawa relays are often found.
The Fujitsu VS 24STBU-5 10A relay series are commonly used for switching in 
1 kW antenna tuners for maritime radio.

73/

Karl-Arne
SM0AOM







----- Original Message ----- 
From: "Guido Tedeschi" <guidoted at gmail.com>
To: "Tom W8JI" <w8ji at w8ji.com>
Cc: <amps at contesting.com>
Sent: Wednesday, June 27, 2007 11:48 AM
Subject: Re: [Amps] Relais for an antenna crossover switch


> Thanks to all for the answers.
> 
> The things are changed, because now I have twelve SPST W2W vacuum
> relais like this:
> http://tinyurl.com/37uv8r
> 
> So, I changed my schematic to this one, with only six 30A relais
> instead of 12 and which is also safe for the radios.
> The new schematic is this:
> http://www.hamlan.org/maco/macosafe.pdf
> 
> I opened the 30A relay and I took some pictures, take a look:
> http://www.hamlan.org/maco/668290120000_1.jpg
> http://www.hamlan.org/maco/668290120000_2.jpg
> http://www.hamlan.org/maco/668290120000_3.jpg
> 
> It seems that there are some good and bad things:
> 1) short wide leads to contacts
> 2) the spacing between contacts and/or coil seems good
> 3) the contacts area seems not so large, especially the NC contacts
> (but I will use two in parallel)
> 4) when the coil is energized, the moving contact press the NO fixed
> contact using only its elastic force (not pressed by the armature)
> 5) when the coil is not energized, the moving contact press the NC
> fixed contact using the elastic force of the armature (it is pressed
> by the released armature)
> 
> I searched for "relay" on towertalk (very good mailing list, I will
> subscribe also to that one!) and I found many very interesting
> messages (many from Tom) about relais and RF.
> 
> Tom, what kind of error you have opening my links?
> 
> Thank you very much to all.
> Ciao and 73
> Guido, ik2bcp
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> 
> 
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