[Amps] Relais for an antenna crossover switch
d.cutter at ntlworld.com
Tue Jun 26 14:12:20 EDT 2007
The contact current rating should be derated for rf. I don't by how much
for this relay, but a vacuum relay I have derates a 30A contact to 6A at
32MHz. In the vacuum relay heat cannot escape so easily, so your relay might
be better from this point of view. This is for continuous current and can
be adjusted by the mark-space ratio, since it is the heating effect that
matters. This is the carrying current, not the switching current, which
should be practically zero in your application. Of course amateurs are well
known for over-running things, so you make a choice about the reliability,
ie life time of the contact. My guess is that this is not going to be a
I would worry more about the voltage rating. I have seen 100W tuners use
small relays with 2-pole 240V rated contacts wired in series to avoid damage
during tune-up or off-tune or accidental conditions which can put much
larger voltages into the system than with ideal load conditions. The
change-over relays I've seen in some rf switches "look" small for the
service, but they may be specially made and not immediately obvious what
they are. Some relay manufacturers have very special contacts which are
good at both ends of the current range, but I haven't seen these in amateur
service. I have seen "ordinary" junk box octal relays used in home brew
equipment with seemingly no ill effects.
It's always a wonder to me how heavy duty contacts work at uV levels in
linear amplifier change-over relays; I'd like to know more on that. I
assume that they are specially made, but some I know of are definitely not.
The gold plating on most relays is to protect them during storage and
amounts to just a few microns. This rubs off very quickly and it is the
continual wiping action that keeps them working. But I still don't know how
they work at the uV level, I'm missing something.
I look forward to hearing from others.
----- Original Message -----
From: "Guido" <guidoted at gmail.com>
To: <amps at contesting.com>
Sent: Tuesday, June 26, 2007 11:30 AM
Subject: [Amps] Relais for an antenna crossover switch
> Hello all,
> sorry for the offtopic (but these relais can be used also for a
> I want to build an antenna crossover switch for two radios and six
> antennas and I have some 30A DPDT relais (the type is Finder 66.82 and you
> can see the specification at http://www.hamlan.org/maco/s66en.pdf).
> Assuming that the logic will avoid that the rigs will be connected to the
> same antenna (sgrunt... ;-) ) and will avoid the "hot switching", do you
> think that I can use these relays with 1500W and a maximum SWR of 3:1
> The relay specification says that the minimum switching load must be
> 1W/10V/10mA, do you think that,after some period of storage without use,
> would be enough some RF (100W) for few seconds to clean the contacts and
> remove the oxide?
> Initially, I designed this series configuration
> (http://www.hamlan.org/maco/macoser.pdf), but I thought that it would be
> safe to depend on too much relais...
> Then, I went back to the classic parallel configuration with some
> to improve the isolation between ports
> At last, after some doubts about having 10A of RF (1500W with 16OHM) on
> single contact, I designed this more simple version
> What do you think about the dielectric (relay plastic case) between
> with 470V of RF (1500W with 150OHM)?
> I know that the layout is very important for the isolation between radios,
> what layout do you think should be the best layout with these relais?
> I think that any HAM transceiver can tolerate without damage, at least,
> 25-30dBm not on the same band, do you agree?
> How much in band signal can be tolerated without damage, do you think that
> 15-20dBm is too much?
> Thanks and 73
> Guido, ik2bcp
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