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[TowerTalk] Re: [FCG] Relays in RF usage

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Subject: [TowerTalk] Re: [FCG] Relays in RF usage
From: (Didier Juges)
Date: Tue Aug 12 16:12:27 2003
At 11:29 PM 8/11/2003 +0000, you wrote:
>I ask:
>1  Can RF and AC be equivalent?  I think not.

RF is AC. However, at high frequencies (RF), skin effects tend to make life
harder on contacts. At HF (<30MHz), skin effect is probably minimal and not
a real concern.
The big difference is between AC and DC, as DC tends to cause long lasting
sparks when the contact opens under load, which eat through contacts in a
hurry. Inductive load DC current would be the worst for a relay contact.

>2.  Unless the relay contacts have high resistance, there should be no 
>factor as to how long they are switched ON.... is this what u mean by "duty 
>cycle"  ?   But the normal usage of "duty cycle" refers to how many times 
>the thing can operate on and off until failure and has nothing to do with 
>current carrying abilities.

Duty cycle typically refers to the amount of time a device is in use over a
certain period of time. If you transmit one minute and listen 3 minutes,
your transmitter's duty cycle is 25%. Electromechanical relays have a life
cycle rating, which is the number of times they can be switched on and off
under a specified load, while the contact resistance remains within spec.

>I say, put in a BIG ONE and if it burns out.... it was too small.  AMATEUR 
>radio, after all.... 73,

That will probably work well at HF, but at VHF and higher, the size of the
relay will actually play against you, as the inductance of the contacts
will cause VSWR. A smaller device will probably have lower inductance and
be faster, which could be necessary to avoid transmitting into an open
before the relay closes.

I am amazed at the relatively small size of the relays used in modern
transceivers to switch the antenna and/or the tuner in and out of circuit.
Those I have seen (in my TS-440SAT for instance) look like 2 or 3A AC
service types if I recall. At 100W and with a 3:1 antenna VSWR, the current
will still be well under 3A, so I guess it's OK, assuming no additional
derating for RF currents versus 60 cycles.

>Charles Harpole


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