[NLRS] [VHF] Tohtsu relay power ratings

Tue Jul 15 21:25:52 EDT 2014

You can get a first approximation of the isolation of any relay by 
examining the contact diameter and contact spacing when open. The 
capacitance will be more than that of the contacts at that spacing. For 
the contact pair, the capacitance is 0.225xA/t where A is the area of 
the contact faces in square inches and t is the contact spacing in 
inches. Change the constant to 0.0885 for area and spacing in 
centimeters. This formula is simplified from the standard handbook 
formula to be only 2 plates and only air for the dielectric. Then figure 
the capacitive reactance and the ratio of that to 50 ohms. That ratio is 
the minimum isolation in when taken as voltage so 20 log10 (ratio). The 
moving bar that carries the moving point will also contribute 
capacitance to the open fixed contact so this computation is optimistic. 
Its not really hard with signal generator and receiver with s-meter to 
measure the true isolation. It takes small contacts on the receive side 
with a wide gap to get good isolation though Dow-key did it sometimes 
with grounds for the receive side contacts.

It is quite practical to use two relays to protect the receiver. One 
with big contacts to handle the transmitter power connected to a second 
relay on the receive side that hooks the receiver input to a termination 
and in a small relay with good isolation but not many watts of power 
handling. Its very critical that the two relays not be spaced a quarter 
or half wave by coax. That will impedance match the capacitive coupling 
high impedance to 50 ohms or some other low impedance and makes the 
isolation difficult to achieve. That can be worse if the big relay 
shorts the receive side but has inductive coupling.

There are a number of Ducommun SMA relays with 12 volt coils that show 
up nearly daily on epay at a reasonable price. SMA rated to 26.5 GHz.

Occasionally RF Relay Store advertises in trade journals, they will sell 
in single lots. Not necessarily cheaply, but they do have a wide 
variety. www.rfrelaystore.com/default.html

73, Jerry, K0CQ

On 7/15/2014 2:03 PM, Mike King - KM0T wrote:
>
>
>> What website were you looking at Mike?
>
>> The manufacturer rates them at 1000/800. That is the information I would use.
>
>
>
>> 73 de Tom, K6VCR
>
>
>
> Tom, here is what I have…
>
>
>
> Radiodan and Nebraska Surplus sales have them rated that way….
>
>
>
> http://radiodan.com/ted_stuff/tohtsu.htm
>
>
>
> http://www.surplussales.com/Relays/rfcoaxialrelays/rfcoax_n.html
>
>
>
> It appears that the consensus from the answers is that these are not suitable for higher power…..
>
>
>
> Does anyone of know if a good source of high power N – SPST relays.  I would like the style that looks like your standard SMA relay, but bigger and handles the power.  Research I have done says that the dowkey and Relcom versions of these have very good isolation and power handling characteristics…the problem is that I can find some of these like on RF parts, but they are 28V.  The voltage is not that big a deal, but I can’t seem to find a place where one could purchase these new if they wanted to…the dowkey website is not all that great….  The old SSBUSA site has the EME brand of relays, look great, but do not seem to be available anywhere that I can find…
>
>
>
> 73
>
>
>
> Mike  - KM0T
>
>
>
>
>
> On Tue, Jul 15, 2014 at 9:06 AM, Mike King - KM0T<scsueepe at mtcnet.net>  wrote:
>
> Anyone have any experience with the Tohtsu CZX-3500?  I purchased a few of these to share feed lines between a few bands...  (50 MHz / 222 MHz) (144/432)
>
> The specs on the relays I purchased turned out not to be not the same as indicated on the
> website that I got them from.
>
> The data sheet for the relays I received say 1000 watts input power PEP for
> 50 and 150 MHz, as well as 800 watts PEP for 500 MHz.
>
> The website indicated 2300 watts PEP on 50 MHz, 2000 watts PEP on 150 MHz
> and 1000 watts cw at 500 MHz.  These are the specs I needed and based my
> purchase on due to the power I will be running.
>
> Anyone know if there are two different versions of this relay?  I have a few old CZX-3500s from a long time ago, these are labeled dc-6ghz and 250 ma at 12v.   Now these new ones just say 230 ma, no freq rating.
>
> I am thinking of returning these, but the price was good.  Anyone know first hand if these handle 1.2 kW cw?
>
> Thanks and 73
>
> Mike - KM0T
>
> Sent from my iPad
> ------
> Submissions:                    vhf at w6yx.stanford.edu
> Subscription/removal requests:  vhf-request at w6yx.stanford.edu
> Human list administrator:       vhf-approval at w6yx.stanford.edu
> List rules and information:     http://www-w6yx.stanford.edu/vhf/
>
>
>
> ______________________________________________________________
> NLRS mailing list
> Home: http://mailman.qth.net/mailman/listinfo/nlrs
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:NLRS at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html