[NLRS] [VHF] Tohtsu relay power ratings

Tue Jul 15 21:36:39 EDT 2014

I had responded to Mike off-list that the Tohtsu datasheet straight
from the mfr is out there,

http://www.tohtsu.com/shouhin.holder/CZX-3500.html

and it does show considerably lower power levels than others have
quoted,  eg,

450W on 50 MHz
400W on 150 MHz
300W on 500 MHz

so, something has probably changed with the design or they took a dose
of truth serum.

I suppose it would probably survive the higher power levels but then
doesn't meet the quoted isolation most likely.

Chris N0JCF

On Tuesday (07/15/2014 at 07:25PM -0600), Dr. Gerald N. Johnson wrote:
> 
> 
> 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
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-- 
Chris Elmquist