Isolation between RF side of antenna bypass relays and control side question

[Chris Wilson]

I have built an RF amp bypass relay box and control board, and am wondering how best to feed the relay coil signal voltages through a dividing baffle. Please see photos at http://www.gatesgarth.com/isolation.zip  The baffle will be a tight fitting aluminium plate, (not a rough bit of paper <hi>), and I need to feed the relay coil leads through from the three relays, to the control board side. Should I use feed through caps in the baffle( I have some 1000pF ones) or just drill holes? What else might I do to keep RF from the control board side? The tags on the relay coils themselves will have 10nF caps across them. The control circuit is also in the zip. Thanks.

[SeanB]

Feedthrough capacitors and ferrite beads right by them, and then a low value inductor and capacitor in a L filter to attenuate further on both sides. Plate really needs to either have rivets holding it at small intervals or it must be soldered to the side as otherwise as it ages the oxide layer will isolate it and make it into a radiator.

[C]

Hi
Quick summary
5 KW <30 Mhz
No Control leads for relays are at ground
 
Am I computing it right, 5 kW output corresponds to 1440 Volts peak-to-peak into 50 Ohms. If so you might want to look at your distances for the possibility of arc over.

A quick look at your box tells me you will not be able to get a great RF seal. At these power levels you may want a better box around the RF section. A really good RF box will have a electrical conductive gasket between an access plate and the rest of the box.

It may be possible to have the RF section of a relay in the HIGH Power shielded box with the rest of the relay in a second shielded box there by reducing the amount of filtering needed on the relay control wires..

You could use shielded twisted pare wire for your relay wires to help keep the RF out of the control leads. The shield grounded at one end and cap bypass at the other.

make it into a radiator.

You do not want to have to spend the time and effort to find a radiator like this, it is a lot of time to find.
Think of that bare conductor carrying your RF I see in the photo as an antenna. Any thing connective could have a voltage induced in it or act like an antenna due to that bare conductor. It is much easer and cheaper to try to keep the RF in the confined area where it is needed then to try to remove if from all the places it is not wanted.

You may even want to add some shielding between the transmit side of this and the receive side if that Amp is not really quiet when not transmitting.

Note a copper clad board will work as a shield and is easy to solder the corners & edges. Will not be as great as proper stuff.

C
 

[Chris Wilson]

Thanks for the replies so far, I should have stated this is initially just for 1.8 to 30 MHz usage, not VHF. I took the easy route of starting with a case I had already made and not used, maybe something solderable would have been more suitable

[JackOfVA]

Any antenna switching arrangement that depends on three independent relays being switched in the right sequence makes me nervous. A relay failure has the potential for expensive damage. And, of course, a relay failure might be either mechanical or the associated electronics driving the relay coils.

For ham use - and I've been a licensed amateur radio operator for 50 years - I've seen worse lashups, but it would not pass muster for a commercial or MILSPEC design.

You've used the design from F1FRV http://f1frv.free.fr/main1b_Amp_Bypass_Relays.html and his suggested single relay DPDT solution seems to me a less risky proposition, recognizing that there is a power limit for DPDT vacuum relays.

If I were going with a multiple relay solution, I would take a serious look at closing the loop by verifying all relay coils have rated current before the transmitter output is enabled. Ideally one would verify relay status by a method that determined the relays are in the proper state rather than just that they had coil energized as relay current does not provide 100% certainty of proper operation.  In the broadcast business, for example, the RF contactors / relays had microswitches operated by the armature so that the relay status could be verified by reading the microswitch closure/open. Only if all microswitches were in the correct lineup could RF be applied -- this was at the 50KW output level.

[C]

hi JackOfVA
Reading your post I got to thinking of what the cost could be of

Dumping 50KW into an open cable or shorted cable.   
Not a real good thing to do I am sure.

With this three relay setup and a minor goof you could have Transmitter to antenna and also output of power amp.  Would probably be bad for something.

C

[Rory]

I agree with Jack about the SPST relays but don't care much for DPDT RF relays to bypass a PA, especially in circuits with the possibility of a failure causing the switch to look into a reactive load. You want lots of isolation between input and output.  Most of the commercial high power (5KW+) HF amps I have dealt with that have T/R switching use a low voltage relay on the input side, which doesn't need to be a vacuum relay at exciter power levels, and the output relays have always been SPDT.

The Jennings type vacuum relays are great for this application because the switched contacts can be sealed inside a shielded RF-tight enclosure while the coil terminals are on the outside, away from the RF.

Here is a link to the type of relays I'm referring to:

http://www.mgs4u.com/RF-Microwave/vacuum-relays-SPDT.htm

Exciter PTT T/R switching timing is a different story. One must never hot switch a PA output relay, even vacuum relays can be damaged by arc-over on the make or break action. It is prudent to design the T/R sequencing logic to incorporate sufficient time for the relays to settle before energizing the exciter, as well as de-energizing the exciter prior to switching the relays to bypass mode.

Another consideration, if the transmitter system is intended for continuous transmit operation, for example in a full-duplex communication system, it is entirely possible that power line dropouts can cause the T/R relays to toggle while the PA is producing RF output. A slow bleed-down control power supply powering the relays, or even additional capacitance designed into the circuit will hold the relays closed long enough for the power to recover, or the PA power supply to bleed down to essentially nothing. 

[C]

Hi guys

I seam to recall a switch setup somewhat like this where the RF path was tested with a very low power RF signal that could handle the open circuit problem with out harm.

Just to get the idea across of what I might have seen. An SWR meter connected to antenna connection all the time with some auto ranging for the power level changes. The output of the power amp with a switchable low power 50 Ohm load. The input to the power amp with a switchable 50 ohm load to protect the transmitter. A switchable attenuator  between this load and the input of the power amp.   
The PTT being a request to the power amp which had total control of this signal. When pressed the circuit would put all the loads & attenuator in place as stage one. Stage two would be to then start bringing up the power to the power amp so that it would function at a very low power level. At the same time it would start the process of getting the relays to the correct positions.
To get to stage three the readings from the SWR Meter would have to be correct for this very low power level. The Power amp has the low power load connected to protect it's output. This load will survive due to the attenuator on the input keeping the output to the safe low level. When the SWR meter finally regesters this low power level you know that the output relays are correct and should be functioning properly, but you have signals to warn you if this is not the case at higher power.
Stage 4 could be dropping one of the connections to the low power load getting it out of circuit before higher power levels have to pass through this point. An we can do this safely as the SWR readings says it's ok working at this level.
Stage 5 might be dropping the input load.
Stage 6 to __ could be removing attenuation from the attenuator and giving the power amp more power so that the output could increase.
     
Come to think of it there could be an antenna auto turner in there also.

Probably some military radio design I have seen.

C

[Chris Wilson]

That's all getting far too complex for me It'll have to run the gauntlet of a slightly less than optimal risk factor I'm afraid. bear in mind all three relays shown actually change together, the "sequencing" is done pre this box, so you have PTT at the microphone, the 3 relays immediately change from RX to TX positions simultaneously, then after a 35 mS delay the amp bias is changed and the exciter goes to TX mode.


 On my Ian White "Tetrode Control Boards" `Ian has used a mini inductor on the boards for the bias supply out. I will look at the BOM and see what he used, I suspect something similar is what I need here on the feedthroughs as a PI L?

I have another quickie I am afraid, sorry.

My in built 24V DC relay coil power supply (linear) has enough output to run other things and it would be handy to bring it out to the tag strip on the case http://www.gatesgarth.com/psu.jpg Do I need to decouple the positive and negative leads from the separate 24V relay PSU section at the tag strip, as well? Cap values? Thanks !

[C]

Hi Chris

In simple terms this antenna switch is assumed to just work with no way for this circuit to say I have a problem do not power up that transmitter.
The way that the circuit is built it does not check for what it could check for problems.

Give you a couple of examples,
There is a problem with the 27V power connections. All the rest of your control system goes through the process to transmit. Just one problem, none of the relays can energize due to power problem. Did you just damage something or is this a no problem that it happened.

One or more of your relay coils open or has a wiring problem so just those relays did not function. What is the damage caused for this problem.

While you may not want to take the time now to build a very fancy system to protect things, it would be good if you could add the easy things you can while your working up to that fancy control system.
I would suggest adding two simple outputs. Transmit_OK and Receive_OK to go back to the rest of your control circuit. For the stupid simple circuit to start with how about making two copies of the circuit around Q1 but leave out Q2 and change J4 of the copies to have ground instead of +27V. Simple open collector logic both saying it's good if the +27v is present. At a later date you could then add some more circuity to check for proper current flow through the relays to improve the safety. At an even later date you could add an input that says I am detecting transmitter power and add that into the transmitter_OK logic.

The question you need to ask your self is it worth the very few parts here to save the high dollar other parts even in this limited faction and also allow easy future improvements. Note: An important thing to happen in protection circuits is to always fail to a safe state.

In case you were thinking complicated SWR Meter, take a look at 
Stupid simple SWR.
I am sure some of the people here may be able to tell you a better way but try this for an idea. For what I talked about you really do not care how good a reading it is just that you get a reading in the ball park. To read the forward wave vs reverse wave (SWR) one simple version is to put two additional wires in parallel to the center conductor of your transmit power coax. You add a termination resistor at opposite ends of the two added wires and a Envelope detector at the other ends of the wires. How could you build this? You could take two pieces of small coax and strip off the shield for a bit in the center. You could then remove the outer plastic cover of a length of your transmitter cable and push back the braid. make a hole in the braid next to the remaining outer plastic on each side and feed the two small cables through the holes so that just a small bit of braid of the small cables will over lap the transmitter cable braid at that end  when you slide the transmitter cable braid back like is was to start. Inside the transmitter cable braid for a bit you will have three wires in parallel. You could make it nicer if add two more holes through the transmitter cable braid for the small cables to come out at that end.

http://en.wikipedia.org/wiki/SWR_meter 

By using small shielded coax above you have an easy way to get in to the high power RF area and back into a shielded area for the remaining circuits for this.

Just some ideas

C

     

[JackOfVA]

If the design is for your own personal use and you don't mind the occasional blow up, have at it.

If it's something to be sold to a paying customer, I would set aside a significant reserve for warranty repairs. Probably would have a chat with my insurance agent as well about liability coverage.

What I was trying to say about "closing the loop" - and other replies have mentioned it as well - is that commanding a function to be executed is not the same as knowing the function has been executed.

Before you apply multi-KW of RF power to a lashup of three relays, a commercially reasonable design would verify that the three relays are in fact in the correct lineup for TX mode.  Thus, the transmitter enable is part of a closed loop control system in that positive verification of correct relay operation is made before the first milliwatt of RF power is applied.  This does not depend on "well, the circuit applied a control signal to the solid state logic that transmits DC voltage to the wire leading to the relay, so the relay must be correctly operating."

This closed loop verification may be accomplished in a variety of ways, some of which are better than others.  I've most often seen (in the AM broadcast transmitter field) auxiliary switches mechanically coupled to the RF contactor armature. And to provide a second degree of reliability, the status auxiliary switches are operated with current through them, some tens of mA, and set up in a configuration so that current flows (through different circuits) whether the associated relays are in position A or B.  Maintaining current through the auxiliary contacts allows the control logic to identify breaks in the wiring and some switch faults and thus throw an alarm condition to the control system inhibiting potentially unsafe operation.

In fact, the relay verification, interlock and other safety/proper operation logic can be the most complicated part of a high power transmitter. And, it's often done, or at least it was when I last looked at it, with electromechanical relays and switches instead of solid state logic so as to reduce the possibility of RF causing false indications by getting into places where it does not belong, either by poor design or by component failure or whatever.

But, as I said, there's a world of difference between building a one-off high power amplifier for your own use versus selling one to a customer. Customers become unhappy when transmitters blow up and if they are earning money with the transmitter, the unhappiness multiplies.   

[KJDS]

You could put some LEDs in series with the relay coils to indicate if they're activated.

You could also put some optocouplers in there that are turned on by the relay current and use those signals to turn on the transmitter.

[Chris Wilson]

OK, thanks, I will consider further interlocks. The control board has voltage present and SWR interlocks, but I may add more. This is a hobby level cost conscious project though, not something to sell to litigious radio station executives, if it blows up, catches fire or fails to work it won't change my life unduly The LEDs sound simple, cheap and easy to add at this late stage though, I like that!


Anyone up for the other question:

My in built 24V DC relay coil power supply (linear) has enough output to run other things and it would be handy to bring it out to the tag strip on the case http://www.gatesgarth.com/psu.jpg Do I need to decouple the positive and negative leads from the separate 24V relay PSU section at the tag strip, as well? Cap values? Thanks !

[vk6zgo]

hi JackOfVA
Reading your post I got to thinking of what the cost could be of

Dumping 50KW into an open cable or shorted cable.   
Not a real good thing to do I am sure.

With this three relay setup and a minor goof you could have Transmitter to antenna and also output of power amp.  Would probably be bad for something.

C

In a Broadcast Transmitter setup,the main damage would be to the switch----the Transmitter would "grunt" & turn itself off.
High Power Transmitters are pretty forgiving,for the following reason:

Much of the complexity of a commercially made Broadcast or Communications Transmitter is jn the protective circuits.

This has been pointed out by JackOfVA .

[JackOfVA]

hi JackOfVA
Reading your post I got to thinking of what the cost could be of

Dumping 50KW into an open cable or shorted cable.   
Not a real good thing to do I am sure.

With this three relay setup and a minor goof you could have Transmitter to antenna and also output of power amp.  Would probably be bad for something.

C

In a Broadcast Transmitter setup,the main damage would be to the switch----the Transmitter would "grunt" & turn itself off.
High Power Transmitters are pretty forgiving,for the following reason:

Much of the complexity of a commercially made Broadcast or Communications Transmitter is jn the protective circuits.

This has been pointed out by JackOfVA .

It was always interesting to be on transmitter duty during a lightning storm - you would see the flash and hear the thunderclap and the transmitter relays would click for a fraction of a second and that was it. It was rare to have to do a manual reset - the automatic overload circuit would detect the fault, shut down the transmitter and then restart after the protection circuit determined that the fault had cleared.

We also had a coax short while I was on duty. This was a 9 tower directional array station, 50 KW transmitter output and the towers had a "dog house" with some impedance matching (far from 50 ohms, just enough to make 50 ohm coax feed feasible), and then a long run of either 1.625 inch or 3.25" copper air line coax cable (depending on the power to the particular radiating element) back to the transmitter building and the main antenna phasing/matching network.  For some reason we never determined, one of the 1.625" diameter coax cables failed at one of the insulating disks - when we dug up the cable there was a nice trail of melted copper bridging the center and outer conductors.  However, the change in impedance from 9 normal phased antennas to 8 phased antennas and one short was not enough to trip the transmitter out permanently. In fact, all I saw at the time was a quick trip/reset but after the reset the common point current was wrong - not a lot wrong, but enough to indicate all was not normal.   It took a TDR to locate the short, and then a back hoe to excavate the coax - buried 6 ft or so deep.  All in all, I think it was about 3 days from fault to repair, during which time we operated under temporary "parameters at variance" authority from the FCC.

Another interesting fault we had involved cable migration.  The night transmitter was an old RCA plate modulated job (the 50 KW was an RCA "Ampliphase" transmitter which combined two phase modulated identical transmitter sections to yield AM. Incredibly complicated exciter but it had low frequency response better than any plate modulated transmitter and was more efficient.)

The 10KW transmitter had a suitably hefty 3-phase power transformer with secondary voltage around 5 or 6 KV. The mercury vapor rectifiers were in the transmitter bay and the power transformer was about 15 or 20 feet distant, and the raw AC was run via coax cable through a conduit between the transformer room and the rectifier bay.  I can't recall the coax type, but it was larger than RG-8 diameter, but smaller than RG-17. Probably close to 0.75 inch jacket diameter.  The 10KW transmitter had been in place for 15 or 20 years at the time.  One night, the afternoon engineer went to flip the transmitter control from standby to operate and the transformer room filled with smoke and the primary side breakers tripped.  One of the secondary winding coax cables developed a center conductor to shield short.  The chief engineer had the failed coax X-rayed and you could see where the center conductor had migrated at a bend to the point where it was nearly touching the shield and was close enough to arc over.  A foot or so either side of the bend was perfectly centered.  The center conductor probably moved 0.01 inch a year for 20 years.

In any event, working as a transmitter engineer at the AM station was a great job at the time - I worked nearly a full time job there and also full time in school as there was plenty of idle time to do homework while babysitting the transmitter.

First photo shows the control console at the transmitter building. At the left foreground is part of the 50 KW ampliphase transmitter.  Further along that wall is the old RCA 10KW high level modulated transmitter. 

Second photo is rather unevenly exposed, but the phaser control panel is at the right, along with monitoring equipment. At the end of the room is the "last ditch" backup transmitter, a 1KW Gates.

These photos are 40 years old and have major color shifts.