Isn't muting enough?

[ND9B]

I have two receivers plus a transceiver sharing the same master speaker. (Speaker outputs are summed together into a master audio amplifier.) I also have a master RX/TX switch with an extra pole that opens the master speaker circuit during TX. Needless to say, this provides excellent muting. The two receivers are paired with transmitters using proper antenna relays. A friend insist that I should be using the mute input on the receivers (something about voltages in the receivers being turned off during TX).

My thought is that it doesn't matter what the receiver is doing during TX as long as you don't hear it.

Any opinions on this?

Bobby Dipole ND9B

[KA0HCP]

If it isn't broke, don't fix it!

[WQ9E]

There are two possible issues:

1. Do you have a reasonable load on the receiver's audio output, I assume your audio amp is line level with associated input impedance?  If the receiver itself is severely overloaded and producing a lot of audio output its audio transformer can be damaged if the load impedance is far higher than the transformer design value.

2. Some receivers may have a slow recovery if RF overload is extreme.  There won't be damage from this but it will result in reduced sensitivity during recovery time.  So in a group or roundtable you might miss part of the beginning of a transmission from a weaker station.

The extent of either problem is largely dependent upon the shielding of the receiver and the isolation of the antenna switch.  

If the individual receiver output transformers have a proper load and no apparent desense (or it doesn't bother you) then run it as is.  Otherwise it would be simple to use the extra pole of your relay to control one or more additional multi-pole relays as needed to mute the receivers using their designed in mute circuits and one of the additional relay poles can then be tasked to muting your master amp speaker (which also should have a proper load so that switching transients don't damage its output transformer).

[W7TFO]

I've seen several RX with burnt-out RF front end (antenna) coils from too much RF from nearby transmitters.

No lightning at the time, either.

73DG

[Steve - K4HX]

Muting won't speak to that problem. That's a TR/changer over issue.

[W7TFO]

Muting won't speak to that problem. That's a TR/changer over issue.

My point exactly.  If the chageover relay doesn't ground the RX feed in transmit, enough RF can leak through to do damage.

As far as muting is concerned, one can usually short a tube rig output without harm, but some SS units will fry into that load.  Best is to dump it into a non-inductive resistance to match the RX secondary.

73DG

[K5UJ]

My thought is that it doesn't matter what the receiver is doing during TX as long as you don't hear it.
Any opinions on this?
Bobby Dipole ND9B

Dear Dipole,

Yeah it matters but it partly depends on how much power you run.  If your rig is a ranger you don't have to do as much.  If your rig is a kw broadcast rig you have to do more.   Personally I try to do everything as if I am running a broadcast rig, so I don't have to fool with precautions if I ever run a broadcast rig. 

Muting varies from rx to rx.  On the 75A-3 the mute switch just disables the audio amp in the rig and does nothing for the RF.  You need to short the antenna terminal on the back of the receiver to ground.  Even then, in my case the receiver would get slammed with me running 300 w.  You have to have a coax relay on the back of the receiver that opens the feed to the receiver as well.  I credit K4KYV with that advice.  Only then did the rx have moderate RF and fast recovery after transmitting.

Robbie Diode

[WA3VJB]

I stand with the comments on making sure your amplifier's audio output still sees a load resistance, and that the speaker isn't just unhooked from your amplifier during transmit.

And I also agree with those who suggest an ungrounded RF input on your receivers can cause problems during transmit.

So no, the method of muting you describe is not "enough" -- you can easily improve upon your changeover and minimize risk of electrical damage along the chain.

In my station I have two receivers using the "diode load" audio output that drives an external amplifier.  Initially I tried a system with the receiver muting circuits correctly activated (and this is accomplished differently for the SP600 and R390A).

But in using the diode connection, the detector still delivered enough audio to drive the amp and cause acoustic feedback into an open mic while transmitting.

That told me there is significant RF coming from the transmitter thru what was an ungrounded, unconnected antenna feedline to the receiver. Grounding that line, and therefore the RF input to the receiver, has solved the problem and cut the perceived risk.

[K1JJ]

Another thing to think about for PTT switching...

For years I have used standard DPDT open frame relays to switch an RF amplifier antenna between transmit and receive.  For example, normally we switch a transceiver from the amplifier input to the antenna - and the antenna from the amplifier output to the transceiver.  (two poles,  two positions)

The danger here is if the two poles were to arc over to each other, there may be full amplifier RF going into the receiver front end. (we are talking about pole spacing, not contacts spacing)  This has happened to me twice over 20 years and in both cases I smoked the receiver front end.  One case looked like lightning damage and was DOA.  It is rare but when there are amplifier parasitics, or whatever, it can happen.  Using two separate relays to distance the poles solves the single relay close-pole problem, but RF can still arc thru the contacts to the RX (or transceiver) circuits. My second crap-out event used two relays, separate poles. The RF even got into the driver circuits and took out my low level ZHL-3A 1 watt amp.

The first sign of trouble is when there is arcing in the antenna relay contacts, usually on AM during unkey.  (and sometimes on key-up too)  

Solution:  Generally, air gap relays can arc. But vacuum relays arc very little internally in the vacuum so are a safer bet. They are generally rated much higher for RF flashovers. Since my last episode I now employ an antenna vacuum relay for any amplifer larger than 100 watts.  It's a little more expensive, but there is much less chance of a catastrophic arc-over into the receiver (or transceiver).  I recently installed new vacuum antenna relays into Dr. Love and Fabio II.  On the receiving end I have at risk an HPSDR system and FT-1000D, so WTF?  Cheap insurance.

OR, at least carefully look at the pole spacing on your exisiting air gap antenna relay(s). Some designed for 60 Hz have very little spacing between poles - sometimes just a little plasdick tab. High power RF will jump these gaps like nothing. Think defensively.

T

[W1RKW]

For RF applications, I usually modify DPDT relays and do away with the wiper wires/contacts by shorting the wipers together with braided wire and clipping the stock wiper wires that connect to the bottom of the relay. You lose some poles but I use 2 relays instead. It shortens the RF path through the relay but also allows you to separate the RCV and XMIT paths. Then you can step time or set make before break delays as well if needed.

[K1JJ]

For RF applications, I usually modify DPDT relays and do away with the wiper wires/contacts by shorting the wipers together with braided wire and clipping the stock wiper wires that connect to the bottom of the relay. You lose some poles but I use 2 relays instead. It shortens the RF path through the relay but also allows you to separate the RCV and XMIT paths. Then you can step time or set make before break delays as well if needed.

Hi Bob,

Yep, that's exactly what I'd been doing too.  It solves the close proximity poles problem  However it can still arc thru all the contacts, even with two separate relays and take out the receiver.   This is what happened when my class E rig failed and took out the receiver and driver.  If there is a really big arc, it will find its way thru somehow...

The safest way is not to have an arc in the first place by using contacts that are way overrated, or in my case, a vacuum relay that is rated at 15KV.

I believe the higher the RF frequency, the more prone to this arcing. Since a parasitic can be at VHF, we have a situation that may require more headroom.

Then again, I have run some 1500 watt amps for decades using standard antenna relay air contacts with no problems. It's really an insurance policy just in case.

T

[W1RKW]

Hi Tom,
Agree.  Vac relays are indeed a good way to avoid that arc stuff.  Even at  a low RF levels one has to be careful especially in an out of tune condition. I've found that with VHF and hence your experience with parasitic situations.  Putting RCV on one relay and XMIT on another helps. tHen you have to worry about where you split both modes. Then of course you have to think about relay failure situation where one relay hangs or craps out altogether.  Modifying the relay is a cheap solution but not the best compared to a vac relay with good spacing.