Why does mic need to be attached to receive?

[Rob_MA]

Does anyone know the history or reason behind why CB radio is the only two-way radio type that needs its microphone connected to receive? My experience is with land mobile and ham gear as well as some VHF marine radios.

I've seen a couple of exceptions to this with some older tube gear but nearly all I grew up with did this. (Don't own anything really new so that may have changed in the last few decades.) I'm curious because no one I've ever talked to knows why this is.

 

[guitar_199]

In my experience the difference is "relay versus electronic" switching.

In older radios there was a relay that actually performed all switching. The contacts on the relay would handle all of the details required to switch between receive and transmit... so all you had to do was have the mic close a switch that grounded the coil of the relay. To be clear, when you pushed the button... you were closing the contacts of a switch to energize the relay. When you let off the button, you were just opening the contacts and the relay would de-energize. If the coil was energized, you were in transmit, otherwise you were in receive mode. When you unhooked the mic, the radio was by default, in receive.

In later electronic switching, the mic switch has a DPDT (double pole double throw) action where when you press the button you are closing contacts for the transmit circuits. When you let go, you release the transmit contacts BUT you are making ANOTHER SET of contacts for RCV. If you remove the microphone.... NEITHER set of contacts are closed...both are open.

For a reason I don't know, it seems that when they went to electronic switching they used the switching to disable the speaker as well as all the other switched circuits. With that... more than likely the radio IS in receive mode, you just can't hear it because the RX switch in the mic being gone disables the speaker.

I hope this makes sense....

 

[snow bird]

I thing It is to Stop feed back! Or you wool be Hearing your self talk all the time

 

[sunbulls]

I agree with Guitar. Relays are also expensive, take up extra space on the board and draw additional current when activated. Like Guitar said, the DPDT mic key is essentially doing the same job without the necessity of needing a relay with all its pitfalls. They are both similar in respect to their reliability. Contacts need occasional cleaning if not total replacement with use. When keying problems arise with an electronic switched radio, most consumers simply replace the mic, some will try to clean the key first. Relay maintenance or replacement on the other hand is usually left to the techs or advanced hobbyist. A faulty relay is an expensive aggravation to the average consumer that often results in a radio being tossed. The choice is simple. Most would rather have a faulty mic than a faulty radio. Who among us doesn’t have a spare mic or two?

 

[guitar_199]

I like Snowbird's point.....
If you left the speaker active you would most likely wind up listening to the audio coming out of the audio amp to modulate the final/driver.

I thing It is to Stop feed back! Or you wool be Hearing your self talk all the time

 

[guitar_199]

I didn't think to say it this way but honestly Sunbulls fed me the idea....

There are a number of things inside the radio that need to be switched to change the radio from transmit to receive.... and it takes multiple switch contacts to do it.

If the radio uses a relay (INSIDE the radio) then the switches on the relay do the switching and the one set of contacts in the mic energizes the relay coil... or NOT.

If the radio is electronic switching...then the multiple switches are in the microphone and and as LONG AS it is hooked up.... you are in receive unless you press the button, then you are in transmit. BUT...if you unhook the mic... everything is off/disabled.

 

[Recon]

Does anyone know the history or reason behind why CB radio is the only two-way radio type that needs its microphone connected to receive? My experience is with land mobile and ham gear as well as some VHF marine radios.

I've seen a couple of exceptions to this with some older tube gear but nearly all I grew up with did this. (Don't own anything really new so that may have changed in the last few decades.) I'm curious because no one I've ever talked to knows why this is.

The Stryker (10 Meter) 94 is one of the few radios that will receive when the mic is disconnected after it has been modified for the 11 meter band.

 

[Captain Kilowatt]

The reason is that circuits are switched using transistors as switches. Supplying a voltage to a common point can enable several circuits at the same time. Some circuits need to be switched on in TX and off in RX and vice versa. Without the mike connected the necessary circuits are not switched on or off properly. Doing it this way is quite cheap and that is likely the driving factor. Other radios such as marine or aviation may be required to RX without a mike for safety reasons I am not sure.

 

[Shadetree Mechanic]

I always thought that it was for a mic that has a built in speaker. Although I have never seen one.

 

[loosecannon]

all of the export style radios will receive with no mic connected.
LC

 

[Rob_MA]

Ah, makes sense that it comes down to money. Dropping the cost of the radio to make it more competitive with little down side in performance.

Just musing about land mobile equipment and there usually is some amount of circuitry on the board to do this. The profit margin in land mobile equipment is larger than CB gear as well as a likely safety issue if it was in public safety service.

Thanks for satisfying the knowledge itch.