My PTT switches are in the sticks on my RV-6. When I push to transmit there is always a moment of static before the transmission is clear. Also some of my other instruments are affected while I am transmitting (i.e. MP indicator as well as the ammeter on my Indicator module). It didn't occur to me to shield these wires. Does anyone know if this is standard practice?
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PTT shielding?
- Thread starter jacksel
- Start date
fmarino1976
Well Known Member
Nope
Normally PTT wires aren't shielded. The little noise you hear is most likely a faulty switch (it is not making full contact until you push hard, etc). The instruments being affected, that could be several things. Either current related (does the voltage on the bus drops when you transmit?) or RF related. Either way, shielding the PTT wires won't do anything to fix the problem.
Normally PTT wires aren't shielded. The little noise you hear is most likely a faulty switch (it is not making full contact until you push hard, etc). The instruments being affected, that could be several things. Either current related (does the voltage on the bus drops when you transmit?) or RF related. Either way, shielding the PTT wires won't do anything to fix the problem.
gmcjetpilot
Well Known Member
Sheild not needed
What fmarino1976 said. Some people do use shield, but its not necessary. I don't know about the noise you hear just as you key the mic. What kind of radio do you have? Is it an old radio?
The PTT on modern radios is a ground signal, separate from the radio receiver/transmitter and audio amp circuits. The PTT ground is isolated through a transistor or electronic switch.
fmarino1976 is probably right about the switch being dirty or poor quality. If its making an intermediate contact, it could telegraphic into the radio, possibly, but I'm not 100% about that. The switch would have to be real lousy or dirty. It's easy to check, by putting in a new, better quality PTT switch.
If a new PTT does not do it, it could be something else on the headset audio side of the system and not the PTT side. I agree shielding on the wire is not needed.
What fmarino1976 said. Some people do use shield, but its not necessary. I don't know about the noise you hear just as you key the mic. What kind of radio do you have? Is it an old radio?
The PTT on modern radios is a ground signal, separate from the radio receiver/transmitter and audio amp circuits. The PTT ground is isolated through a transistor or electronic switch.
fmarino1976 is probably right about the switch being dirty or poor quality. If its making an intermediate contact, it could telegraphic into the radio, possibly, but I'm not 100% about that. The switch would have to be real lousy or dirty. It's easy to check, by putting in a new, better quality PTT switch.
If a new PTT does not do it, it could be something else on the headset audio side of the system and not the PTT side. I agree shielding on the wire is not needed.
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az_gila
Well Known Member
Grounds....
The question I would have is how does the ground for the PTT switch get back to the comm. radio?
While they might not be shielded in practise, many of the radio makers show shielding in their interconnect diagrams. Often, the PTT return path (aka "ground") is shown as the shield for the mic. Lo/Hi wire pair. This would take the return current path directly back to the radio connector.
If this plane does not have a good avionics ground, connected at only one spot to airframe ground, the instrument fluctuations could be possible, and a poor switching action could occur. One more item to check....
So....
Jack.... how does the PTT switch ground get back to the comm. radio?
The question I would have is how does the ground for the PTT switch get back to the comm. radio?
While they might not be shielded in practise, many of the radio makers show shielding in their interconnect diagrams. Often, the PTT return path (aka "ground") is shown as the shield for the mic. Lo/Hi wire pair. This would take the return current path directly back to the radio connector.
If this plane does not have a good avionics ground, connected at only one spot to airframe ground, the instrument fluctuations could be possible, and a poor switching action could occur. One more item to check....
So....
Jack.... how does the PTT switch ground get back to the comm. radio?
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Norman CYYJ
Well Known Member
I agree with the above possible problems. Another problem that you might have is the intercom. I have seen it where when you first push the PTT there is a noise, sometimes it is the audio circuit and an open mic and you are momentarily hearing cockpit noise. If you have an intercom try putting it into the ISO position, if your noise goes away you know the problem.
chrispratt
Well Known Member
And Check your BNC
One more thing to check is your BNC connection to the antenna. Is it locked in place as intended? Don't laugh. Mine was loose once (just not turned all the way to lock position) and it caused static on the radio when transmitting, poor audio reception and my EI gauges would cycle and reset whenever I pressed the transmit button. Worth checking as the fix is simple -- twist it fully into place.
Chris
One more thing to check is your BNC connection to the antenna. Is it locked in place as intended? Don't laugh. Mine was loose once (just not turned all the way to lock position) and it caused static on the radio when transmitting, poor audio reception and my EI gauges would cycle and reset whenever I pressed the transmit button. Worth checking as the fix is simple -- twist it fully into place.
Chris
Agree with Chris above. The key here is your the static at beginning of transmission AND your MP and ammeter gauges are responding to RF noise. The PTT ground is not of source of noise. fmarino1976 is correct that a faulty switch with intermittent contact could cause noise by the "pulsing" if you will of the voltage but the amps are miniscule so unlikely and you would hear intermittent audio as your transmitter goes on/off. More than likely you have an RF leak somehwere, antenna, back of radio/tray ect.
