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PTT shielding

S

szicree

Well Known Member
So I'm fixin' to wire up my audio and going to great lengths to shield out noise when it occurs to me that the ptt wires run all over the place and are (I think) unshielded. Is this ok? What type of wire are folks using for the ptt?

Steve Zicree
The best RV4 ever built :p
 
You don't need to shield PTT wires - on modern radios they are just pulling down a high impedance input to ground. I didn't shield them and my radios sound fine.

Kevin
RV-7A, 50ish hours
 
Any other opinions on req'mnt for shielded PTT?

I was getting ready to run some wires for PTT and downloaded the PS8000B install manual. They call for shielded wire on almost every wire including the PTT wires. Any other opinions on the need for shielded wires?
 
My understanding agrees with what Kevin said. Check with Bob Nuckolls on the AeroElectric list if you want some authority behind the opinion.

erich weaver
 
Kevin is right. PTT wires are simply remote grounds. No shielding is necessary.
 
Maybe...

Mel said:
Kevin is right. PTT wires are simply remote grounds. No shielding is necessary.
Click to expand...

Yes... but when the PTT is not pressed they could act like a long antenna wire directly into the COMM radio....
Depends on the radio internal design, but when you are in RX mode, you want the minimum amount of extraneous noise.

If the maker says shield, then I would....

gil in Tucson

UPDATE ... the first schematic I found on my PC was for the Garmin GNC300 GPS/Comm. Garmin definitely shows a shield on the "Mic Key" line. In correct fashion, the shields are only grounded at one end.

UPDATE 2 ... same for the Garmin 430 - a newer design.
 
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Maybe so. I started wiring radios in the U.S. Air Force in 1964, never shielded a PTT, and never had a problem. YMMV.
 
Tubes

Mel said:
Maybe so. I started wiring radios in the U.S. Air Force in 1964, never shielded a PTT, and never had a problem. YMMV.
Click to expand...

Perhaps the vacuum tube radios are less susceptible to noise... :D

But seriously, if in doubt, following the manufacturer's TSO'd installation instructions is always a good path to follow.... :)

gil in Tucson
 
What do some pro's do?

Since I'm getting ready to order my audio panel from Stark, I called to ask them how they wire the audio harness.

I was told they follow the PS Engineering spec's to the letter - except for the PTT. They do not use shielded wire for the PTT.
 
So little extra effort....

Rick_A said:
Since I'm getting ready to order my audio panel from Stark, I called to ask them how they wire the audio harness.

I was told they follow the PS Engineering spec's to the letter - except for the PTT. They do not use shielded wire for the PTT.
Click to expand...

This is a little different from my previous comment... the audio panel would act as an isolator from the actual radio.

I would guess that they do shield the cable from the audio panel to the actual radio, as Garmin says. In this case, it is only a shielded cable with one extra wire in it...

Again ... builder's choice, but usually the manufacturer knows the best (maybe most likely to work) method.

If you do have a noise problem, it's really difficult to trouble shoot and rectify when the harness is complete and installed in the panel.
Why not just add a little shielded wire? ....the extra cost and effort is pretty minimal... :rolleyes:

UPDATE

As I surmised, they probably are following the PS Engineering documents.

http://www.ps-engineering.com/docs/PMA8000-SR_wiring.pdf

This shows PTT switches not shielded after the wire leaves the mic jacks, but does show the audio panel to radio being shielded. This is for the 8000 Audio panel product...

gil in Tucson
 
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Rick_A said:
Since I'm getting ready to order my audio panel from Stark, I called to ask them how they wire the audio harness.

I was told they follow the PS Engineering spec's to the letter - except for the PTT. They do not use shielded wire for the PTT.
Click to expand...

Ditto for us. Everything else EXCEPT for the PTT wires are shielded from us. We use a black/white twisted pair (just for convenience and commonaility) - black wire is the ground, and the white is the PPT (tip) - that's just how we do it because we buy that pair of wire pre-twisted...and it makes installs easier for the customers.

Cheers,
Stein.
 
Which lines....

SteinAir said:
Ditto for us. Everything else EXCEPT for the PTT wires are shielded from us. We use a black/white twisted pair (just for convenience and commonaility) - black wire is the ground, and the white is the PPT (tip) - that's just how we do it because we buy that pair of wire pre-twisted...and it makes installs easier for the customers.

Cheers,
Stein.
Click to expand...

Stein ... I bet you shield the PTT line (aka Mic. Key) from the audio panel to the comm radios though.... just like the Garmin and PS Eng. installation manuals say to do...

Correct??

gil in Tucson
 
Not needed

Ditto Stein's comments.

The PTT is only grounding the circuit to actuate the transmitter. The ground is either being applied to a digital gate AND/OR or a transistor and neither of them would transmit any RF noise into the radio.

It may be indicated on the instructions as a method of convenience (bundled with other wiring in a single muti stranded cable), or a company general practice of manuf. I have installed many a GA radio and have never seen a report of or heard of any problems.

Obviously, it would also not hurt to do it that if you want. To each his own.
 
Follow the directions?

Comes under the can't hurt. However I do agree most modern avionics are just a ground signal for solid state switche (transistor) to turn on the transmitter. The PTT is not directly in the audio circuit, so not sure how that affects the audio how it acts like an antenna, shielded or not. May be old radios where susceptible to this?

It can't hurt. The good part about using plan wire, is lighter, cheaper and easier to deal with. Most intercoms and radios have so many connections, dealing with shield is a pain.


Now the $24 question what end do you ground the shield, the radio end, the PTT switch end, both ends or neither, leave the shield ungrounded? (just a rhetorical question, talk amongst yourself.)
 
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RTFM

gmcjetpilot said:
....
Now the $24 question what end do you ground the shield, the radio end, the PTT switch end, both ends or neither, leave the shield ungrounded? (just a rhetorical question, talk amongst yourself.)
Click to expand...


Simple,

The end that the Installation Manual shows.... :D

gil in Tucson
 
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