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two radios through same PTT?

prkaye

Well Known Member
Hello,
If I wire two independent com radios (no audio panel) through the same PTT switch, is this likely to cause any problems (weird ground loops etc)?
Thanks!
 
20+ years ago, I added a 2nd radio without an audio panel. The PTT switch was run through a DPDT switch that selected com1 or com2 for the PTT and audio in between the two radios. The switch was located just to the left of the bottom of the top and top of the bottom radio and labeled as Com1 / Com2.
 
Thanks Gary, that sounds like a much more elegant solution than what I was thinking of doing. Would you be able to send me a diagram or sketch of how you wired this? I'm having trouble visualizing how the headphone, mic and PTT circuits for the two radios would be wired through a single switch.
[email protected]
Thanks!
 
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Something different than vintage?

PTTs were (are?) simply a ground on the mic jack tip IIRC. You could have a infinite number of PTT switches/circuits provide the ground path for that function.

Sounds like you’re talking a different approach. Any details you can share would be appreciated.
 
PTTs were (are?) simply a ground on the mic jack tip IIRC. You could have a infinite number of PTT switches/circuits provide the ground path for that function.

Sounds like you’re talking a different approach. Any details you can share would be appreciated.

He's talking multiple radios through the same PTT--not the same issue as having multiple PTTs
 
Yes, what i'm doing may be a bit unusual. I have COM radios, with no audio panel and no external intercom. My primary COM radio has an internal two-place intercom. My secondary radio is a backup only and will only be connected to single-place pilot headset. The two radios will never be used simultaneously. In the event of primary com failure, i'd like to be able to throw a single switch and have my pilot-side headset and PTT change-over to the backup COM. Can this be done in a simple way?
 
A DPDT switch will do this nicely
Mike

Edit
Maybe not. I did not think this through and ignored the audio switching.
 

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I might have misinterpreted what the OP was doing. . If the same headset mic is providing the input to both radios you would have to switch the mic input.

I probably should have left this alone.
Sorry
 
Okay, here’s your answer
1. Use the schematic in post #9, but with the corrections of post #10, e.g., change ‘com power’ to ‘mike’, change ‘com1 pwr’ to ‘com 1 mike input’, ‘com2 pwr’ to ‘com 2 mike input’. This will allow you to select com 1 or (but never both) com 2 for transmitting.
2. Receiving. The issue is, depending on the actual radios, one radio may ‘fight’ (try to cancel) the other radios’ headphone signal. Do not simply parallel the two headphone signals. Instead:
(A) method A. Insert a resistor in series with the wire out of com 1 going to the headphone jacks. Do the same with the wire from com2 to the headphone jacks. Any value of resistor between 220 to 470 ohms should work okay (but use the same value for both). You will notice that, due to these resistors, you will have to turn up the volume some, but most radios have volume to spare.
(B) method B. If you have more than 2 or 3 audio signals to hear (warning tones from autopilot? Traffic warnings from adsb? Warnings from efis?) adding series resistors to all lines will start to drag the volume down, maybe so low you can’t hear it well. In this case you should insert an audio mixer between all the inputs, and your headphones. Such circuits are available commercially, pre built for around $100, or build your own for $5. Since the mixer involves electronics,, you might think of a way to mitigate its failure.

I suspect A would work for you. In flight, position the switch to com 1, adjust com 1 volume as needed. Set com 2 volume fully off. If com 1 fails, move the DPDT switch to com 2, and remember to turn up the com 2 volume. As a bonus, in flight, if you wish to stay on ATC or tower frequency while getting ATIS information, just put the ATIS frequency in com 2 and turn up the volume. You’ll hear both tower and atis. Or, keep com 2 on 121.5 with the volume up, just in case someone calls. Note, depending on how close your two antennas are, you may get an annoying ‘bleed thru’ if you transmit on com 1 with the com 2 volume turned up.
 
Separate PTT

Yes, what i'm doing may be a bit unusual. I have COM radios, with no audio panel and no external intercom. My primary COM radio has an internal two-place intercom. My secondary radio is a backup only and will only be connected to single-place pilot headset. The two radios will never be used simultaneously. In the event of primary com failure, i'd like to be able to throw a single switch and have my pilot-side headset and PTT change-over to the backup COM. Can this be done in a simple way?

Another viable option is to mount a separate PTT on the panel somewhere for the backup radio, given that it is only ever going to be used when the primary radio fails.
 
Another viable option is to mount a separate PTT on the panel somewhere for the backup radio, given that it is only ever going to be used when the primary radio fails.

Good idea, and headset jacks are also pretty cheap. But I'd put in a cheap audio panel, so that then you can use the 2nd radio regularly, even if only to make sure it's working ok.
 
I would just add an audio panel if you have room. You can find a used one for not too much money. Look on ebay or they pop up on here from time to time.

I know some people listen to 2 radios at the same time, but I don't often. Conceptually it is a great idea to be able to listen to the ATIS while being on the active Com frequency. It seems when I try to do that 9 times out of 10 there is so much traffic on the active that I can't catch all of the ATIS information. Certainly there are places where the active would be fairly quiet and it could work. For me it has to be the perfect combination of people talking on both so that I can decipher what is being said. Having the audio from the two mixed all the time wouldn't be a solution that I could live with.
 
Do the Dynon panels work as stand alone, or do they need the balance of the EFIS system to function?

Stand alone. For two Comm radios you do add a single DPDT switch to select the radio you want to talk on. This is covered in the install manual.

Carl
 
Stand alone. For two Comm radios you do add a single DPDT switch to select the radio you want to talk on. This is covered in the install manual.

Carl

Thanks Carl, it took me a few minutes to locate the manual, looks like a well priced option that can run dual coms. I have a Flightcom 403 and I'm not happy with the intercom sound quality. I also want to plumb in the audio alerts from the Garmin G5, GFC autopilot, Aera 760 and 430W. This would give me that option without adding in a mixer as well as music input and dual coms.

Thanks,!!
 
I hate to say it, Phil, but I think you may have lost sight of the forest and while being concentrated on the trees. I say this respectfully. I also say it as a multi-decade avionics guy and an active builder/owner/pilot who is familiar with the area around your home base.

Once it's installed that second comm radio is going along for the ride. It's already costing you everything a radio costs, from acquisition cost to installation cost to the cost of the gas to haul it's weight around and the cost of performance to lift that weight. In short, you've bought and paid for that baby in every respect.

Contrast this with your use scenario. You are trying to establish an "emergency only" configuration which nets you the absolute minimum gain against which to amortize all of the above costs. Maximum pain for minimum gain. Respectfully, this is an unwise approach.

A second comm radio is a very handy thing to have IF you can use both comm radios at the same time. Your current methodology robs you of the ability to reap that extra capability.

As an operational example, you are departing your home 'drome of CYSH headed northbound to Gatineau. You currently have selected 122.7 on your single comm. You are headed toward Ottawa's terminal airspace and need clearance through it. To get that clearance you need to monitor ATIS 121.15 and then be ready to talk to TML on 127.7, all while still being within the radius of CYSH where you still should be monitoring and broadcasting position and intent on 122.7.

The second radio, if properly implemented, would be tuned to 121.15 with 127.7 set in standby. You would select comm 1 as "listen and transmit" and comm 2 as "listen only". Make your position broadcasts using comm 1 as you depart CYSH, while picking up ATIS on comm 2. Once you have "Information X-Ray" you switch so comm 2 is "listen and transmit" and comm 1 is "listen only". Swap frequencies on comm 2 and call up Ottawa TML on 127.7 while actively monitoring CYSH Unicom on comm 1. Cleared into Ottawa TML, now switch back to comm 1 as "listen and transmit" - broadcast your "clear of CYSH" message then swap back to comm 2 to continue communications with Ottawa TML.

This is extremely common functionality in a two-radio scenario. This is what makes having two radios a true communications power tool. Your current implementation, as planned, robs you of the ability to quickly and easily swap back and forth between radios while maintaining a listening watch on both.

In short, your current plan robs you of the true benefit of having two radios. A small incremental investment in an audio panel will provide very significant gains in performance and utility and, ultimately, safety.
 
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A small incremental investment in an audio panel will provide very significant gains in performance and utility and, ultimately, safety.

Mark, I agree with everything you posted, except the conclusion (!) copied above. A single toggle switch (transmit1/transmit2) plus an audio mixer for the headphone output, gives you exactly the same capabilities you described, at reduced cost and panel space.
Note to Ray (re listening simultaneously to ATIS and ATC when ATC is busy): try turning up the atis volume to the point that you can hear it okay over ATC. Pretty much ignore ATC except for listening for your N number. If you hear that, immediately turn down the atis, reply to atc. Some audio panels help you with this, by putting com1 in the left ear and com2 in the right (I know, I just advocated for not buying a full up audio panel!). The human brain is actually pretty good about doing this-ignoring background unless you hear your ‘name’(N number).
 
I had this same setup in one of my planes. 2nd radio output was wired to the first radio's aux in (since the 1st radio had the intercom built in Garmin gtr200) I was able to monitor frequencies and transmit on it if i wanted by flicking a switch. No need for an audio panel
 
I skipped the audio panel in the 6. I had an intercom with 2 unswitched aux inputs. I used a 4PDT switch (PTT, Mic hi, spk hi, stdby comm out) and a SP3T switch (on-on-on) and get all of the functionality of an audio panel with some creative wiring. 1st switch selects radio and 2nd switch is either none, stdby comm or Nav as the second audio feed.

Larry
 
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I had this same setup in one of my planes. 2nd radio output was wired to the first radio's aux in (since the 1st radio had the intercom built in Garmin gtr200) I was able to monitor frequencies and transmit on it if i wanted by flicking a switch. No need for an audio panel

This is fine for general use, but not for a backup; if #1 fails, #2 won’t work either.
 
Thansk guys for all the thoughts. I was afraid the thread would turn into a flurry of messages about how i should be getting an audio panel. I really don't want to wade into this debate because i've thought it through carefully - but here's how i arrived at my decision:
1) My new COM radio is a Garmin GNC-255 Nav/Com that allows me to monitor the standby frequency. This offsets some of the advantages that an audio panel provides. Also, the GNC has an integrated 2-place intercom. I like the simplicity of this.
2) my old COM radio becomes a stand-by for emergencies only. I may never have to use it, so i don't want to spend more than necessary. My original plan was to install secondary headset jacks for this and a panel-mount PTT, which is what I will do. I was asking on this thread about the switching to see if it could be done simply and cleanly.
3) I have already had SteinAir build fully interconnected wiring for my setup this way. To go with an audio panel now would require re-wiring work, meaning delay and $$. I'm not ready to do this right now.
4) Handheld as backup - this opens another can of worms. In Canada, dual radio is not required for IFR, but some TC inspectors insist on it. I have had at least one inspector tell me he won't accept a handheld because it's too much workload to manage a handheld in IFR flight. He's not going by the official TC regs, but I figure it's easier jsut to configure my old COM as an emergeny backup than get into an argument with TC inspectors, potentially delaying my approval.
So, I understand all the comments that I "should" be getting an audio panel, but in my specific case right now, I don't agree. I may put one in down the road, and maybe an auto-pilot too, but one thing at a time.
Back to the original question on this thread, it sounds like the switching arrangement will not be as simple as I had hoped, and so I'm just going to go with backup headset jack wired to the backup radio, and dedicated panel-mount PTT. Simple, clean and no extra $$.
(also i don't like that an audio panel presents a single point of failure. Some people have told me that when an audio panel fails it defaults to COM1, but this assumes a particular mode of failure. If, for example, a circuit within the audio panel becomes fried that makes the audio output ureadable, then this is a single point failure that makes both radios ineffective. My approach has two completely independent COM systems, which I feel meets the redundancy objective more robustly).
 
So if you really want an emergency backup radio, then a handheld is best. Why, because even if you have 2 radios and an audio panel, what happens if the audio panel fails? Or what happens if you to turn off your master because of a fire or short? Handheld is totally separate and battery powered. Sporty's sells a handheld that you can plug your headsets right into it without an adapter cable.
Cheap insurance.
 
then a handheld is best.
Ok. As I said, Transport Canada (at least certain inspectors) doesn't agree, and I really don't want to get into a long fight with them :)
Let's put this topic to bed? The original wiring question has been answered :) We could go on debating "best" and "should" indefinitely ;)
 
You should do what you want. I see nothing dangerous or unsafe. It’s easy for others to spend your money!

But I do have a comment about your previous post about audio panels being a single point failure. Many of the newer ones just use a relay to by-pass #1 com to the pilot’s headset, if there’s no power. So it may be that pilot intervention is required in the event of a failure - the pilot might need to turn off (“fail”) the audio panel to activate (technically, de-activate) the fail-safe relay. Of course, nothing is 100% guaranteed (ask me how I know!) but this comes pretty close.
 
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