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  #1  
Old 06-17-2018, 04:35 PM
RV4Ever RV4Ever is offline
 
Join Date: Nov 2017
Location: Nr London, UK
Posts: 4
Default Dual Radio Installation

Hi,

I am installation 2xTrig TY91 radios in to my RV8. Rather unusually, we are going to use 2xPTTs and no audio panel. The purpose is to allow the use of both radios simultaneously, predominantly for formation flying. I.e. an inter-aircraft radio and an ATC radio, with the two PTTs readily available on the stick.

I am using an audio mixer for the outputs, feeding in to the headphones.

I am slightly concerned though about joining the microphone inputs together for each radio. Will this need any degree of electronic isolation (resistors etc)? Has anyone got any experience of this?

Many thanks.
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  #2  
Old 06-17-2018, 04:52 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 5,355
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You have some reason for concern. Hooking one mike to two transmitters will put the two bias resistors in parallel. This may affect the gain of the mike in some unpredictable way, although my guess is the effect is small. Why not have PTT #2 control a small, DPDT relay, which would direct the PTT signal (ground) and the mike audio to #2 radio?
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  #3  
Old 06-17-2018, 06:38 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
Posts: 3,590
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Here's what the TY91 installation manual states:

========
5.6.6
PTT1/2 Key Input

Two Push to Talk (PTT) inputs are provided, which correspond to the two microphone inputs. Only the corresponding microphone input is routed to the transmitter when the key switch is pressed.

The PTT1 input has priority over the PTT2 input – if the PTT1 switch is closed when the PTT2 switch is closed, the input from Microphone 1 is routed to the transmitter.

The inputs are active low, and will be asserted when the voltage to ground is pulled below approximately 4 volts. The input should be connected to a momentary switch on the yoke or on the microphone.

============

5.6.8
Transmit Interlock

When two VHF radios are mounted in an aircraft the transmit interlock input of one can be connected to the transmit PTT key of the other radio. When the other radio transmitter is keyed, the squelch threshold of this radio is increased to minimise break-through between one radio and the other.

Note: To improve the performance when using two radios, the antennas should be as far apart as practical – for example on the top and bottom of the fuselage.

===============
Dave
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  #4  
Old 06-17-2018, 07:11 PM
Dan Langhout's Avatar
Dan Langhout Dan Langhout is offline
 
Join Date: Mar 2008
Location: Huntsville, AL USA
Posts: 456
Default Simultaneous Transmit

THIS

Quote:
Originally Posted by David Paule View Post
Here's what the TY91 installation manual states:

========

5.6.8
Transmit Interlock

. . . . . . . Note: To improve the performance when using two radios, the antennas should be as far apart as practical – for example on the top and bottom of the fuselage.

===============
Dave
I have two COMM in my RV-7 (connected through an audio panel). Both antennas are mounted on the belly as far apart as I could get them (about 3 feet). The transmitting radio DEFINITELY will bleed over onto the other radio with the antennas that close - doesn't matter what freq you have the radio tuned to. My audio panel isolates me from this problem unless I select to hear both radios at the same time - and then transmit on one of them.
Unless you can put one antenna on the top (difficult to impossible with an -8) and the other on the bottom, I suspect you will not be able to use both radios as you describe.
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  #5  
Old 06-18-2018, 01:40 PM
RV4Ever RV4Ever is offline
 
Join Date: Nov 2017
Location: Nr London, UK
Posts: 4
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Thanks for all of your replies. I was worried about the effect of connecting 1 microphone in to both radio microphone inputs, as BobTurner said (thanks), creating issues with the bias resistors.


I think perhaps the way to go is with a DPDT relay, linking the microphones in to the correct place on a 1-1 basis as required.


I'll report back with results in case anyone is interested.


Good thought RE TX interlock. I had decided we didn't need that, but sounds like the breakthrough might be eliminated nicely by this feature.

thanks for everyone's inputs.
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  #6  
Old 06-18-2018, 03:41 PM
Canadian_JOY Canadian_JOY is offline
 
Join Date: Aug 2007
Location: Ontario, Canada
Posts: 1,777
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A good audio panel is just a MUCH better way to accomplish what you want to do. I know, nobody wants to install an audio panel if they can avoid it. Unfortunately nobody wants to troubleshoot crappy audio when the "homebrew" audio installation doesn't work, as too often is the case.

Better to do it right, from the start, than have to re-do it later the way you should have done it in the first place.

(Sorry for the soapbox nature of my comment... As a career avionics guy I just get so tired of people taking shortcuts on audio then expecting me to sort out the mess they've created. My flat response now to somebody who comes asking me to fix their mess is that I will only work on their airplane if they are prepared for a complete rip-and-replace of the audio system. Life's too short to deal with poor audio. OK, soapbox has been put away now. Please return to your regular programming... LoL)
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  #7  
Old 06-18-2018, 04:22 PM
RV4Ever RV4Ever is offline
 
Join Date: Nov 2017
Location: Nr London, UK
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Canadian_JOY thanks for your post. I totally get it, and normally I would be there with you. However, I cannot find an audio panel that does what I want. I don't want flip-flop switches or channel selectors - 1xPTT for com1 and 1xPTT for com2. If you can enlighten me as to one that might be available... I'm genuinely keen to "do it properly!"

thanks for engaging (on or off your soapbox!)
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  #8  
Old 06-19-2018, 06:10 AM
Tooch Tooch is offline
 
Join Date: Nov 2011
Location: Amelia, Va
Posts: 201
Default Homebrew Audio

My "homebrew audio" sounds great. I have a Garmin GTR 200 radio that has an excellent intercom built in. I run the audio inputs through a usb powered mixer that has the standard 1/8 inch stereo inputs. I have audio alerts from my portable hooked to it. and the alerts from my Dynon D10A. Also music.

I have a portable radio hooked to it also. With this set up, it is very easy to change things.

Now, with that said, if I was starting from scratch,(and I had the money) I would install a dual screen G3X system with 2 radios and a bluetoothed audio panel.

I didn't build my plane and it did not have an audio panel.
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  #9  
Old 06-19-2018, 09:03 AM
Canadian_JOY Canadian_JOY is offline
 
Join Date: Aug 2007
Location: Ontario, Canada
Posts: 1,777
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Most audio panels will support an on-stick momentary switch contact to swap radios for transmitting. I'm not sure if you've evaluated this solution vis a vis your perceived requirements, but for most operations it is often sufficient.

If you do opt to build your own system, you will want to use a multi-pole relay to switch both Mic Hi & Mic Lo between radios. Wire it such that in its normally "relaxed", unenergized state, Mic Hi and Mic Lo are routed directly to COMM 1 to ensure you have a fail-safe design. PTT #2 would be wired in parallel to the coil of the mic switching relay and the PTT input of COMM 2 such that actuating the PTT #2 switch both routes mic signals to COMM 2 and keys the COMM 2 transmitter. Use a relay with high-quality gold contacts and make it easily replaceable because it WILL need to be replaced over time. A relay that uses a standard socket base is your best bet so you can just pop a new relay in without having to mess with wiring (and always carry a spare relay).

If you use an audio summing junction you will find it much easier to balance the headphone volume levels between radios by using the volume control on each radio. Don't just tie the two headphone hi signals from the two radios together as that will provide less-than-optimal results.

Frankly, if you're using the radios so intensively that you have to have two PTT's I would be very worried about doing this while actually flying the airplane, especially in formation. This sounds like a recipe for distraction-related incidents. OK, I guess I've got my soapbox out again, haven't I? LoL
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  #10  
Old 06-19-2018, 09:14 PM
jakej jakej is offline
 
Join Date: Aug 2005
Location: Adelaide, Australia
Posts: 84
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Quote:
Originally Posted by RV4Ever View Post
Canadian_JOY thanks for your post. I totally get it, and normally I would be there with you. However, I cannot find an audio panel that does what I want. I don't want flip-flop switches or channel selectors - 1xPTT for com1 and 1xPTT for com2. If you can enlighten me as to one that might be available... I'm genuinely keen to "do it properly!"

thanks for engaging (on or off your soapbox!)
Check out the Ps-Engineering units, specifically the ones where you can select & TX Com1/2 at the same time (eg Pilot & Copilot) or a ‘mixer’ like the AMX 2A or 4A
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