Garmin 340 Audio Panel Split Com

[O. Sami Saydjari]

Does anyone out there have experience with Garmin's 340 audio panel and
its feature that lets you split the Com 1 and Com 2 so that the pilot
can talk/receive on one while the co-pilot can talk and receive on a
separate frequency on another? It is called the "Com 1/2" button.
On my installation, the co-pilot experienced that Com 2 reception was
blocked with I was talking on Com 1. That is not how it is supposed to
work. I have been told by one avionics shop that it is difficult to not
have Com1/Com2 interference and that I should just write-off this
feature. Another shop said that the Com 1 and Com 2 antenna's had to be
relocated so that they were on opposite sides of the plane (one op top
and one on bottom). I am not sure what to believe at this point. Any
advice or experience that folks have had would be appreciated.

-Sami
N2057M Piper Turbo Arrow III

[Nathan Young]

On Fri, 23 Jan 2004 21:54:40 -0600, "O. Sami Saydjari"
> wrote:

>Does anyone out there have experience with Garmin's 340 audio panel and
>its feature that lets you split the Com 1 and Com 2 so that the pilot
>can talk/receive on one while the co-pilot can talk and receive on a
>separate frequency on another? It is called the "Com 1/2" button.
>On my installation, the co-pilot experienced that Com 2 reception was
>blocked with I was talking on Com 1. That is not how it is supposed to
>work. I have been told by one avionics shop that it is difficult to not
>have Com1/Com2 interference and that I should just write-off this
>feature. Another shop said that the Com 1 and Com 2 antenna's had to be
>relocated so that they were on opposite sides of the plane (one op top
>and one on bottom). I am not sure what to believe at this point. Any
>advice or experience that folks have had would be appreciated.

I'm not an avionics expert, but if the COM1 and COM2 antennas are near
each other (which they generally are on a GA plane), the COM2 antenna
is going to be receiving a ton of energy (relatively speaking) when
the COM1 radio transmits. Even if this energy is on a different
frequency (ex. COM1 on 120.55, COM2 on 124.8) the channel selection
fillters will not be able to supress the transmit energy from COM1.

I've always wondered how aviation radios deal with this. I suspect
they have some sort of level detector on the output of the 1st
(receiver) amplifier stage which shuts off the input if it reaches a
threshold.

If so - this would explain why your COM2 receive cuts out when COM1
transmits.

Well, enough speculation. Perhaps someone who actually knows how the
radios work will respond.

-Nathan

[Thomas Borchert]

O.,

> I have been told by one avionics shop that it is difficult to not
> have Com1/Com2 interference and that I should just write-off this
> feature. Another shop said that the Com 1 and Com 2 antenna's had to be
> relocated so that they were on opposite sides of the plane (one op top
> and one on bottom).
>

We have been told the same thing. Our antennas are both on top and we have
similar problems.

--
Thomas Borchert (EDDH)

[Mark Manes]

I just had a Garmin 340 audio panel installed in my 310. The avionics shop
said that there would be interference between the two if the coms were split
and suggested not enabling that feature. I took their suggestion. I am
very pleased with the audio panel but that would have been a nice feature.


"O. Sami Saydjari" > wrote in message
...
> Does anyone out there have experience with Garmin's 340 audio panel and
> its feature that lets you split the Com 1 and Com 2 so that the pilot
> can talk/receive on one while the co-pilot can talk and receive on a
> separate frequency on another? It is called the "Com 1/2" button.
> On my installation, the co-pilot experienced that Com 2 reception was
> blocked with I was talking on Com 1. That is not how it is supposed to
> work. I have been told by one avionics shop that it is difficult to not
> have Com1/Com2 interference and that I should just write-off this
> feature. Another shop said that the Com 1 and Com 2 antenna's had to be
> relocated so that they were on opposite sides of the plane (one op top
> and one on bottom). I am not sure what to believe at this point. Any
> advice or experience that folks have had would be appreciated.
>
> -Sami
> N2057M Piper Turbo Arrow III
>


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[jerryk]

O. Sami Saydjari wrote:

> Does anyone out there have experience with Garmin's 340 audio panel and
> its feature that lets you split the Com 1 and Com 2 so that the pilot
> can talk/receive on one while the co-pilot can talk and receive on a
> separate frequency on another? It is called the "Com 1/2" button.
> On my installation, the co-pilot experienced that Com 2 reception was
> blocked with I was talking on Com 1. That is not how it is supposed to
> work. I have been told by one avionics shop that it is difficult to not
> have Com1/Com2 interference and that I should just write-off this
> feature. Another shop said that the Com 1 and Com 2 antenna's had to be
> relocated so that they were on opposite sides of the plane (one op top
> and one on bottom). I am not sure what to believe at this point. Any
> advice or experience that folks have had would be appreciated.
>
> -Sami
> N2057M Piper Turbo Arrow III


I have a 340 AP and use the COM1/COM2 switch. We have had no problems.
But, the COM1 antenna is in the tail and the COM 2 antenna is on the belly.

jerry

[Richard Hertz]

I assumed mine was working fine as I have used the "split" feature often.
However, I have never transmitted while the other person was on the number
2 - I will be sure to check it out next time I have a second person in the
plane.


"O. Sami Saydjari" > wrote in message
...
> Does anyone out there have experience with Garmin's 340 audio panel and
> its feature that lets you split the Com 1 and Com 2 so that the pilot
> can talk/receive on one while the co-pilot can talk and receive on a
> separate frequency on another? It is called the "Com 1/2" button.
> On my installation, the co-pilot experienced that Com 2 reception was
> blocked with I was talking on Com 1. That is not how it is supposed to
> work. I have been told by one avionics shop that it is difficult to not
> have Com1/Com2 interference and that I should just write-off this
> feature. Another shop said that the Com 1 and Com 2 antenna's had to be
> relocated so that they were on opposite sides of the plane (one op top
> and one on bottom). I am not sure what to believe at this point. Any
> advice or experience that folks have had would be appreciated.
>
> -Sami
> N2057M Piper Turbo Arrow III
>

[MikeM]

Like Nathan said!

To be able to receive on one while transmitting on the other
requires much technical effort, including separating antennas,
and installing filters/cavities/duplexors in the antenna leads
to the two radios. Not practical in an aircraft, unless it is
a C130.

Mikem


Nathan Young wrote:

> On Fri, 23 Jan 2004 21:54:40 -0600, "O. Sami Saydjari"
> > wrote:
>
>
>>Does anyone out there have experience with Garmin's 340 audio panel and
>>its feature that lets you split the Com 1 and Com 2 so that the pilot
>>can talk/receive on one while the co-pilot can talk and receive on a
>>separate frequency on another? It is called the "Com 1/2" button.
>>On my installation, the co-pilot experienced that Com 2 reception was
>>blocked with I was talking on Com 1. That is not how it is supposed to
>>work. I have been told by one avionics shop that it is difficult to not
>>have Com1/Com2 interference and that I should just write-off this
>>feature. Another shop said that the Com 1 and Com 2 antenna's had to be
>>relocated so that they were on opposite sides of the plane (one op top
>>and one on bottom). I am not sure what to believe at this point. Any
>>advice or experience that folks have had would be appreciated.
>
>
> I'm not an avionics expert, but if the COM1 and COM2 antennas are near
> each other (which they generally are on a GA plane), the COM2 antenna
> is going to be receiving a ton of energy (relatively speaking) when
> the COM1 radio transmits. Even if this energy is on a different
> frequency (ex. COM1 on 120.55, COM2 on 124.8) the channel selection
> fillters will not be able to supress the transmit energy from COM1.
>
> I've always wondered how aviation radios deal with this. I suspect
> they have some sort of level detector on the output of the 1st
> (receiver) amplifier stage which shuts off the input if it reaches a
> threshold.
>
> If so - this would explain why your COM2 receive cuts out when COM1
> transmits.
>
> Well, enough speculation. Perhaps someone who actually knows how the
> radios work will respond.
>
> -Nathan

[Jack McAdams]

My 340 does the same thing if the frequencies are close together. I
can't say what the spread needs to be but have noticed that if they
are close the odds of blocking are more pronounced.

Jack
Sundowner - N6363U


"O. Sami Saydjari" > wrote in message >...
> Does anyone out there have experience with Garmin's 340 audio panel and
> its feature that lets you split the Com 1 and Com 2 so that the pilot
> can talk/receive on one while the co-pilot can talk and receive on a
> separate frequency on another? It is called the "Com 1/2" button.
> On my installation, the co-pilot experienced that Com 2 reception was
> blocked with I was talking on Com 1. That is not how it is supposed to
> work. I have been told by one avionics shop that it is difficult to not
> have Com1/Com2 interference and that I should just write-off this
> feature. Another shop said that the Com 1 and Com 2 antenna's had to be
> relocated so that they were on opposite sides of the plane (one op top
> and one on bottom). I am not sure what to believe at this point. Any
> advice or experience that folks have had would be appreciated.
>
> -Sami
> N2057M Piper Turbo Arrow III

[[email protected]]

On 23-Jan-2004, Nathan Young > wrote:

> I'm not an avionics expert, but if the COM1 and COM2 antennas are near
> each other (which they generally are on a GA plane), the COM2 antenna
> is going to be receiving a ton of energy (relatively speaking) when
> the COM1 radio transmits. Even if this energy is on a different
> frequency (ex. COM1 on 120.55, COM2 on 124.8) the channel selection
> fillters will not be able to supress the transmit energy from COM1.
>
> I've always wondered how aviation radios deal with this. I suspect
> they have some sort of level detector on the output of the 1st
> (receiver) amplifier stage which shuts off the input if it reaches a
> threshold.
>
> If so - this would explain why your COM2 receive cuts out when COM1
> transmits.
>
> Well, enough speculation. Perhaps someone who actually knows how the
> radios work will respond.


You've pretty much nailed it, Nathan. However, the problem is not in the
"channel selection filters" as you call them. Channel selectivity is
achieved with filters at the "intermediate frequency" or "IF" stages.
Before the received signal gets to that point it must be amplified by the RF
preamplifier and frequency-shifted in the first mixer. Both of these stages
have a limited "dynamic range", which means that they cannot pass a
relatively weak signal (as in one from a ground station) in the presence of
a vastly stronger one (as in one emanating from another nearby com antenna
on the same airframe).

--
-Elliott Drucker

[Marco Leon]

My installation works as designed. The avionics shop separated the two
antennas--Com1(GNS430) on top of the fuselage and Com2 (KX-170B) on the
bottom. He said that I could have had them on the same antenna but if I
remember correctly, he said I would have had interference between the two
coms.

It IS a very cool feature that I've used often. However, most are with the
copilot just listening to ATIS and very few with the copilot talking
simultaneously with the pilot. These are usually reporting PIREPs or
speaking with other aircraft.

Marco


"O. Sami Saydjari" > wrote in message
...
> Does anyone out there have experience with Garmin's 340 audio panel and
> its feature that lets you split the Com 1 and Com 2 so that the pilot
> can talk/receive on one while the co-pilot can talk and receive on a
> separate frequency on another? It is called the "Com 1/2" button.
> On my installation, the co-pilot experienced that Com 2 reception was
> blocked with I was talking on Com 1. That is not how it is supposed to
> work. I have been told by one avionics shop that it is difficult to not
> have Com1/Com2 interference and that I should just write-off this
> feature. Another shop said that the Com 1 and Com 2 antenna's had to be
> relocated so that they were on opposite sides of the plane (one op top
> and one on bottom). I am not sure what to believe at this point. Any
> advice or experience that folks have had would be appreciated.
>
> -Sami
> N2057M Piper Turbo Arrow III
>



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