Difference between Com1 & Com2

tuwood

tuwood

Pre-takeoff checklist
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tuwood
I've been reading up quiet a bit on using the radio and various techniques to stay ahead of ATC, such as using Com2 for ground, clearance, ATIS and Com1 for Tower, Approach, etc. Basically Com 1 for the people you need to talk to and Com 2 for the non flight impacting stuff.

Then I got to thinking about the Cirrus I'm flying which has a big white antenna on top for Com 1 and a bent wire antenna on the bottom of the plane for Com2.

Is there a different range for Com1 and Com2, or are they both the same power and receiver sensitivity?
Does being on top or on the bottom make one or the other better suited for certain types of communications?
Or are they both effectively the same and the Com1 for primary communications is just an organization thing?
 
Are you sure that bent wire on the bottom is a COM radio? 'Cause it sounds like an ADF antenna.

Every airplane I've seen with two COM radios has identical antennas for COM1 and COM2. The radios themselves may not be identical.

Using COM2 for "extras" is a common convention, but there is no reason behind it except not having to expend brain cells.

If one of your radios is a lot better than the other (it happens), you might consider using that for anything you transmit, including CD and ground.
 
I think it is just an organization thing. I use COM1 for just about everything were talking is involved and COM2 for stuff that needs to be monitored or to store the next needed frequency as a reminder. On a low wing I have heard they usually have a top and bottom radio antenna and sometimes on the ground you can't use the bottom COM due to the wings blocking signal and when flying the upper may be blocked sometimes.
 
The model (or age) of the radio might affect reception of weak stations. Different models could also potentially have different power output; I don't know whether that is standardized. In practice, problems due to that are rare.

An antenna on the top of a metal aircraft could have a small null zone directly below you, which could momentarily cause weak reception for a station in that location, but other than that, it shouldn't make much difference.

Whether a radio is connected as com 1 or com 2 makes no difference unless there is a defect in the connectors or cabling.
 
Both of my Com radios go through a bent wire antenna on the bottom of the plane. In my opinion it's better to have it there then on top, after all you aren't talking to space.

The antenna WAS on top of the plane, but it got moved to the bottom when I had my Garmin GTN-650 installed. Now the GPS antenna is on top where it belongs. Different radios have different reception needs too, 10 watts of power vs 16 watts, etc. That may also influence the type of antenna required.
 
MAKG1 said:
Are you sure that bent wire on the bottom is a COM radio? 'Cause it sounds like an ADF antenna.

Every airplane I've seen with two COM radios has identical antennas for COM1 and COM2.
Click to expand...

An ADF on a Cirrus? ;)
 
MAKG1 said:
Are you sure that bent wire on the bottom is a COM radio? 'Cause it sounds like an ADF antenna.

Every airplane I've seen with two COM radios has identical antennas for COM1 and COM2. The radios themselves may not be identical.

Using COM2 for "extras" is a common convention, but there is no reason behind it except not having to expend brain cells.

If one of your radios is a lot better than the other (it happens), you might consider using that for anything you transmit, including CD and ground.
Click to expand...

This is what the COM2 radio looks like on the bottom of the Cirrus (inverted of course)
av-17-01.jpg

COM1 looks similar to this one (on top)
2480.jpg


It could very likely be a combined antenna providing COM1 and some other service.
 
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ok, thanks for the replies everyone. I run into these weird questions every now and again that I can't find on the internet. :)
I'm an old Radar guy from the Navy so I try to understand how everything works from an engineering aspect as well as operations. I suspect Cirrus (low wing aircraft) designed it with one on top and one on bottom so you could switch if you needed better coverage on one side or the other.
 
Antennas and location:

The bent ones have poorer radiating patterns than the other. They are bent only for increased clearance and installed on the bottom.

The reason to separate them is to reduce interference between the two coms. In theory you could use both radios simultaneously with two crew.
 
I think it's just a preference thing. I teach my students basically what you do with comm 1 and 2.
 
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I have heard CSIP recommend using the top while on the ground and the bottom in the air.
I don't do that.

My bottom radio is for my destination and my top radio is my "working radio"

I don't want to mess with the radio while looking for the field and setting up for the pattern so I set lower radio for that before I takeoff.
I do all the in flight switching on the upper radio. That's just the routine that works for me though.
 
My #1 radio (part of the GNS480) and #2 (SL30) are essentially identical, but the 480 is integrated with the database. I tend to use #2 for AWOS and for some CTAF uses and #1 for everything else. Just figure out what works well for you.

When I was based at IAD, I put the departure frequency in one radio and use the ohter for all the ground operations and the tower. Then to switch to departure (a busy time), I just had to flip the switch on the audio panel.
 
I've had interference between the two radios with both antennas mounted on top in the locations provided by Piper. The com panel kills the receiver in the radio that you are transmitting on but not on the other radio. It didn't matter much what frequency the non-transmitting radio was tuned to. I found later that there was an option that could have been installed in the radios (TKM) to provide for killing both receivers when either transmitter was keyed. Separating the antennas top and bottom should reduce this problem.
 
Whatever works best for you.

For what it's wort here's my typical setup.

Enroute (if I'm IFR) I'll typically have my current and last ATC up top on com 1, and my unicom and AWOS being monitored for my destination on com 2, I'll leave the AWOS/ATIS on and once I can pick it up I'll request direct the favoring approaches IF/IAF and start planning for that approach, now I start monitoring the Unicom and flipping back to the weather every once in a while, and for use before I hit the first segment of the approach.



On the ground, I'll have tower and departure on comm one.

I'll pick up the wx and get my clearance on comm two, then have ground on comm two, as I approach any runway I'll be monitoring both tower and ground (even though at many medium sized airports it's the same guy).

If it's a uncontrolled airport il have Unicom and weather on comm two and the freq I will get ATC on in comm one, more or less like my enroute configuration.




As far as antennas go, I have both my comm one and two next to each other on the top of the plane (cessna amphib) and a spare third comm (bent style) on the belly.

For which antenna for which frequency, doesn't really matter, if comm two was my top radio and comm one was my bottom radio I'd reverse what I said above, I don't go off which radio has a antenna in what location, it's just a top and bottom thing, works better in my noodle. I should add that my comm one and two are both GNS boxes, if you had a GNS and a old crap radio I might favor the better box for the more important stuff like talking to ATC.
 
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Almost every airplane I've ever flown, one of the two radios, is better than the other. Pretty much every airplane I've seen with a 430/KX-155 combo is a good example of that. The KX-155 for one reason or another always has better range both RX and TX.

I'm not one to stage every frequency in all the radios. Whichever radio I like better is the radio I use for all communication. I will stage whatever the *next* frequency is in STBY if I suspect I'm changing to that frequency. If I'm enroute cruise then whichever radio works the best has the current controller in active, the last controller in STBY, and 121.5 being monitored on the other radio with the AWOS/ASOS for the destination as STBY.

I teach students basically the same thing. I do not teach them to stage 4 frequencies. I think that's just over complicated and confuses people when they need to switch which radio is transmitting. Plus a lot more to go wrong with volume knobs and whatnot.
 
Yeah, Im a one radio pilot too. Maybe its because my plane only has one radio.

I do miss being able to listen to the ATIS while still on with ATC though. Thats the biggest handicap of only one radio.
 
bnt83 said:
Antennas and location:

The bent ones have poorer radiating patterns than the other. They are bent only for increased clearance and installed on the bottom.

The reason to separate them is to reduce interference between the two coms. In theory you could use both radios simultaneously with two crew.
Click to expand...

I haven't seen the aviation version modeled in NEC or similar, but as I recall, at VHF bending a quarter wave doesn't do all that much to the antenna pattern. Even bending it back on itself.

Adds some more blobs to it but isn't that significant a pattern change inside the typical ranges and powers we're using in aviation.

Got a source for the claim, or better a modeled radiation pattern chart? I don't feel like finding the Windows machine to model it in NEC right now.

jesse said:
Almost every airplane I've ever flown, one of the two radios, is better than the other. Pretty much every airplane I've seen with a 430/KX-155 combo is a good example of that. The KX-155 for one reason or another always has better range both RX and TX.
Click to expand...

I'm going to be sad when we eventually have to pull one of the Kings to make room for a Garmin GPS/Comm.

I think the Garmin Comm stuff underperforms on the RF side of things, and the audio sounds like typical modern audio paths designed by someone with a computer running L/C filter modeling software and who never ever looked at the resulting audio on a scope or just with a decent ear. Flat and no dynamics.

Older King radios were designed by analog engineers and it shows in the audio quality of the audio amplifier chain and filters.

If it weren't for the frequency sequencing, I'd be tempted to put the NON-Comm version of the Garmin in and keep the Kings. But maintenance shops that actually know how to work on the Kings and some of the parts are both slowly headed downhill too. :(
 
jesse said:
Almost every airplane I've ever flown, one of the two radios, is better than the other. Pretty much every airplane I've seen with a 430/KX-155 combo is a good example of that. The KX-155 for one reason or another always has better range both RX and TX.
[snip]
Click to expand...

One plane I used a lot in training had a 430 and a KX-155. I noticed the 155 would pull in VORs from significantly further out than the 430 as well.

John
 
I have a 430/155 stack, but the 155 is still TX INOP, so I wouldn't know which would have better TX range. Both seem to RX about the same on both the Nav and Com side for me.
 
jesse said:
Almost every airplane I've ever flown, one of the two radios, is better than the other. Pretty much every airplane I've seen with a 430/KX-155 combo is a good example of that. The KX-155 for one reason or another always has better range both RX and TX.

I'm not one to stage every frequency in all the radios. Whichever radio I like better is the radio I use for all communication. I will stage whatever the *next* frequency is in STBY if I suspect I'm changing to that frequency. If I'm enroute cruise then whichever radio works the best has the current controller in active, the last controller in STBY, and 121.5 being monitored on the other radio with the AWOS/ASOS for the destination as STBY.

I teach students basically the same thing. I do not teach them to stage 4 frequencies. I think that's just over complicated and confuses people when they need to switch which radio is transmitting. Plus a lot more to go wrong with volume knobs and whatnot.
Click to expand...

That is what I do too.

Additionally, I'm surprised it took as long as it did before someone said that one radio is typically better than the other. The only airplanes I've been fortunate enough to fly that the radios are equal in are nearly brand new. Anything with more than a few years on it probably has one good radio and one marginal one. Fly the plane long enough and you'll figure out which one is which when you either get a bunch of noise or a controller saying they can't hear you.
 
My Narco is a vastly better performer for COM than my 430W Garmin. Always has been.
 
mondtster said:
,,,Additionally, I'm surprised it took as long as it did before someone said that one radio is typically better than the other...
Click to expand...

I'm not sure whether it's typical, but the possibility was mentioned in post #4.
 
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