Antenna questions

[shawnd]

Got a couple of antenna questions I am hoping folks here can help answer:

1. Anyone know the model number for the stock rod and ball type transponder antenna that typically sits in the belly? Modern equivalent is a Comant CI-101 but not sure. The stock ones are wider and have a Type N connector I believe but would love to get the spec sheet for it.

2. What I thought was a COM antenna turns out to be a new one for me: DM C 63-3/A which operates in the 138 - 174 MHz (above all normal aviation comm frequencies). Any ideas on what this was used for?

My airplane is a 1986 M20K 252 TSE. @M20Doc maybe you know it off hand?

Thanks!
Shawn

[1980Mooney]

3 hours ago, shawnd said:

Got a couple of antenna questions I am hoping folks here can help answer:

1. Anyone know the model number for the stock rod and ball type transponder antenna that typically sits in the belly? Modern equivalent is a Comant CI-101 but not sure. The stock ones are wider and have a Type N connector I believe but would love to get the spec sheet for it.

2. What I thought was a COM antenna turns out to be a new one for me: DM C 63-3/A which operates in the 138 - 174 MHz (above all normal aviation comm frequencies). Any ideas on what this was used for?

My airplane is a 1986 M20K 252 TSE. @M20Doc maybe you know it off hand?

Thanks!
Shawn

it is a COM antenna.
 

“The DM C63 series antennas are VHF communication antennas designed for high mechanical strength with machine tapered aluminum alloy radiating elements. These vertically polarized antennas cover the frequency range of 118-137 or 138-174 MHz for both transmitting and receiving applications.

The DM C63-1/A and DM C63-4/A are designed for mounting on top of the fuselage. The DM C63-2 and DM C63-3/A are low profile "bentback" radiating element designs for mounting on the bottom of the fuselage. They are well suited for helicopter installations.”

https://www.dallasavionics.com/cgi-bin/products.cgi?master=avionics&category=antennas&man=edo&url=dmc63-3a.html

Edited May 11, 2022 by 1980Mooney

[EricJ]

7 hours ago, shawnd said:

Got a couple of antenna questions I am hoping folks here can help answer:

1. Anyone know the model number for the stock rod and ball type transponder antenna that typically sits in the belly? Modern equivalent is a Comant CI-101 but not sure. The stock ones are wider and have a Type N connector I believe but would love to get the spec sheet for it.

2. What I thought was a COM antenna turns out to be a new one for me: DM C 63-3/A which operates in the 138 - 174 MHz (above all normal aviation comm frequencies). Any ideas on what this was used for?

My airplane is a 1986 M20K 252 TSE. @M20Doc maybe you know it off hand?

Thanks!
Shawn

Just search some place like aircraftspruce.com for "transponder antenna" and you'll get a bunch of options, from the TED unit for $23, a RAMI AV-22 for about $85 that claims a TSO, or a RAMI AV-11 that doesn't mention a TSO for $30, etc., etc.   Most of them have BNC connectors and should work fine.

[shawnd]

5 hours ago, 1980Mooney said:

it is a COM antenna.
 

“The DM C63 series antennas are VHF communication antennas designed for high mechanical strength with machine tapered aluminum alloy radiating elements. These vertically polarized antennas cover the frequency range of 118-137 or 138-174 MHz for both transmitting and receiving applications.

The DM C63-1/A and DM C63-4/A are designed for mounting on top of the fuselage. The DM C63-2 and DM C63-3/A are low profile "bentback" radiating element designs for mounting on the bottom of the fuselage. They are well suited for helicopter installations.”

https://www.dallasavionics.com/cgi-bin/products.cgi?master=avionics&category=antennas&man=edo&url=dmc63-3a.html

@1980Mooney Dallas Avionics mentions the "cover the frequency range of 118-137 or 138-174 MHz for both transmitting and receiving applications.". That would make sense, however, Aircraft Spruce's description only cites: "These vertically polarized antennas cover the frequency range of 138-174 MHz for both transmitting and receiving applications. " and a follow-up questions mentions:

Q: The DM C63-2 is listed in the alternate items tab. Does that mean this they are capable of the same frequencies?

The DM C63-3/A Antenna has a Frequency Range 138-174 MHz while the DM C63-2 has a Frequency Range of 118-137 MHz.

https://www.aircraftspruce.com/catalog/avpages/c63-3_antenna.php?clickkey=535835

I also found a PDF that agrees with Aircraft Spruce:
https://www.pilotshop.com/catalog/pdf/11-02765specs.pdf

[shawnd]

1 hour ago, EricJ said:

Just search some place like aircraftspruce.com for "transponder antenna" and you'll get a bunch of options, from the TED unit for $23, a RAMI AV-22 for about $85 that claims a TSO, or a RAMI AV-11 that doesn't mention a TSO for $30, etc., etc.   Most of them have BNC connectors and should work fine.

Yep did - hence got the CI-101 :-) Trying to figure out the model number to verify specs given the other antennas on this bird.

[1980Mooney]

1 hour ago, shawnd said:

@1980Mooney Dallas Avionics mentions the "cover the frequency range of 118-137 or 138-174 MHz for both transmitting and receiving applications.". That would make sense, however, Aircraft Spruce's description only cites: "These vertically polarized antennas cover the frequency range of 138-174 MHz for both transmitting and receiving applications. " and a follow-up questions mentions:

Q: The DM C63-2 is listed in the alternate items tab. Does that mean this they are capable of the same frequencies?

The DM C63-3/A Antenna has a Frequency Range 138-174 MHz while the DM C63-2 has a Frequency Range of 118-137 MHz.

https://www.aircraftspruce.com/catalog/avpages/c63-3_antenna.php?clickkey=535835

I also found a PDF that agrees with Aircraft Spruce:
https://www.pilotshop.com/catalog/pdf/11-02765specs.pdf

This for your VHF COM right?… I think you need DM C63-2 for the belly application.  I have three COMs and the third antenna is on the belly.  I think it is a DM C63-2  

Edited May 12, 2022 by 1980Mooney

[Will.iam]

Got a pic of the belly antenna?

[shawnd]

1 hour ago, Will.iam said:

Got a pic of the belly antenna?

Indeed. It definitely is a C63-3/A.

 

[EricJ]

12 hours ago, shawnd said:

Yep did - hence got the CI-101 :-) Trying to figure out the model number to verify specs given the other antennas on this bird.

Are you worried about the hole size and connector being different?         

That's pretty curious about the DM C63-3 being spec'ed actually outside of the VHF air comm bands.    That said, antenna frequency responses are very broad and don't drop off very quickly, so even if spec'ed for 138 MHz and upward, it would be expected (obviously) to work at adjacent lower frequencies, just with less gain.   

[shawnd]

2 hours ago, EricJ said:

Are you worried about the hole size and connector being different?        

Wanted to make sure CI-101 is the right one in terms of frequency range etc. There are no markings on the existing antenna at all

 

2 hours ago, EricJ said:

That's pretty curious about the DM C63-3 being spec'ed actually outside of the VHF air comm bands.    That said, antenna frequency responses are very broad and don't drop off very quickly, so even if spec'ed for 138 MHz and upward, it would be expected (obviously) to work at adjacent lower frequencies, just with less gain.   

Wondering what it's used for - I have two RAMI AVT-4 style antennas on the top so I believe both my COM1 and COM2 are on the roof. I do have ADF and WX but its unlikely its for either of those. Tracing the line is going to be challenging so asking first

 

[EricJ]

5 hours ago, shawnd said:

Wanted to make sure CI-101 is the right one in terms of frequency range etc. There are no markings on the existing antenna at all

Any of the "transponder antennas" from aircraft spruce or generally elsewhere (for aviation apps) will be the right frequency.    It's a very small chunk of spectrum, so they're all tuned for it.

5 hours ago, shawnd said:

 

Wondering what it's used for - I have two RAMI AVT-4 style antennas on the top so I believe both my COM1 and COM2 are on the roof. I do have ADF and WX but its unlikely its for either of those. Tracing the line is going to be challenging so asking first

 

The band specified for the DM C63-3 is not really an aviation band, but it's probably a decent generic VHF antenna and seems to have been used for bottom-mount applications, probably because it is short relative to the mounting surface.   It is possible that it is no longer connected to anything or maybe somebody put it on a splitter with one of your other VHF comm antennas just to get better coverage below the aircraft.    It may also have been a floater with a loose cabe connection in the cabin somewhere to connect a handheld in case you need it.   

[Pinecone]

15 hours ago, EricJ said:

so even if spec'ed for 138 MHz and upward, it would be expected (obviously) to work at adjacent lower frequencies, just with less gain.   

To be a bit pedantic, gain is not affected by matching issues.  And total radiated power is minimally different.  As long as the transmitter has some matching hardware to handle the reflected power.  

And matching does not affect reception.

Advanced class ham operator here.

 

[shawnd]

8 hours ago, EricJ said:

   It may also have been a floater with a loose cabe connection in the cabin somewhere to connect a handheld in case you need it.   

Thanks Eric - yeah need to go dig around behind and under the panel to see if there are any open ends and then do some continuity testing.

[EricJ]

7 hours ago, Pinecone said:

To be a bit pedantic, gain is not affected by matching issues.  And total radiated power is minimally different.  As long as the transmitter has some matching hardware to handle the reflected power.  

And matching does not affect reception.

Advanced class ham operator here.

 

It's not an impedance matching issue it's just the frequency response of the antenna.    If it is tuned (dimensionally) for 138-174 MHz it'll have less gain at frequencies outside that range as the response tapers off.

35 year career wireless R&D engineer here.   

Why this particular antenna is indicated to be tuned for that range and TSO'ed for VHF aviation comm use is unknown to me.   I suspect it's just that it's a little shorter so that it fits underneath for belly mount.

[Pinecone]

22 hours ago, EricJ said:

It's not an impedance matching issue it's just the frequency response of the antenna.    If it is tuned (dimensionally) for 138-174 MHz it'll have less gain at frequencies outside that range as the response tapers off.

35 year career wireless R&D engineer here.   

Why this particular antenna is indicated to be tuned for that range and TSO'ed for VHF aviation comm use is unknown to me.   I suspect it's just that it's a little shorter so that it fits underneath for belly mount.

Tuning dimensionally is also impedance matching.   How do you tune an antenna dimensionally?  Normally, but checking SWR.

 

[EricJ]

1 hour ago, Pinecone said:

Tuning dimensionally is also impedance matching.   How do you tune an antenna dimensionally?  Normally, but checking SWR.

 

They're related but they're two separate things.   Antenna dimensional tuning is to match wavelength, impedance matching is to match the transmission line impedance.   Often there's a transformer inside the antenna to cover big disparities between the natural impedance of the antenna and the transmission line.   Tuning SWR just aligns the particular impedance differences for that particular unit at the test frequency.

In this case tuning the antenna for a slightly higher chunk of the VHF spectrum provides a shorter antenna (since the wavelength is shorter), so it fits under the airplane a little easier without dragging on the ground.    At least, that's the only reason I can think of for doing that and using it in the comm band.

[Pinecone]

I am not familiar with antennas with their own matching network.   In that case, if you can tune the matching network for proper SWR for the radio final stage, the antenna length is immaterial.

 

[EricJ]

12 minutes ago, Pinecone said:

I am not familiar with antennas with their own matching network.   In that case, if you can tune the matching network for proper SWR for the radio final stage, the antenna length is immaterial.

 

The antenna length affects its resonance which determines its frequency response.

The impedance of air (free space) is about 377 Ohms, the antenna physics matches that.    Transmission lines (cables) these days are 50 or 75 Ohms or whatever.   The mismatch between the antenna-to-free-space impedance of 377 Ohms, the antenna physics, and the transmission line has to be handled between the antenna and the transmission line for best efficiency.   Usually it is a transformer inside or at the antenna.   

Those are the basics, but there's all kinds of tricks and hijinks that are often done.   This is a pretty simple case, though.

[PT20J]

That matching network is what's in those shacks you see at the bottom of AM radio towers. My first summer job after getting my 1st Class Radiotelephone (now General) license in high school was baby sitting a 50kW AM station transmitter and logging the base current readings every half hour. Really boring, but it paid well and I spent most of the time in the studio recording music from the library to play in my car.

Skip

[larryb]

I'm thinking the DM C63-3/A Antenna, 138-174 MHz, is intended for land-mobile application in aircraft. Think of a police helicopter talking to dispatch and ground units. It is not intended for aircraft communications. It will work, but not as good as the proper antenna. Either the airplane had a land-mobile radio in the past or this was simply installed by mistake. If it were me I'd replace it with the proper air-band antenna. 
 

[EricJ]

23 minutes ago, larryb said:

I'm thinking the DM C63-3/A Antenna, 138-174 MHz, is intended for land-mobile application in aircraft. Think of a police helicopter talking to dispatch and ground units. It is not intended for aircraft communications. It will work, but not as good as the proper antenna. Either the airplane had a land-mobile radio in the past or this was simply installed by mistake. If it were me I'd replace it with the proper air-band antenna. 
 

It is TSO'd (C37d and C38d) as VHF comm radio equipment "within the radio frequency range 117.975 to 138.000 Megahertz".   Aircraft spruce sells it as a VHF air band comm antenna: "The DM C63-3/A is a low profile "bentback" radiating element designed for mounting on the bottom of the fuselage."
 

Those TSOs don't have any performance requirements, just documentation requirements.   A lot of TSOs are like that in that they have no performance requirements.

https://www.aircraftspruce.com/catalog/avpages/c63-3_antenna.php

 

Edited May 16, 2022 by EricJ

[larryb]

TSO or not, the DM C63-3/A Antenna is the wrong antenna for the application here, which is air-band communications. It is the correct antenna for land-mobile communications. If you want to install your 2m ham radio in your plane, this would be the correct antenna.

 

If you check the manufacturer datasheet linked in that spruce posting above, you'll see the TSO only applies to the aviation band versions. So the text in the Spruce product description that says the TSO applies to the -3/A and -4/A versions appears incorrect.

 

 

 

https://www.aircraftspruce.com/catalog/avpages/c63_2.php

[EricJ]

26 minutes ago, larryb said:

TSO or not, the DM C63-3/A Antenna is the wrong antenna for the application here, which is air-band communications. It is the correct antenna for land-mobile communications. If you want to install your 2m ham radio in your plane, this would be the correct antenna.

 

If you check the manufacturer datasheet linked in that spruce posting above, you'll see the TSO only applies to the aviation band versions. So the text in the Spruce product description that says the TSO applies to the -3/A and -4/A versions appears incorrect.

 

 

 

https://www.aircraftspruce.com/catalog/avpages/c63_2.php

Yes, the AS description for the unit isn't accurate, sadly.    Regardless, it's sold and used for aircraft as a VHF comm antenna, presumably for belly mounting (as described) since it is shorter due to the shorter wavelengths in the higher band.    It'll still work in the comm band, just the gain will be a little lower.   The belly position will likely more than make up for the lower gain in wing shadowing, and I suspect some people may have connected an upper antenna and a belly antenna with a splitter for more consistent coverage if they were experiencing shadowing.   It's not a "wrong" antenna to use for VHF air-band comm, it's just a different tradeoff selection.

[larryb]

The 138 Mhz antenna is basically the same height as the 118mhz one. The difference in size is the length of the whip after the bend, about 4.4" shorter. I don't see the shorter horizontal section of whip being an advantage in any aircraft belly mounting situation.

FWIW I have the CI-122 bent whip on the belly of my Mooney. The shop had to move one of my comm antennas off of the roof to make enough room for the 2 Aspen RSM's and the garmin GPS anntenna and meet all of the separation rules.

Spruce sells a number of antenna's that are not used in standard aircraft communications, such as these. At least in the AV-14 the text tells you it's not intended for the aviation comm band. It is up to the user to buy the proper antenna for their application. 

https://www.aircraftspruce.com/catalog/avpages/av14.php
The AV-14 is similar to the AV-10 using the same 4-bolt mounting base except it has a shorter radiating whip element. The antenna is designed to operate in the Public Service and Business Bands 138-174 MHz.

https://www.aircraftspruce.com/pages/av/antenna_uhf/comantuhf200.php
UHF 450 - 470 Mhz 

 

 

[EricJ]

Yup.    Either will likely work in either band, just a loss of gain outside of the tuned region.    No telling how it wound up on the OP's airplane without some sort of documentation or something.