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MisfitSELF

DME vs Transponder Antenna Locations

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I own a 1984 M20J that has mostly stock avionics and two blade antennas on the  lower section just aft of the cowling.  They are right behind the exhaust on the right side and the engine drain tubes on the left.  I'm 99% sure that one is for the transponder/ADS-B and the other is for the DME.

Can anybody tell me which antennas are which?

Pardon if this has been covered already.  A search of of "antenna", "DME", and "locations" didn't turn up what I was looking for.

Thanks,

Bruce

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In short, not by visual examination.  The two antennas operate in the same frequency band.

An avionics shop with a ramp test set can probably sniff out which is the transponder antenna. 

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Hi, these are identical antennae. You will likely need to trace the coax wiring to see which one is which.

You mechanic may have tested the transponder and may know which one is which from that testing.

 

 

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Yeah, I can ask my Avionics guys (who installed the ADS-B), but I'm afraid he may have just unplugged the old coax and plugged it into the new unit.  I tried tracing the coax from below and above with the dash cover off, and it's such a rat's nest of wires, that I can't positively tell which is which.  I was hoping that there was a "stock answer" since one of the antennas goes to my old KNS-80.

Bruce

Edited by MisfitSELF

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And yes, I know they are identical antennae.  DM NI 70-2 with a band of ~950 to 1220Mhz.

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Mine are both mounted on the pilot side for some reason. One immediately behind the cowl, the other just behind the wing.

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Or, you could lower the belly panel, disconnect one of the cables from its antenna.  

Then transponder testing via a specific code in conjunction with your control tower folks.

Just a thought.:mellow: 

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Just now, MooneyMitch said:

Or, you could lower the belly panel, disconnect one of the cables from its antenna.  

Then transponder testing via a specific code in conjunction with your control tower folks.

Just a thought.:mellow: 

Just an FYI, but never test a transmitter without a load (antenna) attached.   It can damage the Power Amplifier in the transmitter.

 

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15 minutes ago, EricJ said:

Just an FYI, but never test a transmitter without a load (antenna) attached.   It can damage the Power Amplifier in the transmitter.

 

Thank you and possibly so.  With that, I’ve accidentally left coax disconnected after installation of panel, flown, discovered inop., only to discover my error.  Connected and no damage other than ego :lol:

Maybe I was lucky, eh? :P

Whoops, I just remembered an occasion when xponder was not being rec’d by ATC.  Eventually discovered coax wire broken at BNC.  Repaired and life was good again.   Still lucky. 

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2 hours ago, MooneyMitch said:

Thank you and possibly so.  With that, I’ve accidentally left coax disconnected after installation of panel, flown, discovered inop., only to discover my error.  Connected and no damage other than ego :lol:

Maybe I was lucky, eh? :P

Whoops, I just remembered an occasion when xponder was not being rec’d by ATC.  Eventually discovered coax wire broken at BNC.  Repaired and life was good again.   Still lucky. 

The older transponders can survive not being connected to an antenna, but I’m told that the newer all digital ones cannot. When I had my old KT76A replaced with a GTX 345 for ADS-B the avionics shop warned me about that and the tech even put a warning tag on the coax at the antenna end stating not to power up the transponder with the antenna disconnected . 

On my 1994 M20J, the DME antenna is on the pilot’s side and the identical transponder antenna is on the copilot’s side. I don’t know if this was standard at the factory or not. 
 

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19 minutes ago, PT20J said:

The older transponders can survive not being connected to an antenna, but I’m told that the newer all digital ones cannot. When I had my old KT76A replaced with a GTX 345 for ADS-B the avionics shop warned me about that and the tech even put a warning tag on the coax at the antenna end stating not to power up the transponder with the antenna disconnected . 

On my 1994 M20J, the DME antenna is on the pilot’s side and the identical transponder antenna is on the copilot’s side. I don’t know if this was standard at the factory or not. 
 

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Thank you.  Yep, I've got an oldie indeed.  Still works for me. 

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I would just tone it out with a toner.

 

On another point, my antenna aft of the exhaust got melted. I had no DME so there was a blank on the pilot side. I moved the transponder antenna to that side to get out of the exhaust flow.

 

https://www.amazon.com/ELEGIANT-Multifunction-Collation-Telephone-Continuity/dp/B01HCQSHNG/ref=sr_1_3?dchild=1&keywords=Cable+toner&qid=1586134953&smid=A1RS0JMDI1W43Y&sr=8-3

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See..................we love MS and MSer's.  It is all things Mooney and beyond.  Lots and lots of great information, ideas and fun!!  :wub:

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