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Peter(at)sportingaero.com
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 Posted: Tue Jul 18, 2017 1:38 pm Post subject: Relative position of Transponder and DME antennas |
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I am retrofitting a panel mount DME into a low wing metal airplane and trying to figure out where the antenna should be installed.
There are several complicating factors, particularly that it is very difficult to route the antenna coax from the panel aft past the spar. The transponder antenna is already placed under the fuselage between the spar and the firewall under the passenger’s knees with one comm (comm 2) antenna in a similar position under the pilot’s knees. There is a position for the DMA antenna just ahead of the spar on the center-line, but this puts it around 15” from both of the other existing antennas. This busts the 36” between antennas rule of thumb – but so does the existing comm & txpdr antennas with no apparent ill effect. However, King recommend the DME antenna should be more than 3ft from a comm antenna and 6ft from a txpdr antenna. One method to install the antenna towards the rear of the aircraft could be to route the coax alongside the comm 1 coax (RG400) for 5ft, but that appears to be poor practice.
The DME is being installed to support IFR operation and will be used only infrequently, but when it is needed it will during an instrument approach and so reliable operation from (say) 10 or 15 miles is highly desirable. Any degradation in the current good comm or txpdr performance is undesirable, no matter what the phase of flight.
My questions are, - Will the installed performance of the DME be noticeably degraded if the antenna is mounted 15” away from an existing comm and txpdr antenna?
- Will the installed performance of the existing comm and txpdr be noticeably degraded if the DME antenna is mounted 15” away from both of their existing antennas?
- If the DME & comm RG400 feeders are run in close proximity for 5 feet will they mutually interfere with each other?
Thanks for any advice,
Peter
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donvansanten(at)gmail.com
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| Posted: Tue Jul 18, 2017 2:15 pm Post subject: Relative position of Transponder and DME antennas |
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Peter,In my RV all the antennas cables except
the GPS run together down the cold side of the firewall and back in the wire tunnel. I had to drill s couple holes in the spar carry through bit Van's daid this was no issue. All the cables are within an inch of each other. They are all RG400. The length of the runs where they ate together is roughly six feet.
Antennas are 2 wingtip (archer) 2 com 1transponder/adsb out 1 adsb in 1aprs
Two GPS, 1xm and the GPS for the aprs are under tbe cowl. All have worked perfectly for nine years now with the atchers working up to 50 miles if over 8000 feet altitude.
On Jul 18, 2017 2:43 PM, "Peter Pengilly" <Peter(at)sportingaero.com (Peter(at)sportingaero.com)> wrote: | Quote: |
I am retrofitting a panel mount DME into a low wing metal airplane and trying to figure out where the antenna should be installed.
There are several complicating factors, particularly that it is very difficult to route the antenna coax from the panel aft past the spar. The transponder antenna is already placed under the fuselage between the spar and the firewall under the passenger’s knees with one comm (comm 2) antenna in a similar position under the pilot’s knees. There is a position for the DMA antenna just ahead of the spar on the center-line, but this puts it around 15” from both of the other existing antennas. This busts the 36” between antennas rule of thumb – but so does the existing comm & txpdr antennas with no apparent ill effect. However, King recommend the DME antenna should be more than 3ft from a comm antenna and 6ft from a txpdr antenna. One method to install the antenna towards the rear of the aircraft could be to route the coax alongside the comm 1 coax (RG400) for 5ft, but that appears to be poor practice.
The DME is being installed to support IFR operation and will be used only infrequently, but when it is needed it will during an instrument approach and so reliable operation from (say) 10 or 15 miles is highly desirable. Any degradation in the current good comm or txpdr performance is undesirable, no matter what the phase of flight.
My questions are, - Will the installed performance of the DME be noticeably degraded if the antenna is mounted 15” away from an existing comm and txpdr antenna?
- Will the installed performance of the existing comm and txpdr be noticeably degraded if the DME antenna is mounted 15” away from both of their existing antennas?
- If the DME & comm RG400 feeders are run in close proximity for 5 feet will they mutually interfere with each other?
Thanks for any advice,
Peter
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