Radio COM Bleed Over / Leakage

[hello55]

Hello All,

Have a continuous VHF COM1 COM2 radio bleed over/transmission leakage when the Transmit frequency and receiving frequency separation is kept 0.400 or less.

For example Tx COM1 (121.900) and Rx COM2 121.500 there is transmission leakage over receiving end. In order to diagnose the fault if it is MMDR (KTR-2280) , Antenna or wiring following tests are carried out with dummy load (with antennas' not removed) and with actual antenna configuration.

The VHF antennas installed are on top fwd and aft of fuselage with a separation of 17 feet.

The results indicate following ;

It eliminates the audio panels interference radiated between the MMDRs, coax routing, etc.The only thing left is that the transmitting radio is radiating enough RF through it's antenna
to be received by the non-transmitting radio's antenna. Though the antenna placement is more than the recommended separation ( i.e > 8ft).
But, if we put a dummy load in place of the antenna, the problem disappears.
I am wondering how the 0.400 or less separation between two COMs is behaving or resulting in continuous noise/bleed over.

Any thoughts from the forum for this




TESTS:

Frequencies used (.400 MHz apart)
Comm #1 118.100 122.100 126.000 134.000
Comm #2 118.500 122.500 126.400 134.400
Comm #2 118.100 122.100 126.000 134.000
Comm #1 118.500 122.500 126.400 134.400
TX on Comm #1, bleed on Comm #2
TX on Comm #2, bleed on Comm #1
RESULT: Using the aircraft antenna’s, we experienced bleed in all frequencies. Tried all frequencies listed on both Comm #1 and Comm #2 and bleed was present on the receiving end.

Dummy load installed in antenna #1
Frequencies used (.400 MHz apart)
Comm #1 118.100 122.100 126.000 134.000
Comm #2 118.500 122.500 126.400 134.400
Comm #2 118.100 122.100 126.000 134.000
Comm #1 118.500 122.500 126.400 134.400
RESULT : With the 50-ohm load, it did not matter which frequency was used, there is no bleed at all.

Dummy load installed in antenna #2
Frequencies used (.400 MHz apart)
Comm #1 118.100 122.100 126.000 134.000
Comm #2 118.500 122.500 126.400 134.400
Comm #2 118.100 122.100 126.000 134.000
Comm #1 118.500 122.500 126.400 134.400
RESULT : With the 50 ohm load it did not matter which frequency was used, there is no bleed at all.

[snips]

Have a look at the specs for the radios and do the link loss sums.
10 watts output power =10dB=40dBm
Rx sensitivity approx -110dBm
Difference is 150dBm

Co channel/adjacent channel rejection 60dB
So you're looking at achieving 90dB of isolation between the antennae

The figures are an educated guess but basically you're driving the receiver's front end into overload and causing the break through

[Zaphod Beblebrox]

Is there a possible chance of significant antenna base plate corrosion? If there is significant corrosion, the antenna can float "off ground" and it's operating impedance is no longer 50Ohm but some higher value. This could cause significant SRW on one or both antennas.

A lot of SRW can cause interference.

Just a thought.

[tpng conehead]

Sounds like antenna bonding to me. Have you done a VSWR check of the antenna's from the T/R.
Might sound silly, but are the antenna attaching screws tight, and antenna is bonding to the airframe. I know some guys use the mounting screws only for the bonding. I was taught a clean ( no paint etc ) under the antanna base to airframe.
Dayton Granger make a two pack antenna sealant with aluminium balls to act as the conductive path for bonding. his stuff is the best i have worked with.

[FlightSpanner]

Don't have access to the KTR manuals so cannot check, Do they have a Simulcom function? attenuates the receiving Comm when the other VHF comm transmits, to stop this issue.