Two questions related to VHF radio performance came in for this month’s Ask a Tech feature, and both problems could yield similar solutions. The new owner of a Citabria brought the airplane to his mechanic to have a new Trig Avionics standalone comm radio installed. The other with an older Cherokee had his shop install a new Garmin radio.
Unfortunately, both owners are having issues receiving anything unless they’re close to the station, and the Citabria owner says controllers complain of weak and garbled transmissions. These aren’t the problems you’d expect after writing a $5000 check for installation.
But what many owners don’t realize is that the radio is only part of the equation when it comes to solid performance. The antenna system, which includes the actual antenna, coaxial signal cable, RF connectors and the ground plane on which they’re mounted, all play a significant role in VHF radio performance. For fabric aircraft, it’s often a dilemma.
Follow the manual
That’s a starting point, where manufacturers generally have specific requirements for wiring and the physical installation of the antenna system.
If there isn’t a sufficient ground plane, performance will likely suffer, even for non-transmitting antennas like the ones used for GPS navigators and ADS-B position sources. Most install manuals suggest the conductive ground plane be a minimum diameter of 8 inches. While that sounds like plenty, you might not have the area where you want to mount the antenna. For fabric aircraft, technicians often fabricate a ground plane using heavy foil tape or other metallic surfaces for a solid bonding of the antenna. Since there might be limited structural space available for mounting antennas on fabric aircraft, the installation of some systems just might not be possible. Composite aircraft present similar challenges, plus you’ll need to find a shop that’s versed in working with the composite structure.
There’s also coaxial antenna cabling that might be problematic, and in aircraft that haven’t seen radio upgrades in a long time, total replacement of the old RG58 cabling with low-loss shielded cabling is in order. In many cases, the cabling that attaches the antenna to the radio could be as old as the aircraft and is subject to deterioration and wear from years of vibration, heat and cold. Consider too, that old coaxial cable is likely non-shielded RG58 (or older) and could be the source of interfering noise that’s worked its way into your radios and audio system.
Sometimes even the best cable and routing practices will result in performance issues, especially on small airframes with antennas in close proximity. One common issue is GPS receiver blocking from the VHF comm, especially with older analog radios. But there are remedies. If there is interference that jams the GPS signal when you transmit, you can install a 1.57452 GHz notch filter on the coax cable as close to the back of the comm radio as possible.
Last, when investing in a new radio, plan to go all in. Even though the teardown effort to access the cable (removing the entire interior, including headliner in some cases) can mean days of shop labor, it’s a task worth doing. Shortcuts are a setup for grief and money spent on troubleshooting.
