COM Transceiver problem and SWR test

Question

I have a VHF COM problem. I have a new composite structure aircraft (a Berkut) with an older VHF transceiver, (a Bendix King KX-155 TSO), that has significantly reduced range in certain situations. The VHF antenna is in one of the two winglets, and, obviously, does not have ground plane attached to it. The symptoms are somewhat bizarre.

I am based at Prescott's Love field, and most of the time we depart on runway 21, where the approach end of the runway is at the north end of the field, which is only about 6,000 feet from the control tower. When I taxi out to the quick check area prior to takeoff, the radio works fine both transmit and receive, until I am about 4-5000 feet away from the tower. After that, for a few feet further away, I can hear the tower, but they can no longer hear me. When I get to the end of the taxiway (run-up area) I can no longer hear the tower at all. When I taxi back (closer to the tower), both transmitter and receiver begin working again.

Once I am airborne, the transmitter and receiver both work satisfactorily within 20-25 nm from the field but start to degrade as I move further away. On cross-country flights, where I attempt to maintain communications with ARTCC (for flight following), the transceiver routinely fails at longer ranges before I have reached the transition point where the current sector controller would normally transfer me to the next controller.

I went to the local Avionics shop to get some expert help. The tech there performed an SWR test on my aircraft, and a bench test on the radio. Both were normal. They happened to have an identical model KX-155 they had removed from another aircraft sitting on a shelf, that they let me borrow. I replaced my unit with theirs and experienced virtually identical symptoms. That does not of course indicate that the problem is NOT in the unit itself. (both units could have similar faults in them), but it is not likely.

The normal SWR test raises my first question. Does the SWR test only send the RF pulse down the core of the coaxial cable? i.e., if there was a break, or crimp or other failure in the shielding, would the RF reflection from the SWR test also show an issue from that? and secondly, (because another avionics tech suggested this to me) can a failure/degraded, crimped or corroded) shielding on the RG-58 coax cable cause an issue like this? And if so, is there another test I can perform on the aircraft to definitively diagnose this issue?

I need to diagnose this as I am planning on a complete avionics suite upgrade (to the Dynon HDX suite), including their HF transceiver and COM control panel, and I don't want to install that complete system if the issue is actually in the coaxial cables or the antenna until I have fixed this.

Answer 1

First, let me try to clarify some of the theory and terminology.

The coax cable is an unbalanced system. That is (in theory) all the energy is carried on the conductor and nothing on the shield. With the standard antenna, the conductor is connected to the antenna element and the shield to the ground plane. Without the shield, the energy can be radiated outward with the ground plane shaping the pattern.

The length of the antenna element is typically a 1/4 wavelength of the frequency in the center of the operating frequency band. The 1/4 wavelength will maximize the radiated energy. If the antenna was "perfect" and you were on the design frequency, the VSWR would be 1:1. At the top and bottom of the band, VSWR is typically 1.2:1 or 1.3:1. Any flaw in the cable or the antenna will cause the VSWR to increase. It's a measure of reflected energy which occurs when there is an impedance mismatch. A flawed ground plane won't have a material impact on VSWR unless it is close enough to the antenna element to act as a partial shield.

But I don't know what kind of antenna you have. Many composite aircraft use a dipole antenna embedded under the skin to compensate for the lack of a ground plane. The dipole antenna looks like the "T"-shaped antenna that sometimes comes with FM radio receivers. The difference is in the aircraft with vertically polarized AM signals, it is turned sideways so the top of the "T" is vertical.

The dipole doesn't need a ground plane as it is a balanced system. In a balanced system, the energy is balanced across both conductors (no shield.) With a dipole antenna, you have to convert the unbalanced signal on the coax to a balanced signal before the antenna. This is done with a balun (a BALunced to UNbalanced converter.)

If you have a dipole antenna, the VSWR will be a good measure up to the balun. If there's an issue with the dipole past the balun, it may not be detected by the VSWR test. The best way to detect an issue is examination of the antenna (which is often difficult when it's bonded to the skin.)

One option that may or may not help is to test with an FFT based cable analyzer. With them, you can see any mismatches and they can tell you how far down the cable the issue is located.

Your other choice is to replace the antenna if practical.

Answer 2

It is common for an unbalanced monopole antenna on a coax feed line without a ground plane to exhibit weird radiation patterns & behavior like this. Walkie-talkies use monopoles but their feedlines are vanishingly short so they escape these effects. In a metal airplane, the coax shield is connected to the fuselage skin to form a ground plane or counterpoise and the antenna behavior is thereby tamed and made predictable.

The usual way to furnish a fuselage-mounted monopole with a counterpoise in a nonmetallic plane is to glue a square of copper sheeting to the inside of the skin and mount the antenna in the center of the square, with the coax shield connected to the copper.

You can also glue long strips of copper tape in a radial pattern extending outward from the antenna mount to accomplish the same effect.

In either case, the dimensions of the copper sheet or the lengths of the copper radials need to be adjusted to get the best electrical match between the feed line and the antenna, which will suppress radiation from the feedline shield. This is called pruning and shortwave enthusiasts who make their own monopole antennas have to do this.

Suppressing radiation from the feedline is important because it is accompanied by high voltages on the shield and significant amounts of radiation inside the fuselage. you can get bad shocks and RF burns if you touch the shield, and the radiated energy will seriously disrupt other electronic devices in the cockpit. Baluns are commonly used by ham radio guys to block "RF in the shack" but their design and use is something of an art.