Hello Walt,
I haven't tested an ELT antenna for broadband Com performance but I have tested a simple wire type antenna in the condition you mentioned.
Some may see this as going beyond Walt's question but the data points out the importance of a quality antenna system and how it contributes to solid, reliable communication.
All data was gathered with Anritsu lab grade equipment calibrated with traceable standards.
Test one:
This antenna is a simple bent wire type antenna designed to be mounted on the underside of the aircraft. This antenna is available from several suppliers priced in the range of $40 to $80.
The data plot indicates the antenna was cut for the band center of 127 MHZ and provides its best performance at that frequency, moving away from 127 MHZ the losses increase and performance falls off.
The VSWR is less than 3.237 to 1 over the entire com frequency.
Test Two
This VHF com antenna has a built in impedance matching network and is designed to provide broad frequency response over the entire 118 to 136 MHZ com frequency.
The data plot indicates a VSWR less than 1.56 to 1 over the entire com band.
Comparing the two data plots:
Test one indicates the antenna impedance varies from 15.44Ω to 161Ω
Test two indicates the antenna impedance varies from 32.05Ω to 78Ω
Modern Com transceivers are designed to produce optimum performance with a 50Ω load. As the impedance increases or decreases from 50Ω the communication performance will be less than optimum.
Since the receiver and the transmitter are using the same antenna, the receiver will be much more tolerant to impedance changes and will normally offer reasonable performance over the entire Com band with a simple wire antenna even with the extreme impedance range.
Things become more complicated from the transmitter side, the radio designers must deal with many factors including heat producing devices in a very limited space. Typical efficiency of the transmitter power amplifier is in the range of 60%, in other words to produce 6 watts of output requires starting with 10 watts, the other 4 watts is dissipated as heat.
For example many VHF Com radios are rated at 6 watts output with a 50Ω load.
Ohm's Law states to produce 6 watts of energy in a 50Ω load requires 17.32 volts at .346 amps. From a radio designers point of view, many build in devices to limit the power amplifier current and voltage slightly higher to insure it meets the rated spec's and call it a day.
This performs well till the antenna impedance changes. Using the numbers above, assume the load increases to 160Ω, with the voltage limited, the current drops to .108 amps and the unit produces 1.87 watts of output with good portion of that being reflected back (because of the high VSWR) to the power amplifier which again has to be dissipated as additional heat.
Because a VHF Com radio uses amplitude modulation (AM), the power amplifier of the radio must operate in the linear portion of it's envelope to a provide a clean distortion free, modulated RF wave. Changing the load to extremes above has the potential to move the power amplifiers envelope from a linear to a non linear portion of operation. This would come across the Com frequency as garbled, hard to understand transmission and may be heard on several other frequencies also.
Several times a month I receive questions like this: I can hear ATC fine in all portions of the Com band but ATC can not hear me at the lower and upper portions of the band, in all cases they are using a simple wire antenna.
A simple wire antenna has it's place if you use a narrow portion of the band and the antenna is cut for the frequency.
