Using a single feeder cable for both a 20-meter and a 40-meter inverted V antenna can introduce certain challenges

Bharatiya Amateur Radio

Here’s an analysis of the setup and some advice for optimizing performance:

Challenges with a Single-Feeder Cable

Impedance Matching:

Each antenna has its optimal impedance at its resonant frequency (20m or 40m band).
When connected to a single feeder cable, impedance mismatches can occur when transmitting or receiving outside the resonant frequency of either antenna, leading to power loss and reduced efficiency.

Coupling Between Antennas:

Since both antennas are connected to the same feeder, mutual coupling might cause interference. This can distort the radiation pattern and reduce efficiency.

SWR (Standing Wave Ratio):

A single feeder cable might result in a higher SWR on some frequencies, which can strain your transmitter and reduce power output.

Impact on Efficiency

Transmitting Efficiency:

Efficiency is highest when the antenna is resonant. If either antenna is not matched properly due to the shared feeder, transmitting power will be lost as heat in the tuner or mismatched feeder.

Receiving Efficiency:

While receiving, mismatched impedance can lead to signal loss or reduced sensitivity, particularly for weak signals.

Advice for Improvement

Use a Proper Balun:

Install a 1:1 or 4:1 balun at the feed point to improve impedance matching and reduce feedline radiation.

Antenna Switch or Trap Design:

Consider using an antenna switch or designing a trapped antenna system to select the appropriate antenna for the desired frequency band.

Separate Feedlines:

If possible, use separate feeder cables for each antenna. This avoids impedance and coupling issues.

Antenna Tuner:

Employ a good-quality wide-range antenna tuner at the transmitter end to handle mismatches and optimize performance.

Test SWR and Radiation Patterns:

Use an SWR meter or antenna analyzer to measure the SWR on both bands. If issues are observed, adjust the antenna lengths or feeder setup.

Consider Fan Dipole:

Replace the dual inverted V with a fan dipole design. This allows each band to have its resonant dipole element, with a single feeder cable, while minimizing interaction.

Conclusion

While a single feeder cable can work with compromises, separating the feedlines or using traps is generally better for maintaining high transmitting and receiving efficiency. Testing and fine-tuning will help you achieve optimal performance. If you have an SWR analyzer, use it to determine the actual efficiency of your current setup.