Multiband Antennas

While it is possible to use one antenna for multiple frequencies, there are some considerations in obtaining the best results.

An 88 foot inverted-vee dipole is probably the most efficient of all multiband antennas when driven by open wire line and matched with a balanced antenna tuner. The 88 foot dimension is used to prevent multiple lobes of radiation on the higher frequencies.

To call the matching device an antenna tuner may be a misnomer. It is actually a device to match the output impedance of the transmiter (50 ohms) to the varying input impedance of the open wire line (50 to 2000 ohms).

The surge impedance of the open wire line will be about 400 to 600 ohms depending on what it is and/or how carefully it is constructed. The more astute will realize that an 88 foot balanced dipole antenna is not resonant on any frequency in any ham band and will not be a match to 400 or 600 ohms on most frequencies that will be used. The result is that the open wire line can exhibit some very high SWR levels. The mismatch of the open wire feedline to the antenna cannot be overcome by the antenna tuner in the shack. The high SWR will remain on the open wire line but will not result in any significant loss because open wire line has extremely low loss under all conditions (except driving rain).

Another all-band antenna scheme is the simple 80/40 meter trap dipole. This antenna is driven with coax and has greater losses than open wire line but it can be used on all bands with SWR less than 3:1 and does not require an antenna tuner.

The third possibility is the use of a Windom style antenna. This antenna performs about the same as a normal dipole or inverted vee. It has the advantange of offering a reasonably constant impedance to the feedline over a wide range of frequencies. This impedance is around 600 ohms. A match to transmission line can be made through the use of a matching transformer and matching transformers can be used at both ends of the transmission line. The result is that an antenna tuner is not required.

A fourth possibility is to install multiple dipoles to the same feedline. This type of antenna has been called a fan dipole in the past but it is not a real fan dipole. Fan dipoles are single band antennas that have multiple elements all cut to the same frequency, connected to the same feedline at one end and seperated into a fan configuration at the other end. The purpose of fan dipole is to make it broadbanded on a single band.

A fifth possibility is to install half wavelength wires (for other bands) in close poximity (within 6 inches or so) of a normal dipole desiged for a different band or frequency. I use two such antennas. One to cover 80 and 75 meters. The other to cover 40 and 30 meters. The 75 meter dipole is driven. The 80 meter coverage is obtained by installing the half wavelength wire along side the 75 meter dipole. The result is an SWR of 1.2:1 at 3.97mhz and 3.60mhz. Frequency shifts, 75khz to either side of either SWR point results in an SWR of 2:1. This antenna is not really multiband or even broadband, but it does cover specific frequncies of interest on 80 and 75 meters. The 40/30 meter version shows similar performance but it covers both bands with SWR under 1.5:1 on both bands.

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