What is Doublet Antenna for Multiband Operation?

What is Doublet Antenna for Multiband Operation?

Doublet antenna is a form of dipole antenna with a balanced feeder like an open wire feeder or ladder-line used along with an antenna tuner to provide multi-band operation. VU3TPW is using a doublet antenna to cover HF bands from 80 m to 6 m. Open wire feeder can operate with standing waves along its length and effectively becomes part of the antenna, enabling operation over a wide range of frequencies. As the two wires in the open wire feeder carry equal and opposite currents, their radiation cancels out. Doublet Antenna is popular among radio amateurs who wish to cover several bands with a single antenna.

Basic principle is that each half of the top and each wire in the balanced feed line must be of equal length. Antenna top is cut to resonate at any particular frequency to suit the location. Typical length of one section of the top section is quarter wave length of the lowest frequency one is planning to operate on. Hence it will be 20 m each if you are planning to operate from 80 m upwards. It has been mentioned that top section λ/2 can be reduced in length to about 3λ/8 retaining good efficiency.

As the impedance of this antenna can be over a wide range an antenna tuning unit with a wide range and a balanced output connection is needed. Otherwise a balun is needed along with the antenna tuner, to transition from unbalanced to balanced line. VU3TPW has used 4:1 and 1:1 baluns in series, connected to 1.5 m RG 213 cable leading to the antenna tuner. It is worth remembering that a balanced open wire feeder cannot run through a building and needs a transition like the RG 213 cable into the building. If the combined length of one limb of the doublet antenna and the feeder is an odd multiple of the electrical quarter wavelength of the frequency, to be used, it has the best impedance match. Typically it will be 50 ohms, mostly resistive.