What is an Oblong Antenna?

Oblong antenna, also known as oblong loop has a rectangular shape. When mounted in the vertical plane, it has a horizontal polarization, with signals along the broadside of the plane of the wire loop. It has about 2.6 dB directional gain perpendicular to the plane of the wire loop. It has a deep null along edge or side on directions. Maybe we can use that for a fox hunt or amateur radio direction finding contest! If the edge or side of the loop is pointed towards the transmitting station and rotated to a position where the signal is almost unreadable, that pinpoints the direction of the station. Obviously, maximum gain will be if the antenna is rotated to a perpendicular position with broadside towards the transmitting location.

Though the oblong antenna is more popular on 6m and 2m, it can also be made for the 70cm UHF amateur radio band. Width of the antenna is one sixth of the wavelength of the resonant frequency and the height one third of the wavelength. It is often used for portable operations like Summits on the Air or SOTA as it is light weight to carry up the hill. Velocity factor of the wire has to be considered while arriving at the dimensions for the antenna on each band. A typical oblong antenna for 6m band with 50.2 MHz as nominal frequency will be 1017 mm wide and 2000 mm tall. VK1AD has suggested a total length of 6034 mm of 18 AWG wire. If you are interested in homebrewing one, detailed instructions are available at VK1AD’s webpage, with nice photographs as well.

An article by IN3AQK gives details for a 144 MHz oblong loop, which is 36 cm x 69 cm, again made of wire. That article also gives a picture of a coax balun, meant for suppressing common mode current on the outer surface of the coaxial shield. VK1AD has also suggested the use of a 1:1 current choke, though the pictures show direct connection of BNC plug to a banana BNC adapter. Banana BNC adapter is a convenient way of connecting the wires at the feed point as it does not need soldering. While VK1AD had used wooden dowels as a supporting frame at upper and lower ends, IN3AQK has used PVC pipes and seems to be using SO239/PL259 combination instead of BNC. Feed point impedance of the oblong loop antenna is mentioned as 50 ohms so that a matching transformer is not needed, unlike in the case of a delta loop. Others have made oblong loops, mainly for higher frequencies, with aluminium pipes as well, with a spacer at the feed point made of non-conducting material.

DL6GCA has built a quadruple oblong using the DK7ZB design for VHF, with a gain of 5.6 dBd in two directions. As you may be aware, dBd is gain over a dipole, which itself has a gain of 2.15 dBi with respect to a theoretical isotropic antenna with a perfect spherical radiation pattern all around. DK7ZB calls oblong antennas as quadlong antennas. The quadruple oblong made by DL6GCA for 2m SSB was 29.5 cm wide and 286.5 cm tall. Horizontal 3 dB angle was 88 degrees, a fairly wide beam width. It had a feed point impedance of 47 ohms. While the less than 1.5 SWR bandwidth was calculated at 3.25 MHz, actual measured was over 4 MHz. DK7ZB has also suggested adding reflector elements. All these seemed to be too complex at my first look at oblong antennas. May be I will return to them at a later date when I am ready to try out one.