Vhf Skeleton Slot Antenna

The skeleton slot was developed in the UK for TV use soon after WW2. Someone had worked with slot aerials on aircraft, and the use of a half wave vertical slot in a very large piece of sheet metal, results in radiation very much the same as a horizontal dipole. 2m Skeleton Slot yagi Stack. 145MHz Skeleton Slot yagi Stack 145MHz Slot yagi. The skeleton slot was developed in the UK for TV use soon after WW2. Someone had worked with slot aerials on aircraft, and the use of a half wave vertical slot in a very large piece of sheet metal, results in radiation very much the same as a horizontal dipole. This is a great antenna for 2-meter repeaters, which require vertical polarization. The copper pipe loop portion of the skeleton-slot antenna is the equiva-lent of the solid plane in a classical slot antenna with the vertical space between the loops forming the slot. The whole antenna is folded vertically into a cube to make it omni-directional. With that close a coupling the slot and its flanking pair of D1's appear as a single exciter. The following image shows the skeleton slot and upper & lower D1: the Publications This design was initially published in the 'YU VHF UHF Bulletin's Special edition Antennas in 1980.

I am always interested in trying different antenna designs, especially if they are simple to construct and provide increased functionality. While perusing some old issues of QST magazine online I found a series of articles that discuss a design called the Folded Skeleton Sleeve. The design is a unique way to build a dual-band resonant dipole or groundplane vertical. The articles appear in the May 2011, October 2011, October 2012, December 2013, and March 2015 issues of QST magazine.

I was particularly interested in this antenna design because a simple resonant dual-band antenna could be very useful for deployment at Field Day or for EMCOMM purposes. Other multi-band antenna designs exist and can perform quite well (windoms, off-center-fed dipoles, G5RVs, non resonant end feds, dipoles fed with window line, etc.), however, most of these require a wide range antenna tuner to achieve a decent SWR on multiple bands. Other designs, such as trap dipoles, can be heavy and cumbersome with multiple points of failure. The folded skeleton sleeve design exhibits non of these limitations.

Design

The folded skeleton sleeve at first looks like a standard folded dipole, however, the top radiator is not continuous. Two notches are cut along the top of the window line to create the parasitic element that allows for operation on the higher frequency band.

A 75M / 40M antenna should be perfect for both EMCOMM (these are the most common HF bands used for emergency communications) and Field Day. A 40M / 20M antenna is equally perfect for Field Day and the combination of the two provides a lot of operating versatility from two simple antennas that cover the three busiest Field Day bands. I also decided to construct a 40M / 30M antenna for use as a portable antenna for digital communications.

Construction

I built the antennas using 18AWG stranded copper-weld 450 Ohm window line (Wireman #553) and folded dipole insulator kits (Wireman #804) which make fantastic strain reliefs for securing the window line. I also made my own 1:1 baluns in a similar design to what I have done before, except this time I used FT-150A-K toroids and 18AWG wire which allowed me to make the baluns smaller in size while still being adequate to handle 100W. To house the baluns I used Bud Industries PN-1322-DGMB NEMA 4X enclosures. These are well made boxes and they feature convenient mounting tabs that are easily bolted to the center insulator.

75/40 Bandwidth

75 Meter Band

• 2:1 SWR: 3.68-3.785
• 3:1 SWR: 3.63-3.86

40 Meter Band

• 2:1 SWR: 7.18-7.238
• 3:1 SWR: 7.1-7.3

While the bandwidth of this antenna is not particularly wide, it is easily matched to the radio’s 50 ohm output with practically any antenna tuner.

My ham radio club used the 75/40 at our Field Day site for the duration of the event. While obviously intended for use on 75 & 40 meters, the antenna was used on the higher bands as well with the help of a wide range antenna tuner. Over the course of field day this setup resulted in over 350 CW contacts.

40/30 Bandwidth

40 Meter Band

Vhf Skeleton Slot Antenna Parts

• 2:1 SWR: 7.158-7.33
• 3:1 SWR: 7.073-7.448

30 Meter Band

• 2:1 SWR: 9.93-10.24

This antenna exhibits better bandwidth than the 75/40 and even reaches an SWR of 1.1:1 on 30 meters.

40/20 Bandwidth

This antenna is by far the best design of the bunch. This configuration results in an SWR of under 2:1 across the entirety of both the 40 and 20 meter bands.

Antenna Winders

Since ladder line can be annoying to work with since it doesn’t coil easily, I decided to build some winders from 1/2 inch PVC pipe to keep the finished antennas organized. I built a larger one for the 75/40 antenna and smaller ones for the 40/20 and 40/30 antennas. I am really pleased with how these turned out and plan to build more for use with other antennas; they are a fantastic way to avoid a tangled mess.

A slot antenna consists of a metal surface, usually a flat plate, with one or more holes or slots cut out. When the plate is driven as an antenna by an applied radio frequency current, the slot radiates electromagnetic waves in a way similar to a dipole antenna. The shape and size of the slot, as well as the driving frequency, determine the radiation pattern. Slot antennas are usually used at UHF and microwave frequencies at which wavelengths are small enough that the plate and slot are conveniently small. At these frequencies, the radio waves are often conducted by a waveguide, and the antenna consists of slots in the waveguide; this is called a slotted waveguide antenna. Multiple slots act as a directivearray antenna and can emit a narrow fan-shaped beam of microwaves. They are used in standard laboratory microwave sources used for research, UHF television transmitting antennas, antennas on missiles and aircraft, sector antennas for cellular base stations, and particularly marine radar antennas. A slot antenna's main advantages are its size, design simplicity, and convenient adaptation to mass production using either waveguide or PC board technology.

Structure[edit]

Vhf Skeleton Slot Antenna System

As shown by H. G. Booker in 1946, from Babinet's principle in optics a slot in a metal plate or waveguide has the same radiation pattern as a driven rod antenna whose rod is the same shape as the slot, with the exception that the electric field and magnetic field directions are interchanged; the antenna is a magnetic dipole instead of an electric dipole; the magnetic field is parallel to the long axis of the slot and the electric field is perpendicular. Thus the radiation pattern of a slot can be calculated by the same well-known equations used for rod element antennas like the dipole. The waves are linearly polarized perpendicular to the slot axis. Slots up to a wavelength long have a single main lobe with maximum radiation perpendicular to the surface.

Antennas consisting of multiple parallel slots in a waveguide are widely used array antennas. They have a radiation pattern similar to a corresponding linear array of dipole antennas, with the exception that the slot can only radiate into the space on one side of the waveguide surface, 180° of the surrounding space. There are two widely used types:

• Longitudinal slotted waveguide antenna - The slots' axis is parallel to the axis of the waveguide. This has a radiation pattern similar to a collinear dipole antenna, and is usually mounted vertically. The radiation pattern is almost omnidirectional in the horizontal plane perpendicular to the antenna over the 180° azimuth in front of the slot, but narrow in the vertical plane, with the vertical gain increasing approximately 3 dB with each doubling of the number of slots. The radiation is horizontally polarized. It is used for vertical omnidirectional transmitting antennas for UHF television stations. For broadcasting, a cylindrical or semicircular waveguide is sometimes used with several columns of slots cut in different sides to give an omnidirectional 360° radiation pattern.
• Transverse slotted waveguide antenna - The slots are almost perpendicular to the axis of the waveguide but skewed at a small angle, with alternate slots skewed at opposite angles. This radiates a dipole pattern in the plane perpendicular to the antenna, and a very sharp beam in the plane of the antenna. Its largest use is for microwave marine radar antennas. The antenna is mounted horizontally on a mechanical drive that rotates the antenna about a vertical axis, scanning the antenna's vertical fan-shaped beam 360° around the water surface surrounding the ship out to the horizon with each revolution. The wide vertical spread of the beam ensures that even in bad weather when the ship and the antenna axis is being rocked over a wide angle by waves the radar beam will not miss the surface.

History[edit]

The slot antenna was invented in 1938 by Alan Blumlein, while working for EMI. He invented it in order to produce a practical type of antenna for VHF television broadcasting that would have horizontal polarization, an omnidirectional horizontal radiation pattern and a narrow vertical radiation pattern.^[1][2]

Prior to its use in surface search radar, such systems used a parabolic segment reflector, or 'cheese antenna'. The slotted waveguide antenna was the result of collaborative radar research carried on by McGill University and the National Research Council of Canada during World War II.^[3] The co-inventors, W.H. Watson and E.W. Guptill of McGill, were granted a United States patent for the device, described as a 'directive antenna for microwaves', in 1951.^[4]

Other uses[edit]

In a related application, so-called leaky waveguides are also used in the determination of railcar positions in certain rapid transit applications. They are used primarily to determine the precise position of the train when it is being brought to a halt at a station, so that the doorway positions will align correctly with queuing points on the platform or with a second set of safety doors should such be provided.

See also[edit]

• Microwave Radiometer (Juno) (has a slot array antenna)
• RIMFAX (radar for Mars rover has slot antenna design)

References[edit]

Vhf Skeleton Slot Antenna For Sale

1. ^Blumlein, Alan (1938-03-07), 'Improvements in or relating to high frequency electrical conductors or radiators', British patent no. 515684
2. ^Burns, Russell (2000). The life and times of A.D. Blumlein. Institution of Engineering and Technology. ISBN0-85296-773-X.
3. ^Covington, Arthur E. (1991). 'Some recollections of the radio and electrical engineering division of the National Research Council of Canada, 1946-1977'. Scientia Canadensis: Canadian Journal of the HIstory of Science, Technology and Medicine. 15 (2): 155–175. doi:10.7202/800334ar.
4. ^Watson, William Heriot; Guptill, Ernest Wilmot (6 November 1951), Directive Antenna for Microwaves, retrieved 20 December 2016

External links[edit]

Vhf Skeleton Slot Antennas

• 'Slot Antennas'. Antenna Theory.
• Slotted Waveguide Antennas Antenna-Theory.com