The Jampro JSL-V Band III VHF Slot Antenna delivers performance. The detail in manufacturing and tuning for your pattern ensures premium performance. Additionally, pressurized feed lines protect them from the from the harshest of environments. The sealed radome also protects the antenna against the weather. Skeleton slot An antenna consisting of a full- wave rectangular loop higher than it is wide. It is fed via a transmission line split to connect to the center of each vertical side. It acts as two horizontally stacked bent dipoles in phase and was used as the driven ele- ment structure of a VHF stacked Yagi array popular in the 1950s. At VHF and UHF, most antenna systems are derived from the dipole or its complement, the slot antenna. Many antennas are based on half-wave dipoles fabricated from wire or tubing.
Introduction
This paper was presented at the 40th Annual West Coast VHF/UHF conferenceheld on May 5-7 in Cerritos, California. Some editing was done to make itsuitable for this web page.
Abstract
Slot antennas can be used for fixed stations, satellite ground stationsand beacons. With proper mounting, a slot antenna can also be used for 'microwavemobile'. With a 16-slot total, the antenna can have 10-12 dbi gain.
Slot antennas can be built from surplus waveguide sections, which will givean ominidirectional pattern (and horizontal polarization). This paper offersa computer-aided method to calculate the proper dimensions for the slotsand their locations.
Because the antenna is of one-piece construction, it is rugged and can bebuilt cheaply, requiring only access to a reasonably precise drill pressor milling machine.
Getting Ready
The first step in construction of the antenna is to select a section ofwaveguide which covers the desired frequency. Approximate waveguide dimensionsfor several microwave bands are listed in table 1. If your waveguide doesn'tmatch the dimensions exactly, go ahead and run the program. It will tellyou if the waveguide is too small (if you attempt to operate a waveguidebelow its cutoff frequency, it will have unacceptable SWR and losses).
| Usable Frequency (Ghz) | Typical Waveguide | Long Dimension 'a', inches | Short Dimension 'b', inches |
|---|---|---|---|
| 1.12 - 1.7 | WR650 | 6.5 | 3.25 |
| 1.7 - 2.6 | WR430 | 4.3 | 2.15 |
| 2.2 - 3.3 | WR340 | 3.4 | 1.7 |
| 2.6 - 3.95 | WR284 | 2.84 | 1.34 |
| 3.3 - 4.9 | WR229 | 2.29 | 1.145 |
| 3.95 - 5.85 | WR187 | 1.872 | .0872 |
| 4.9 - 7.05 | WR159 | 1.59 | .0759 |
| 7 - 11 | WR102 | 1.02 | 0.51 |
| 8.2 - 12.4 | WR90 | 0.9 | 0.4 |
| 10 - 15 | WR75 | 0.75 | 0.375 |
| 18 - 26.5 | WR42 | 0.42 | 0.170 |
Running the Computer-aided Design File
Once the waveguide has been selected, just enter its cross-sectional dimensionsand the desired operating frequency into the Mathcad program file slot_ant.mcd or slot_a2.mcd. In order to edit the file and make the calculations, you must have a copyof Mathcad, running under Microsoft Windows.
The program file references the slot antenna dimensions shown in Figure1. Note that all calculated dimensions are given in both inches and millimeters.For the best pattern, a pair of ground planes (wings) can be added to eitherside of the antenna. The wings should be flush with the top of the waveguideand extend at least a half-wavelength below the bottom slot. The wings needto protrude out from the sides by several wavelengths, if possible.
Machining the Slots (2 methods)
Drill Press - Just drill many overlapping holes, then file out the slots to the desireddimensions.
Milling Machine - The ideal method: fast and accurate, and less prone to an accident of drillingor filing.
Note that the front -face and rear-face slots are exactly across the waveguidefrom each other. You should be able to 'see through' any front/back pairof slots if the antenna is constructed correctly.
Feeding the Antenna
The simpliest feed for the slot antenna is to use a coaxial-to-waveguidetransition. These transitions are also available (sometimes) on the surplusmarket, having BNC or Type N connectors.
There are two versions of the Mathcad slot antenna program. It was discoveredthat the original program running on Mathcad 3.1 had some problems whenusers tried to run it on later versions, namely version 6.0, etc. For usersof the older Mathcad, use slot_ant.mcd, otherwise use slot_a2.mcd. Many thanks to Steve Muther, WF6R, for getting the program running inMathcad 6.0.
Some worked examples
About Mathsoft's Mathcad
Download Mathsoft's Mathcad browser
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- Events
HF Skeleton Slot - An antenna system with a difference.
For many years at this QTH I have tried different wire antennas. Not being interested in rotary beams of the usual type, I have always use fixed wire antennas.
Over the years I came to the conclusion that for my particular purposes I needed something with gain that would work in more than one direction.
Having tried vertical beams and phased verticals all hanging from a nylon line, I decided to use the same nylon line to hold up a Lazy H antenna.
The Lazy H works very well of course, Bi-directional, low angle and gain, BUT its a bit too big to rotate to fill in the directions 90° from its broadside line of fire and I don't have the room to put up a second Lazy H.
It was while playing around with this antenna using the MMANA-GAL modeling program that I tried bending the ends of the top dipole down and the ends of the bottom dipole up, still keeping the open wire phasing line between the two dipoles and fed in the center. I eventually ended up with the ends of the dipoles joined together and the horizontal top and bottom parts of the dipoles very much shortened and tried feeding at the junction of the dipole ends, getting rid of the phasing line between the two dipoles.
This arrangement now reminded me of the VHF skeleton slot antenna that I had seen in an old RSGB handbook, but a search on the Internet turned up nothing for an HF version.
More adjustments in the MMANA-GAL program showed that the antenna should perform well on all bands from 30 meters to 10 meters.
To get the low angle take off, the bottom of the antenna needs to be about 15 feet (4.5M) off the ground. It will still work if the bottom is lower but with a higher angle of take off.
I purchased a Pump up mast 50 feet (15m) and a small cord operated rotator. The top horizontal spreader is fixed to this rotator and the whole antenna hangs down from there.
The MMANA-GAL program can be found here.
Vhf Skeleton Slot Antenna For Sale
Below are the dimensions I used for the Lazy-H antenna.
The plot below shows the gain and angle of elevation of the Lazy-H, as computed with MMANA-GAL. The antenna works well but is difficult to rotate.
The photograph below shows the antenna installed at my QTH.
The top spreader is 50 feet above ground. The bottom about 20 feet above ground level.
HF Skeleton slot
The drawing above shows the dimensions of my Skeleton Slot.
It can be easily rotated. It is bi-directional. It has high gain, low angle of elevation, and can work on all bands from 10 to 30 MHz.
Below are the plots of the Slot for the bands 20m, 17m, 15m and 10m.
Below is shown a 'BowTie' version.
It has slightly higher gain and lower angle of elevation.
The corresponding plots are shown below.
This picture shows the bowtie version at GW3RQT(SK)'s
The small marine ply support for the spreaders can be seen in the center.
The short horizontal bamboo in the center is to support the dog bone insulator and the open line feeder together with the vertical wires coming from the four corners.
Alan GW3RQT(SK) uses the same antenna but with smaller dimensions as his mast is only 30 feet high.
It is designed for 15m but works well on all bands from 10 to 20m.The plots are shown below.
He uses spreaders for the diagonals mounted on a rectangular piece of marine ply with U bolts.
The center is then raised to the top of the mast which gives a height of 15 feet from the bottom of the antenna above ground.
Constructional details
The antenna is very easy and inexpensive to make. For the sake of clarity, the constructional details are on another page.
Conclusions
The skeleton slot antenna is very cheap and simple to make - mine consists of three bamboo spreaders and copper wire!! It is fed in the middle of the center wire with open wire line which is taken to an antenna tuning unit in the shack.
Results have been excellent and quite a lot of DX has been worked on digital modes using 50 watts even through the odd pile up .. With such a broad horizontal pattern I only need to turn the antenna 90° to get pretty well full coverage of the world . For example with the antenna in a NE/SW direction, stations from K4 down to LU and JA down to VK6.
Have fun 73 Don G3VCG
Vhf Skeleton Slot Antenna System
My e-mail address is here