2 Meter Skeleton Slot Antenna
- 2 Meter Skeleton Slot Antenna Combo
- 2 Meter Skeleton Slot Antenna Tuner
- 2 Meter Skeleton Slot Antenna Receiver
- 2 Meter Skeleton Slot Antenna Mounts
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.
Construction & Working of Slot Antennas. The use of slot antennas is well understood through its working principle. Let us have a look at the structure of a slot antenna. When an infinite conducting sheet is made a rectangular cut and the fields are excited in the aperture (which is called as a slot), it is termed as Slot antenna.
- 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.
- Showed me a skeleton-slot antenna for 440 MHz. G6JP ex-plains “.further experiments showed that a thin rod bent to di-mensions of 5λ/8 by 5λ/24 exhibited similar properties of a true magnetic dipole slot”. See skeleton slot on left side of Fig 7. Figure 7 – Skeleton-Slot element can be used as a driv.
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
- 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.
- 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.
2 Meter Skeleton Slot Antenna Combo
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.
2 Meter Skeleton Slot Antenna Tuner
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.
2 Meter Skeleton Slot Antenna Receiver
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
My e-mail address is here