There Ain't No Free Lunch
OK. The English is bad but the title says it all. So many hams are looking for that 'all band, does everything' HF antenna.
On VHF and UHF the 'tuning' of an antenna is far less critical than on HF. The wavelengths at 144 MHz and above provide a naturally wide bandwidth so that you assemble the antenna and, in most cases, it just works. Nearly all transmitting antennas at VHF and above are resonant types.
- Common Data for Questions 8-9: A printed resonant slot antenna is designed at 2.45 GHz on a one side metal FR4 substrate with e r = 4.4, h = 0.16 cm and tan d = 0.02. The width of the slot is 4 mm.
- Slot antennas are an about λ/2 elongated slot, cut in a conductive plate (Consider an infinite conducting sheet), and excited in the center. This slot behaves according to Babinet's principle as resonant radiator.
There are basically only two classes of HF antennas: Resonant and Non-Resonant. Let's look at resonant antennas first.
Resonant antennas include (but are not limited to) monoband dipoles, monoband and trapped verticals, mono-band and trapped multiband Yagis, and specialized multiband antennas like fan and parallel dipoles. Resonance may be designed into these antennas by the use of traps, linear loading, stubs, or by the natural resonance of the length of the radiator. With these antennas, resonance occurs only in narrow chunks of spectrum.
The holes are not resonant at 10 GHz, so they do not radiate as readily as resonant slots. The result is a high 'Q' sharply tuned antenna. The radiating elements on opposite sides of the waveguide are further apart, so the omnidirectionality is not as good.
Non-resonant antennas include (but are not limited to) long-wires, un-trapped multiband verticals, off-center fed dipoles, and other compromise antennas. These antennas typically require a wide-range antenna tuning unit (ATU).
On HF, the wavelengths are long to very long and resonance becomes more critical. A dipole on 80 meters may have a useful SWR bandwidth of only 60 kHz or so. If you want to work 75 phone with an 80 meter CW antenna, you'll need an ATU (better referred to as a transmatch) to compensate. All resonant HF antennas – ALL OF THEM – used outside their resonant bandwidth require the use of a tuner. If you are looking for an antenna that will cover 160 through 6 and work efficiently… that hasn't been invented yet.
Non-resonant antennas may be force-fed using ATUs in conjunction with baluns or feedline current chokes. Baluns and chokes will keep RFI out of your shack and allow the tuner to force-feed the non-resonant antenna so that power is radiated instead of being lost in standing waves or impedance losses. For example, 43 foot verticals are 43 feet long to avoid accidental resonance. In other words, they're designed to be totally non-resonant. Their balun or unun and associated ATU allow them to work across a very wide spectrum. The lower the frequency, however, the poorer the efficiency of these antennas becomes.
Compromise antennas require compromise solutions and support. There ain't no free lunch!
10 GHz Omnidirectional Antenna
Ken Vickers G3YKI
So what's the difference?
This antenna uses round holes rather than slots. They are much easier to make! A normal slotted waveguide antenna has the slots in the broad face of the waveguide. The broad face (of a vertical waveguide) has both vertical and horizontal currents, so a vertical slot has to be used if you want to radiate only horizontal polarisation. The short face carries only horizontal currents so the shape of the holes does not matter much.
I searched all the antenna books I could find, but no one mentioned an antenna like this. But after I convinced myself that it should work, I made one, and it did work.
What are the Disadvantages?
The holes are not resonant at 10 GHz, so they do not radiate as readily as resonant slots. The result is a high 'Q' sharply tuned antenna.
The radiating elements on opposite sides of the waveguide are further apart, so the omnidirectionality is not as good.
The vertical spacing of the elements is greater than one wavelength in free space which leads to less than optimum gain for the length of the antenna, and larger sidelobes in the vertical plane.
How to make it.
Dimensions relate to WG16 for 10.368 GHz.
The waveguide must be exactly a multiple of one half guide wavelength long between short circuits at the operating frequency. The holes are 3/8' (9.5mm) diameter (largest possible)and spaced one guide wavelength apart, starting one quarter guide wavelength from the short circuit. Holes at the same height on opposite faces will give opposite contributions in the directions of the broad face, resulting in a null in that direction. To get the side lobes, as in the pattern shown, the holes on opposite faces are offset by one half wavelength vertically.
- Common Data for Questions 8-9: A printed resonant slot antenna is designed at 2.45 GHz on a one side metal FR4 substrate with e r = 4.4, h = 0.16 cm and tan d = 0.02. The width of the slot is 4 mm.
- Slot antennas are an about λ/2 elongated slot, cut in a conductive plate (Consider an infinite conducting sheet), and excited in the center. This slot behaves according to Babinet's principle as resonant radiator.
There are basically only two classes of HF antennas: Resonant and Non-Resonant. Let's look at resonant antennas first.
Resonant antennas include (but are not limited to) monoband dipoles, monoband and trapped verticals, mono-band and trapped multiband Yagis, and specialized multiband antennas like fan and parallel dipoles. Resonance may be designed into these antennas by the use of traps, linear loading, stubs, or by the natural resonance of the length of the radiator. With these antennas, resonance occurs only in narrow chunks of spectrum.
The holes are not resonant at 10 GHz, so they do not radiate as readily as resonant slots. The result is a high 'Q' sharply tuned antenna. The radiating elements on opposite sides of the waveguide are further apart, so the omnidirectionality is not as good.
Non-resonant antennas include (but are not limited to) long-wires, un-trapped multiband verticals, off-center fed dipoles, and other compromise antennas. These antennas typically require a wide-range antenna tuning unit (ATU).
On HF, the wavelengths are long to very long and resonance becomes more critical. A dipole on 80 meters may have a useful SWR bandwidth of only 60 kHz or so. If you want to work 75 phone with an 80 meter CW antenna, you'll need an ATU (better referred to as a transmatch) to compensate. All resonant HF antennas – ALL OF THEM – used outside their resonant bandwidth require the use of a tuner. If you are looking for an antenna that will cover 160 through 6 and work efficiently… that hasn't been invented yet.
Non-resonant antennas may be force-fed using ATUs in conjunction with baluns or feedline current chokes. Baluns and chokes will keep RFI out of your shack and allow the tuner to force-feed the non-resonant antenna so that power is radiated instead of being lost in standing waves or impedance losses. For example, 43 foot verticals are 43 feet long to avoid accidental resonance. In other words, they're designed to be totally non-resonant. Their balun or unun and associated ATU allow them to work across a very wide spectrum. The lower the frequency, however, the poorer the efficiency of these antennas becomes.
Compromise antennas require compromise solutions and support. There ain't no free lunch!
10 GHz Omnidirectional Antenna
Ken Vickers G3YKI
There is no shortage of information on how to design slotted waveguide antennas, but to actually make them can become a problem without relatively sophisticated machining facilities, especially when you get to the higher frequencies. For example, the slots in an X band antenna might be about 1mm wide by 12 mm long and difficult to make by hand. This antenna was made in a few hours and requires no more than a hand drill, saw and file.
So what's the difference?
This antenna uses round holes rather than slots. They are much easier to make! A normal slotted waveguide antenna has the slots in the broad face of the waveguide. The broad face (of a vertical waveguide) has both vertical and horizontal currents, so a vertical slot has to be used if you want to radiate only horizontal polarisation. The short face carries only horizontal currents so the shape of the holes does not matter much.
I searched all the antenna books I could find, but no one mentioned an antenna like this. But after I convinced myself that it should work, I made one, and it did work.
What are the Disadvantages?
The holes are not resonant at 10 GHz, so they do not radiate as readily as resonant slots. The result is a high 'Q' sharply tuned antenna.
The radiating elements on opposite sides of the waveguide are further apart, so the omnidirectionality is not as good.
The vertical spacing of the elements is greater than one wavelength in free space which leads to less than optimum gain for the length of the antenna, and larger sidelobes in the vertical plane.
How to make it.
Dimensions relate to WG16 for 10.368 GHz.
The waveguide must be exactly a multiple of one half guide wavelength long between short circuits at the operating frequency. The holes are 3/8' (9.5mm) diameter (largest possible)and spaced one guide wavelength apart, starting one quarter guide wavelength from the short circuit. Holes at the same height on opposite faces will give opposite contributions in the directions of the broad face, resulting in a null in that direction. To get the side lobes, as in the pattern shown, the holes on opposite faces are offset by one half wavelength vertically.
A tuning screw is required to set the resonant frequency. The antenna is fed at the centre by a small probe in the centre of the broad face, rather like any other co-axial to waveguide transition.
Results
The antenna was set up on a tripod with receiver and the vertical and horizontal radiation patterns were measured.Vertical and horizontal Radiation Patterns of a 6 Element Antenna.
Polarisation
The antenna was also measured with a vertically polarised source to confirm that it really was working as a horizontally polarised antenna. The response was generally about 20 dB down on the horizontal response. Considering that my 'test range' was surrounded by trees, some of which would be illuminated with RF and scatter it with random polarisation, I would not expect any better.
More Pictures Let me know if you found this interesting,
especially if you build one!
Resonant Slot Antenna Booster
Slot Antenna Resonant Frequency Formula
[email protected]Resonant Slot Antenna Signal
