Frequently Asked Questions
Is the Light Beam series of antennas really a variation of a Cubical Quad Antenna?
No, Light Beam antennas are 3-wire folded dipoles that are uniquely configured. At first sight they might be easily mistaken for a Cubical Quad. However, unlike a Quad, the Light Beam antenna is not a closed loop. Another distinguishing feature of the Light Beam antenna is that it is about ½ the size of a Quad antenna for the same frequency. This is because the Light Beam antenna is a half-wave antenna and a Quad is a full-wave antenna.
How was the Light Beam antenna design optimized?
We struck a balance between antenna height, impedance, gain and Front-to-Back ratio. You trade one for another as the dimensions of the antenna are changed.
What is the Gain and Front-to-Back ratio of the antenna?
Gain is approximately 10 dBi and the Front-to-Back (F/B) ratio is greater then 20 dB, in most cases. The Light Beam antennas were designed using NEC2GO antenna modeling software. The designs were optimized to operate at 30 feet above average ground. At that height, the gain and F/B ratio will vary slightly depending on the antenna’s design center frequency. Actual gain and F/B ratio achieved in use will depend on many factors including ground characteristics and surrounding objects.
Why does the Gain and F/B ratio vary at different antenna operating frequencies?
Actually, the gain and F/B ratios would be the same at all design frequencies if each antenna were in free space but, because all were designed to operate at 30 feet above average ground, each antenna is not at a height that is at the same multiple of the operating wavelength and is, therefore, interacting with the ground in a different manner. In other words – if each of the antennas was placed at a height of 1/2 wavelength instead of at 30 feet, their performance characteristics would be identical.
Why was 30 feet chosen as the design operating height of these antennas?
Our thinking was: Hams need a lightweight, compact antenna that is sturdy and will operate efficiently at a height easily available to most amateurs. The Light Beam antennas are small and light enough to easily mount on a rooftop tripod or can even be mounted within an attic space. A simple pole structure can also be used to support the antenna at 30 feet. These mounting methods have much less visual impact than a standard tower designed for larger, heavier antennas.
What is the feed point impedance of the Light Beam antennas?
The antennas are designed to approach Zo = 50 ohms resistive at the designed operating frequency. A slight reactive component is present due the interaction of the parasitic element and to differences in soil permeability and conductivity at various locations, antenna true height above ground, surrounding structures, etc.
How does one eliminate the reactive component of the antenna without changing the element length?
This is easily done without raising and lowering the antenna to adjust element lengths. Since the Light Beam antennas are balanced antennas, a balanced feed-line transformer connected to a 1 to 1 balun arrangement or a balanced antenna tuner can be used to easily match the antenna to a 50 ohm transmission line. Most Hams already have what they need in their shack for this purpose.
How is a balanced feed-line transformer made?
A balanced line transformer can be made from most types of balanced feed-line. I like to use 450 ohm ladder-line because it is relatively cheap and is easy to use. You can make your own 600 ohm balanced line even cheaper and it will work just as well. Using the Velocity Factor of the balanced feed-line, determine the physical length of the balanced line transformer section as follows:
- The length of a half wave in the balanced line is equal to the free space half wavelength multiplied by the balanced line’s velocity factor. The velocity factor for commercially available balanced (and coaxial) transmission lines can be found in the technical specifications for the line being used. Using the velocity factor for the balanced line, calculate the length of a ½ wavelength section of line and then cut the balanced feed-line to an even multiple of half wavelengths at the operating frequency.
The impedance of the balanced feed-line does not matter because the loss for an open wire or ladder line type balanced line is extremely low – regardless of the balanced line’s SWR.
How is the reactive component of the antenna impedance minimized?
The reactive component can be minimized using several methods. One is to simply adjust the length of the balanced line transformer between the antenna feed-point and a 1 to 1 balun to minimize the SWR on the 50 ohm line after the balanced to unbalanced conversion. Start with a balanced feed-line that is slightly longer than an even multiple of a half wavelength and trim the balanced feed-line length until the SWR on the 50 ohm line is acceptable. If you are using a balanced antenna tuner, that has a wide adjustment range, to feed the Light Beam – the length of the balanced line is not critical and need not be a multiple of a half wavelength. Simply connect the balanced line to the balanced output of the antenna tuner and adjust the tuner to minimize the SWR on the 50 ohm line feeding the tuner. At HF frequencies and for reasonable coax lengths Ð an SWR of 1.5 to one is acceptable.
What minimum SWR should I see?
You should see an SWR somewhere between 1.1:1 to 1.5:1
Can I use coaxial cable to feed the antenna?
Yes. The antenna is a balanced antenna like any dipole so a 1:1 Balun should be used between the balanced feed-line transformer and the unbalanced 50 ohm coaxial cable from your transceiver. If you have a fairly long distance between the transceiver and the antenna, I recommend you either use low loss coax or you can maximize the length of the balanced feed-line and minimize the length of coaxial cable. This approach will reduce coaxial feed-line loss. Balanced feed-line has very little loss. Be sure to use a balanced line length that is an even multiple of ½ wavelengths in the line using the balanced line’s velocity factor to calculate the ½ wavelength physical line length as described above.
Why worry about feed-line loss?
Light Beam antennas are very efficient, low loss antennas. Over 90% of the power you put into the antenna is radiated. Using a feed-line that has loss greater than 0.5 dB is just a waste of power.
Why is the Light Beam Plus antenna series so strong and stealthy?
Clear (transparent) polycarbonate plastic is used for the antenna support structure. This plastic has a very high tensile strength and outstanding impact resistance. It is used in bullet-proof glass and aircraft windshields. It is also useable over a wide temperature range. In addition, the antenna’s support and wire configurations result in mechanically stable triangular structures that maximize strength.
Why do you make the Light Beam antenna series when the Light Beam Plus antenna series has the same radio frequency performance?
The Light Beam antenna is less expensive to manufacture and We can pass the savings on to you through a lower selling price. The Light Beam antenna and the Light Beam Plus antenna do have the same gain, F/B ratio and impedance. Unfortunately the fiberglass Light Beam antenna is not as stealthy as the polycarbonate Light Beam Plus antenna. Both antennas are the same compact size but the Light Beam antenna is more visually apparent.
How much power can the Light Beam Plus and Light Beam antennas handle?
All antennas manufactured by Light Beam Antenna & Apparatus, LLC can easily handle full legal power input. They have been tested at 1200 Watts.
What is the bandwidth of the Light Beam Plus and Light Beam antennas?
The bandwidth is greater than 100KHz. at the 2:1 SWR points. The bandwidth becomes greater as the design frequency of the antenna is higher. In other words, the bandwidth of the Light Beam antenna designed for 10 Meters (LB-10M), is greater than the bandwidth of the Light Beam antenna designed for 20 Meters (LB-20M).