Antenna FAQ

Frequently Asked Questions

What cable should I use for my antenna installation?

Larsen offers a full selection of quality cables in popular RG-174, RG-58 and RG-8 sizes. Cables are produced to Larsen’s exact specifications by recognized suppliers.

How much loss results from each connector installed on the cable?

Connector loss is negligible with respect to the overall cable loss. Some manufacturers suggest adding 0.1 dB for every 2 connectors within a cable system for planning purposes.

What connectors do you have?

Larsen can supply antennas with almost any connector or without a connector for many applications. Standard connectors are Type N, BNC, FME, TNC, SMA, SMB, MCX, and MMCX. Cable size dictates which connectors may be used on certain antennas.

How much loss results from each connector installed on the cable?

Connector loss is negligible with respect to the overall cable loss. Some manufacturers suggest adding 0.1 dB for every 2 connectors within a cable system for planning purposes.

Can I cut the coaxial cable? And if not, how do I store excess cable?

Yes, the coaxial cable can be cut without degrading performance. Excess cable can be stored in any manner as along as the coiled cable does not exceed the specified bend radius.

If I use existing coax from a damaged, currently installed antenna with a new coupling box and whip, are their any concerns?

As long as the cable and connectors are in good condition, they should not need replacing.

Explain antenna propagation and the proper grounding planes for the different types of antennas available.

Ground planes are required to properly impedance match quarter-wave or collinear antennas launched directly from a conductive surface. The first element relies on image theory, described as a virtual antenna of equivalent length and spacing below the ground plane performing cooperatively with the physical antenna to provide impedance matching for far field radiation. Half-wave or 5/8-wave radiators can be stacked vertically to achieve higher gain. The proper ground plane dimensions are defined by a ground plane which is much larger (in both directions) than the antenna length.

Explain the theory behind portable antenna design and their length with respect to operational wavelength.

Traditional portable antennas are generally less efficient, short radiators impedance matched to the radio circuitry and case. At higher frequencies (shorter wavelengths) such as 800/1900 MHz, end fed designs can accommodate true 1/2-wave radiators that are independent of the case and user, providing efficient dipole radiation characteristics.

Is antenna length important to reception?

Antenna length is critical to maximize performance and provide the proper radiation characteristics. The length of an antenna has an important influence on characteristics such as directivity and bandwidth.

Will dual band antennas work as well as single band or do they match the antennas in the middle of the frequency range?

Properly designed dual band antennas will provide a good impedance match over both bands. Dual bands are designed for resonance and proper phasing in each band and not centered between the bands. However, compromises must be made with respect to the radiation efficiencies in each band. Single band antennas are optimized for single band performance and typically perform better than dual band antennas in their respective band of operation.

Explain the theory behind portable antenna design and their length with respect to operational wavelength.

Traditional portable antennas are generally less efficient, short radiators impedance matched to the radio circuitry and case. At higher frequencies (shorter wavelengths) such as 800/1900 MHz, end fed designs can accommodate true 1/2-wave radiators that are independent of the case and user, providing efficient dipole radiation characteristics.

With my low-band antenna mounted on the roof of my car I had great VSWR. I moved it to a trunk bracket and now it doesn´t tune properly. Why?

The antenna is responding to the ground plane size and shape in the immediate field of the whip. Ground plane effects are significant at low frequencies, because a quarter-wave is fairly long for low-band frequencies.

I'm using one of your rubber duck antennas on our electronics box. Our system hasn´t been working right. When I tested the antenna VSWR I found it was very high. What´s wrong with your antenna?

We need to understand what kind of rubber duck you are using. Most require a good ground plane for good performance. If your box is made of plastic, fiberglass or other non-metallic materials, there may not be enough ground plane. If your box is metal, but is small in wavelengths, there may still be inadequate ground plane. One solution may be to use a half-wave antenna if your application will allow the additional height. Half-waves work independent of a ground plane.

What type of antenna should I use for point-to-point data transmission?

Directional antennas should be used for point-to-point transmission. The type of directional antenna depends upon the power output, cable type and length, height, distance, data rate and terrain.

What do you recommend for antenna grounding & lightning protection?

For tower-mounted antennas, there should be a good ground wire (#2/0) attached between the tower base and a single-point earth ground. (There is no need for a separate ground wire running along the tower!) For roof-mounts, the mast should be grounded to the steel structure of the building if possible. If no connection to the building is possible, then a large diameter wire may be run directly to earth ground. Lightning protectors should be added to the coax cable between the antenna and the amplifier or other radio equipment unless built-in to the amplifier.

What polarization should I use?

Most Point-to-Multipoint WLAN systems use vertical polarization. This allows the use of inexpensive vertical omnidirectional antennas. Point-to-Point systems may use either vertical or horizontal polarization as long the same polarization is used at each end. Horizontal polarization may perform slightly better when transmitting through a forested area, otherwise there is very little difference in propagation effects.

Why is a wavelength (or fraction of) in coax cable physically shorter than what I calculate from the formula for wavelength?

The formula you used to calculate the wavelength was probably meant for calculating a “free space” (air) wavelength. In fact, RF energy moves more slowly in a transmission line than it does in air because the materials used in cable slow it down. Therefore, a wavelength in cable takes up less length. The appropriate formula for the calculation of the guided wavelength is: where is the dielectric constant of the material (1 for the air, and 1 for other types of material). This value increases as a function of the frequency.

I put an ohmmeter between the center contact and the outer ring at the mount (bottom) end of a Larsen coil and measure a dead short. On another of your coils I measure an open between the center conductor on the bottom of the coil and the whip. Don´t these ohmmeter results mean the coils are bad?

Not necessarily. DC performance of a matching coil seldom relates at all to how well it works at radio frequencies. Some coils employ a tap a few turns from one end yielding excellent RF performance even though it´s a dead short for DC. Other coils use capacitors that block DC current, and although they´re open to DC they conduct RF just fine. If in doubt about your particular coil, contact our technical support staff.

What is the best mobile antenna mount to buy?

It all depends on your use. The important thing is ensure the whip extends above the roof line of the vehicle. This will prevent the car from blocking, distorting or shading the signal pattern of the antenna.

I finally saved enough to buy my first new car! What type of antenna can I buy that won't require drilling a hole in the car?

Glass mount antennas are easy to install and do not require drilling a hole. Just be sure the antenna is mounted high up on the glass and not over defogger wires or on reflective window tint as this could impede performance.

I use a pick-up truck as my business vehicle. I really need to make sure i have good transmission and reception. What type of antenna do I need?

The best choice for optimum performance is a roof mount antenna. At this height the antenna is free from any vehicular obstruction. The roof also acts as a ground plane for the antenna and actually becomes part of the antenna’s electrical system. Just remember, this antenna type does require drilling a hole in the vehicle.

Can an antenna be cut to any length or does it have to be kept at its original length?

The antenna cannot be cut and is specifically designed to resonate at the design frequencies with the proper phasing between the elements. The coupling box is also designed to match the antenna to the Cellular/PCS frequencies which is critical to maintaining the antenna system performance.

Does slightly bending the whip reduce antenna performance?

Slight bends should not affect the antenna significantly.

I moved my on-glass antenna from my old car to a new one. It worked great on the old car, but I have a very high VSWR on my new car? Why?

Your new car probably has passivated glass — glass impregnated with tiny metal particles which render the glass incapable of coupling RF between the inside and outside couplers.

How can I tell if the glass on my vehicle is suitable for one of Larsen´s on-glass antennas?

First, look near a corner of a window for words meaning or relating to “sun” or referring to ultraviolet or other forms of radiation. If you find terms like “Soft-Ray”, “EZE-Cool”, “Solar-Coat”, “Solar-Cool”, “Insta-Clear” your glass is probably passivated and won´t work with on-glass antennas. If you don’t find these helpful words on the glass and want to be 100% certain, ask a 2-way or cellular shop technician to test your glass with a capacitance meter.

Many factors can affect the performance of on-glass antennas.
Glass Thickness: KGs are designed for glass with a typical thickness of .138″ – .158″. Anything greater or less will result in a frequency shift. Most automobile manufacturers have side and rear glass panels within this tolerance. If the mounting surface is thicker or thinner, this value will change the dielectric constant and change the resonant frequency of the antenna.
Height on glass: The lower on the glass the antenna is placed, the greater the reflection from the roof. This will cause the VSWR to increase.
Tint: Various tints affect the dielectric constant causing a higher VSWR, poor reception or no performance.
Dielectric Constant: The couplers (inside / outside) function as a capacitor. The distance and glass material content between these two plates affects the capacitive value.
UV Glass: Many newer vehicles are manufactured with UV resistant or passivated glass containing metal flakes which destroy the capacitive function. One way to tell if your vehicle has passivated glass is to look in the window for the manufacturers’ stamp. If the brand has the word SOLAR in it, the glass is not adaptable for RF purposes.

Does the black paint around the edge of the window reduce antenna performance?

No. The black mast paint used around the perimeter of the rear window glass on domestic vehicles has not been shown to be detrimental to the glass mount installation. The only difficulty in this preferred installation is the inside to outside alignment of the coupling box to base foot.

Does it make a difference if I mount the antenna on a corner of the window instead of the center of the glass?

The best location is near the center of the roofline. If the installation requires mounting to the corner, there should not be significant performance degradation. Always install the antenna as high on the glass as possible.

Does it make a difference if the antenna is mounted on the front or back window?

Glass mount antennas are designed for tempered automobile glass with a nominal thickness of 5/32″. Front windshield glass is a laminated safety glass, which is thicker and causes some degradation to the VSWR match. Expect a VSWR match to be nominally 2:1.

Can I put a glass mount antenna on glass treated with rain-x?

No. Chemical properties of Rain-X reduce the holding ability of the adhesive tape used on glass mount antennas by over 50%. If Rain-X or another acidified alcohol surface treatment has been applied to the glass, it must be removed from the mounting area. Try Bon Ami cleanser or Ultra Brite flouride toothpaste.

Any quick tips for installing 'on-glass' antennas?

Remember: Install only on INSIDE of window. Do not mount antenna on any dark tinted area or on an area where an after market tinting film has been applied. Do not install over in-glass AM/FM dipole antenna. Do not install on glass with metallic content (passivated glass i.e. “solar-coat”, “solar-cool”). If possible, avoid installing antenna on curved glass. Clean inside mounting area with alcohol pad provided in the installation kit. Wipe off excess alcohol. Do not allow alcohol to dry on the glass. If window film is apparent, use a mildly abrasive detergent-type cleanser. Do not use ammonia-based or similar type of glass cleaner. Vehicle glass should be near room temperature (70° F / 21° C) prior to installation and remain at this temperature for 3 days following installation for maximum antenna-to-glass bond strength.

What is the difference between unity gain, 3 db gain and 5 dbi gain?

Unity (0 dBd), 3 and 5 dBd designs differ by the number of elements incorporated to achieve increased gain. Gain increase is achievable by stacking multiple elements in a collinear manner to compress the vertical plane pattern and direct more energy toward the cell site.

Some antenna gain measurements are listed as dbd and some are listed as dbi. What is the difference and which is better?

All antenna gain measurement are relative to something else. dBi is a measurement relative to a theoretical isotropic radiator. This is considered, to a point, antennas that radiate in a perfect spherical pattern of energy. dBd is measured in relation to a center-fed half wave dipole antenna. This is one of the simplest antennas to construct and is a convenient standard of measurement

Why is gain important?

Gain is an important measure because certain gain figures are better suited to particular usage environments. For strictly urban use, a unity gain antenna is ideal. Because a unity gain antenna send the maximum signal above the horizon at a high angle, it is excellent for areas where there are tall buildings and for mountains as well. For people who live in suburban or rural areas, 3 dBd gain is recommended. The 3 dBd gain antenna concentrates the maximum amount of signal at the horizon while maintaining a good high-angle signal. If you are a cellular user, there is no need to buy an antenna stronger than 3 dBd gain as the USA cellular infrastructure is designed for optimum performance using 3 dBd gain antennas. If you are using a Land Mobile Radio (LMR) system and are in a rural area, a 5dBd gain antenna is suggested as this will help to provide the maximum amount of signal at the horizon with very little high-angle signal. Finally, the gain is a practical value which describes the capability of an antenna to concentrate energy in a particular direction of space.

Larsen doesn't specify gain for its portable antennas. Some manufacturers do. Why is this?

Virtually all portable antennas compromise gain for small size and the convenience size affords the user. True antenna gain results from directivity and the use of efficient (low-loss) materials. Directivity requires a certain amount of “aperture” or physical size. Larsen devotes particular attention to impedance matching. We employ low-loss materials in order to optimize portable performance within the constraints presented by small size.