Antennas

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Question

  1. Are RFS coaxial cables and connectors compatible with Andrew cables and connectors?
  2. Can I mount one of your antennas upsidedown?
  3. How far apart must I mount my transmit and receive antennas?
  4. How much antenna downtilt is required for proper coverage at my site?
  5. I am measuring high reflected power. How can I tell if the antenna is defective?
  6. The antenna's radome has become frayed. How can I restore its surface?
  7. There are some new specifications being applied to Dual Polarized antennas employed in diversity applications. What are the definitions for the terms PQR and Tracking?
  8. What is "Marine" gain?

Answer

  1. Our CELLFLEX® "LCF" (formerly FLC) foam cables and connectors are compatible with Andrew Heliax "LDF" foam cables ½ inch, 7/8 inch, 1-1/4 and 1-5/8 inch sizes only. Our Superflex 1/4 inch and ½ inch is compatible with the Superflex Andrew cable.
  2. Inverted mounting of antennas is often used to conserve tower space or to provide RF isolation. Some of our antennas are designed for in-field changes between upright and inverted mounting to address these requirements. Some of the others require physical modifications. Of primary concern would be the drain hole. If the particular antenna has one, it must be sealed and then drilled at the opposite end. Unfortunately these alterations will void the product warranty; so first check with our sales department to see if the antenna is available from the factory as an inverted model. This will circumvent two other potential problems because we will also electrically invert the antenna internally. This assures that any downtilt or null fill features remain pointing downward.
  3. First you should contact the receiver manufacturer to find out how much out-of-band signal it can tolerate. Then you can determine how much attenuation of the transmitted signal is necessary. The distance required for a specific isolation varies with operational frequency and antenna gain. Lower frequencies and higher gains require greater distances. Horizontal spacing distances may exceed practical tower applications. The best orientation is vertical separation when tower space permits it. This places each antenna in the null of the other antenna's pattern. Refer to the Antenna Spacing Chart in the technical section of our catalog for details.
  4. Electrical and/or mechanical downtilt is normally used to accomplish two objectives. Tilting the vertical pattern downward can either improve close-in coverage or restrict the range to minimize co-channel interference. If you know the antenna's vertical beamwidth, its height, and the desired coverage distance you may find our X-tilt program on the Celtools page useful.
  5. Refer to our application note Evaluating Antennas.
  6. Refer to our application note Antenna Painting Instructions.
  7. Polarization Quality Ratio (PQR) is a measurement that encompasses both orthogonality and cross-polarization discrimination in one measurement. Since poor cross-polarization discrimination and loss of orthogonality both lead to a decrease in diversity quality, it is important to make this a single measurement. This measurement requires two radiation patterns with the source antenna oriented at two orthogonal polarizations, co-polar and cross-polar to the nominal polarization. The same two source polarizations must be used for both antenna ports, and four patterns will result. Over the 3 dB and sector beamwidths, the worst case ratio between the two orthogonal polarizations shall be the figure used. This will be done for both antenna ports. (For example, an antenna under test has two ports - one at +45 degrees slant polarization and one with - 45 degrees slant polarization. For the first port, a measurement of the radiation pattern is made with the source antenna oriented at +45 degrees and then a second measurement is made with the source antenna oriented at -45 degrees. For the beamwidth of interest, the PQR is the worst case ratio between levels of the two orthogonally polarized, radiation patterns. The measurements are then repeated for the -45-degree port. Polarization is defined looking in the direction of propagation.) While this is not a direct measurement of orthogonality, it indicates a deviation from the desired polarization, in conjunction with cross polarization discrimination, and provides a measure of the quality of the polarization of the antenna. It is important to orient the source antenna to within +/- 1-degree of the polarization required, or significant errors can result.

    Pattern Tracking is defined and the maximum deviation in dB of the patterns of the two orthogonal ports over the 3 dB beamwidth. There will be separate values for azimuth and elevation planes.

  8. Very few people in the land mobile business have heard this term. We include a brief note in our Celwave marine antenna catalog. This is published by our facility in Hillerod, Denmark which manufactures a line of marine antennas. Essentially, 3 dB is added to the actual dBd or dBi numbers to arrive at "marine" gain figure. Why? Legend has it, some marketing person justified putting larger numbers in his catalog by claiming antennas perform better over water. This forced competing manufacturers to comply in order to sell marine antennas to buyers who go by the numbers. It can be very difficult to convince a purchasing department that your 10 dBd antenna achieves the same performance as another's 13 dBd antenna. If the same product is sold in two markets it could have a two specification sheets. The land mobile sheet would list the gain at 10 dBd, while the marine sheet would list the gain at 13 dBd. Our Cel-1 and Cel-3 cellular antennas are an example of this. Perhaps one of our customers has heard a different version of this story. Let us know and we will share it.

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