Antenna FAQ

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Antenna FAQ's (Frequently Asked Questions)

Q. I moved my on-glass antenna from one car to another. It worked great on the old car, but now I have a very high VSWR? Why? A. Your new car probably has passivated glass --- glass impregnated with tiny metal particles that render the glass incapable of coupling RF between the inside and outside couplers. Q. How can I tell if the glass on my vehicle is suitable for on-glass antennas? A. First, look near a corner of a window for words that mean "sun", relate to sun, or refer to ultraviolet or other forms of radiation. If you find terms like "Soft-Ray", "EZE-Cool", "Solar-Coat", "Solar-Cool" or similar terms, your glass is probably passivated and won’t work with on-glass antennas. If you want to be 100% certain, and you don’t find helpful words on the glass, ask a 2-way or cellular shop technician to test your glass with a capacitance meter. Q. Why is a wavelength (or fraction of) in coax cable physically shorter than what I calculate from the formula for wavelength? A. 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. Q. 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? A. 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. Q. I’m using one of your rubber duck antennas on our electronics box. Our system hasn’t been working right, and when I tested the antenna VSWR I found it was very high. What’s wrong with your antenna? A. We need to understand what kind of rubber duck you’re using. Most of them 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. Q. I put an Ohmmeter between the center contact and the outer ring at the mount (bottom) end of a 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? A. 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 that yields 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. Q. I just installed a ________ antenna on my car, and I measure reflected power of 2 Watts with 55 Watts forward. Is this is o.k? Should I have 2 Watts reflected? A. Ideally we want no reflected power at all, but that’s seldom realizable in practice. Your antenna is performing to specification, because the percentage reflected power you’re measuring is only 3.6% of forward power. That means 96.4% of the power is transmitted. Most antennas have a VSWR specification of 1.5 or 2.0. Your example is equivalent to a VSWR of 1.47. Q. How do wireless repeater antennas work? A. The average vehicle is a reasonably good barrier to radio waves getting in and out. A wireless repeater creates a leak so to speak in the barrier. A signal from a handheld phone induces a tiny current in the inside antenna mounted on the glass. That current is directly coupled to an outside gain antenna and is radiated. Of course it works in reverse on received signals. For most installations, signals coupled through the two-antenna-glass coupler media are stronger than signals which must penetrate the vehicle unassisted. Q. Some Manufacturers do not specify gain for its portable antennas, whereas some manufacturers do. Why is this? A. Virtually all portable antennas compromise gain for small size and the convenience that 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.