Elucidations and explorations
The September 2000 “Technically Speaking” column, on transmitter noise and receiver desense, requires some clarification, based on feedback from an experienced reader. His comments provide additional useful insights to topics covered in the past few months as well. This month we also have a question from a reader regarding contact corrosion problems with paging transmitters. If you have comments, questions, suggestions or information that you feel might benefit the readership of MRT, please feel free to send them to me at the mailing address or email address listed below, or submit them via the MRT Web site, www.mrtmag.com. Also, if you have found a good Web site that might offer tutorials or calculation tools that would benefit readers of MRT, please pass them along to me. The good leads will be published on the MRT Web site or in future columns.
First up is an excellent response to some of the topics covered in the September 2000 issue by William J. Kessler, P.E. Kessler has more than 40 years of experience in electronic and communications engineering, including university-level teaching, research projects, and extensive consulting engineering. He is a principal in Kessler and Gehman Associates, Telecommunications Consulting Engineers. His feedback concerns the measurement and/or calculation of isolation between antennas. You may wish to refer back to the figures used in the September column, which appeared on pages 18 and 20.
Kessler writes [text edited for space; figures are reprinted on page 20.-Ed.]:
“I noticed the following omission regarding the application of the free-space isolation formula:
“This formula is applicable only when both antennas are located well inside the Fraunhofer region of each antenna. The Fraunhofer region, (far field region) is that region where the propagated wave front from one antenna to the other antenna is essentially a plane wave.
“By contrast, the Fresnel region (near field region) is where the propagated wave front adjacent to the receiving antenna is spherical. This can result in an unacceptable phase variation of the signal at the receiving antenna due to radiation of the signal from different parts of the transmitting antenna. The line of demarcation between the near field and far field region is rather diffuse. However, most engineers have accepted the demarcation distance (d) to be equal to or greater than twice the dimension (D) of the larger antenna squared divided by the operating wavelength (l). That is:
“Therefore, when referring to the application of the free-space attenuation formula, the restriction that the distance separating the two antennas should be equal to, or greater than, the value of d as given in the previous equation.
“Your Figure 1 example is correct in that the two 6dB gain antennas operating at 150MHz are 500 feet apart. A 6dB-gain antenna at 150MHz (l 5 6.56 feet) would be at least 20 feet long. Therefore, according to the free-space propagation formula, the minimum separation distance would be:
“Clearly, the 500-foot spacing in your example in Figure 1 exceeds the minimum required horizontal separation between the antennas.
“With regard to Figure 2, which illustrates the vertical separation (V) between two gain antennas as measured from center-to-center, not all engineers are in agreement on how the spacing should be measured. Some engineers measure the separation distance between the top of the lower radiating element and the bottom of the upper radiating element. For antennas exhibiting gains of 6dB to 9dB, the substitution of the smaller distance into the formula would yield a lower isolation between the antennas than the substitution of the larger center-to-center measure.
Perhaps these engineers simply want to play it safe and use the separation measure that yields the lowest isolation figure. The problem I see with the end-to-end separation measure is that it yields an isolation figure that is independent of the size or gain of the vertically spaced antennas. On the other hand, I have reason to believe that the isolation figure calculated using the center-to-center separation distance between high-gain antennas yields a calculated attenuation (isolation) that is too high.
“I have calculated the isolation between a single dipole and a gain antenna (with as many as eight dipoles) by separately calculating the power coupled into the single dipole from each of the dipole arrays, ranging from two dipoles to eight dipoles and summing the result. In each case, the resulting isolation between the antennas falls between the calculated values for the end-to-end and the center-to-center separation measurement. As a result, I now calculate the isolation using the formula for both the end-to-end and center-to-center measures and use the average of the two.
“Although I agree with your statement in the Figure 3 caption that it is better to measure the isolation between two antennas that are spaced both vertically and horizontally, there is a formula for calculating the isolation for this case, where:
S 5 slant isolation.
V 5 vertical isolation.
H 5 horizontal isolation.
u 5 angle in degrees as defined in drawing.
“The formula appears reasonable because for terminal values of u it reverts to the horizontal separation formula for u 5 0 and the vertical separation formula when u 5 90.”
Kessler also included a graph comparing different methods of determining the isolation between collinear antennas or arrays. The graph is shown on page 18.
The next comment comes from a reader whose question is printed here in hopes of triggering some feedback with suggestions and “what you do” to deal with a common problem. Here is his question:
“We are a paging company that has about 2,000 Motorola Nucleus transmitters throughout the USA. The Nucleus has an external battery that keeps the exciter and controller running when there is a power failure. The printed circuit board contacts on the power supply that provide the charging for the external batteries are gold- plated and the connector is tinned. We have been having lots of problems with corrosion and reduction with that connection. The connection becomes a high resistance. The batteries charge OK, but when the power fails and the controller needs the 2A to run, the resistance causes the station to reset.
“Someone suggested we use Stabilant 22A to eliminate the corrosion problem. They claimed it worked great when they worked for Motorola. Motorola will not dig into this because they discontinued the product. Do you have any suggestions on how to deal with the corrosion problems?”
OK readers, if you have any suggestions that might help a fellow reader with this problem, please email me at the addresses given, and I will pass the information along to the readership in a future column.
You are always invited to use this column as a forum to pass along information, ideas and suggestions that might benefit your fellow technicians working in the land mobile radio field.
Until next time – stay tuned!
