For many years since I was first licensed in the late 1960's I have made myself aware of radio coverage primarily for vehicular mobile stations but, to some extent for fixed base stations as well. I think it is eminently useful for Ham's to think of and appreciate there particular "radio picture" as they attempt communication with others. Thinking firstly of VHF mobile installations, lets consider how 1) frequency, 2) transmitter power, 3) receiver design, 4) antenna type, and 5) relative location effect communications.
When I was first licensed my main mode of mobile operation was on the 6 Meter wavelength band. I used a 10 Watt output Amplitude Modulated (AM) transceiver, and a quarter wavelength whip antenna mounted on the left rear corner of my car. I mention the specific antenna mounting because optimum radiation from such an antenna propagated best or most efficiently over the right front corner of my vehicle. With such an installation I could easily and regularly communicate from Anaheim to the extent of the coastline, and as far inland as areas such as Corona and Pomona. When I next installed a 25 Watt Frequency Modulated (FM) transceiver the same coverage was possible with the addition of more easily copied received signals, due to the inherent noise limiting of the receivers design. All of this communication was "simplex" with no repeater assistance, and by its nature "point to point". These days there are many repeaters on 6 Meters, and a 100 Watt mobile station in the LA basin can talk as far as Frazier Mountain with little difficulty.
When I first started relying more on 2 Meter FM in my car I became painfully aware of the limitations of signal coverage as effected by geography, and also by the typically reduced noise limiting not available from generally available Ham market transceivers. My antenna while improved in both relative gain and mounting location, still provided only basic 25 mile radius coverage with a 35 Watt transmitter and single conversion receiver. The receivers design was probably the most prominent reason for this but, its fair to say, 2 Meters does not provide nearly the same sort of communication quality as does the 6 meter wavelength band! Most of my 2 Meter operation was via mountain top located repeaters. Lets discuss typical repeater installations, and how signals can be enhanced, and what should be expected for a given installation.
First as has been mentioned receiver noise limiting is a must! Many 2 Meter receivers these days provide something on the order of 10 to 20 decibels of limiting. A commercial FM receiver from companies such as Motorola will by contrast provide 40 or more dB's of noise limiting. 50 Watts is probably a desirable power output to deal with most circumstances as one drives around in all areas of the basin. One hundred Watts or so might be nice from several areas but, often as little as 25 Watts will do. Keep in mind that both these figures are either plus or minus the 50 Watt mark by 3 dB's.
Mobile antennas for 2 Meters are quite varied in availability and gain but, typically a 5/8 wavelength whip antenna mounted on the roof of the vehicle will work quite well. Any antenna mounted below the level of the roof will experience notable signal deficiencies. Some antennas such as the Hustler "Collinear" though will work very well even when mounted on a vehicles bumper. We will lastly consider how the various "electronic sites", the common name for radio repeater locations, cover the basin but first lets talk about signal propagation at various frequencies.
We have already made some empiric observations about 6 and 2 Meter band signal propagation but, let me amplify on this. Using a Motorola MoCom transceiver with 100 Watt output and a automobile roof mounted "base loaded" quarter wavelength whip, I was able to maintain quality communication with a base station in Perris, California along the entire route as I drove from my home in Mira Loma to the Goldstone Tracking facility near Barstow. The Perris station was using a 40 Watt Pace transceiver. Using this same installation I could regularly talk to mobile stations in Fullerton, while I was in Mira Loma, and as far away as Redondo Beach at the edge of the LA basin. On 2 Meters I would expect 1/3 to half this distance to be possible given the same sort of radio "hardware".
On the Ultra High Frequency (UHF) band of 70 centimeters propagation can be quite interesting. Using a Motorola U44BBT "T-Power" transceiver. I regularly maintained full duplex communication with a Remote Base or Auxiliary Link Station mounted atop a multi-story building in Newport Beach's Fashion Island. I could do this from almost anywhere in the LA basin. This radio was in no way a "super rig"! The receiver was fairly deaf with 4 microvolts of "Front End" sensitivity for 20 dB's of signal quieting, and the transmitter put out about 12 Watts. The Remote Base ran about the same power output but, probably had a few dB's better receiver. My antennas were quarter wavelength whips on the roof of my Ford van. Present day Ham equipment on this band can easily equal this performance, except for being full duplex in receive and transmit!
On the 222 MHz. or 135 centimeter band I have had generally good results. The mobile radio I most often used was a Midland 13-513. This radio provides 20 Watts transmitter output, has about a 2/10th microvolt receiver that provides something like 10 dB's of limiting. I should mention though that the noise floor is very low on VVHF, as it is on UHF. The antenna I used was a 5/8 wavelength whip atop my van. With this installation talking to Remote Bases in the Whittier Hills or even the flat lands of Santa Ana's Mount Foon was no problem. Communication was nearly always full quieting over nearly the entire LA Basin and Orange County. This sort of performance is also quite available with any decent mobile radio today.
Factors of propagation that can effect most of these frequencies are:
I have listed these in the order of most typically observed instance or usage, and will discuss them in detail below. Never be fooled with the old saw that "VHF is line of sight only"!
In the Los Angeles Basin Thermal Inversion is a common if not painful occurrence. This effect is what causes the smog to thicken and blanket the basin. It also keeps VHF propagation similarly "ground locked" below the inversion ridge. Or, at the least, its fair to say that it often causes a significant signal attenuation between a station near sea level and one that is atop one of our 5000 foot mountains or higher. This can be visually demonstrated by submerging a pipe or straight pole beneath surface of a swimming pool. The water deflects or refracts the image of the pole in much the same way as the Inversion Layer bends a radio frequency signal.
Tropospheric Ducting is an occurrence that reveals itself quite seasonally in the months of Spring. During this time repeaters from distant areas can be heard as if they are local. This is often the bain of low level repeater owners. I know of one such group that every year complains of a repeater in Ventura County for "upping their power". I also know of a wonderful occurrence in 1976, when the folks of the Johnstone Peak 146.820 MHz. repeater turned off their machine for nearly a week so that Ham's in LA could talk to the ".82" machine on Diamond Head in Hawaii.
While this method of communication is seldom exercised or employed, Knife Edge refraction can allow VHF stations to communicate over mountain ranges. I have noticed that my 1282.925 MHz. repeater knife edges quite nicely down into the desert near Adelanto and points north out toward Boron California. My machine is located along Highway 18 in the San Bernardino mountains, and has a major ridge that separates it from the desert.
One last thing that should be mentioned about the upper UHF range, which most Hams commonly think of as Super High Frequency (SHF) or microwave. 1200 MHz. has very intermittent or difficult signal path usage for mobile applications. Even though there is virtually no noise floor, and FM receivers seem to have 50 dB's of limiting, signals can be up, down, and anywhere in between dependent on local foliage. You can have a nice full quieting signal from a repeater, then park across the street from a tree that is opposite the repeaters direction, and the signal vanishes as it is clouded by the moisture surrounding the tree! Handi-talkies don't make it on 23 centimeters!
Here is a listing of common locations or "electronic sites" for repeaters and Remote Base Stations. As you communicate with any of these peaks or ridges, think of where you are keeping in mind other hills and the like that may be between you and the radio site. Get used to reading your S meter as you use these different "machines", especially if your radio provides a truly logarithmic S meter. Also, ask Ham's on the repeater how well your signal is quieting the repeaters receiver.
Dial around, and use these mountain top systems to learn how radio propagation works in the Los Angeles basin area.
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© 2002