freddaniel
Member
YES, you can eliminate most receive dropouts on handhelds by replacing your base or repeater antenna with a non-polarized or circular polarized antenna. Most dropouts occur due to cross-polarization of the received signal. Remember your radio theory that states as the angle of signal arrival rotates past 45 degrees causing up to 3 dB signal loss, continuing the rotation to 90 degrees will cause up to 30 dB signal loss, otherwise a factor of 1000. See attached chart. To illustrate, if you rotate your handheld 360 degrees, 50% of the rotation is 0 to 3 dB loss and the remaining 50% will be 3dB to 30dB loss. I do not want to oversimplify the issue to just trying to maintain vertical polarization while using the handheld. The fact remains unless you are visually in a line-of-site with the transmit antenna, you will not be receiving a vertically polarized signal. As you wander farther from the transmit antenna, the signal WILL bounce and the polarization WILL change in unpredictable ways. Signal multipath will also occur with various polarizations and amplitude. It is generally recognized that 85% of the time, the signal polarization will not arrive vertical, so it would be a waste of time trying.
To make things worse, LMR manufacturers have driven the market away from the simplicity of analog, into the vastly more profitable digital realm. Unlike analog receivers that produce noise when a signal fade occurs, allowing the user to simply reposition the radio for better reception, digital receivers provide no indication of signal fade, but simply go silent. This causes the typical problem where one of two handheld users, walking together, hears a conversation and the other does not. They assume one radio is bad and sends it off for service.
You could also say multipath could cause the problem, but it is far worse with digital than analog, since analog does not require 100% bit-error-rate compliance to simply work. With analog, unless the multipath signal is exactly the same amplitude AND 180 degrees out of phase, it will not corrupt understanding of the signal but may make it sound strange.
Even though the LMR industry has known this for more than 30 years from cellular experience, the major manufactures typically do not offer dual-polarized or circular polarized antennas in the LMR frequency ranges. The typical solution is to simply flood the area with the maximum transmit ERP to overcome the issue, rather than engineering for it. This causes greater co-channel interference between markets due to the interference contour expanding, thereby reducing the availability of spectrum.
Let us pressure our manufacturers to make these antennas!
To make things worse, LMR manufacturers have driven the market away from the simplicity of analog, into the vastly more profitable digital realm. Unlike analog receivers that produce noise when a signal fade occurs, allowing the user to simply reposition the radio for better reception, digital receivers provide no indication of signal fade, but simply go silent. This causes the typical problem where one of two handheld users, walking together, hears a conversation and the other does not. They assume one radio is bad and sends it off for service.
You could also say multipath could cause the problem, but it is far worse with digital than analog, since analog does not require 100% bit-error-rate compliance to simply work. With analog, unless the multipath signal is exactly the same amplitude AND 180 degrees out of phase, it will not corrupt understanding of the signal but may make it sound strange.
Even though the LMR industry has known this for more than 30 years from cellular experience, the major manufactures typically do not offer dual-polarized or circular polarized antennas in the LMR frequency ranges. The typical solution is to simply flood the area with the maximum transmit ERP to overcome the issue, rather than engineering for it. This causes greater co-channel interference between markets due to the interference contour expanding, thereby reducing the availability of spectrum.
Let us pressure our manufacturers to make these antennas!
