Total posts : 45366
[quote:2433e61d8c=”RadioheadC”]…what is a reasonable signal strength (based on your table) that a car radio should be able to pick up (a) with decent listenablilty and (b) as a marginal but recognizable signal (i.e. threshhold before disappearing into the static of background noise, or even another station)? Would that be like 2 mv/M for (a) or another number?[/quote:2433e61d8c]
A good car radio has an FM sensitivity of about 1 uV for monaural signals, in the absence of interference. Full stereo sensitivity is much higher, maybe around 500 uV, below which a typical auto receiver “blends” progressively back to its monaural mode. Without interference (ie, electrical noise and/or other stations), a 1 uV/m field will produce fairly quiet reception for monaural signals — maybe an audio S/N of around 30 dB. The receiver needs maybe 10X that to drive it into full limiting and best S/N (maybe 60dB).
[quote:2433e61d8c]Also, what does that third (rightmost) column mean? Is that the number of mV/M which is 6 dB above the middle column?[/quote:2433e61d8c]
Yes. This should be considered an upper limit for received signals at that distance, which can occur only when the direct wave from the antenna and a reflection of it from some nearby surface are equal, and in phase. This rarely happens in reality, though. And if the two are out of phase, they will cancel, giving the receiver nothing to receive.
[quote:2433e61d8c]Finally, what (if any) is the dB gain of a half-dipole anetenna?[/quote:2433e61d8c]
If you meant a 1/2-wave dipole, its peak gain is 2.15 dB above an isotropic radiator. An isotropic radiator is a theoretical, reference antenna with equal gain in all directions. It doesn’t exist in the real world, but is a useful reference when evaluating other antennas.
[quote:2433e61d8c]And what kind of signal loss can one expect feeding it, say with 25 feet of RG-8/U or RG-58 and common Radio Shack style (but soldered) PL-259 connectors?[/quote:2433e61d8c]
The efficiencies of 25-foot lengths of RG-8 and RG-58 are both 90% or better at 100 MHz. But the line loss really doesn’t matter — it is the field radiated by the antenna that counts. Even if the line loss was 90%, the tx power could be raised to compensate for it, and the peak field from the antenna would be the same 250 uV/m at a distance of 3 meters as if there was no line loss at all.
But this means you must be able to accurately measure the maximum field in any direction 3 meters from the antenna, which takes some good test equipment and skills, not to mention the difficulty of doing that while the antenna is 25 feet or more off the ground.