Hi Martin,
Martin Usher wrote: > our radios > have very little effective range at street level in a built-up area (but > a lot more effective range above the houses and trees, that's why > someone even a mile or more away can bring us down). >
How would the offending TX a mile away have "more signal strength" at altitude, and the one at the local field not demonstrate the same phenomena?
The signal strength falls off the square of the distance (double the distance - signal strength has fallen off at least 4 times) in both cases.
Vertical strength of signal (in dBm) versus the horizontal strength is different due to the GND plane (earth in this case) and antenna polarization (E-field). This is why attempts to measure effective range (actual - with the TX antenna extended) while the aircraft is still grounded (someone drives away with the TX while you stand there) are ineffective (probably 7-8x GND range).
Again...a (any) RX will respond to an on-frequency carrier that demonstrates the higher dBm (signal strength).
I'm not sure why folks wish to gravitate to the notion that somehow the system we have in place is so bad. Granted, if spread spectrum could be made to work it would move towards (but not entirely elliminate) being shot down by an unauthorized TX, and maybe ("BIG" maybe) reduce extraneous interference within the bandwidth chosen (which in itself islimited by what is available to us within the existing spectrum).
But what else would it do? It would cost more, the majority would have to trash equipment that utilize an integrated RF front end, and everyones RX's. I find it hard to imagine both system protocols working harmoniously as the local field.
Some seem to believe that because a couple of projects that utilize spread spectrum are out in the domain that it's just around the corner...it's not. To be able to comment requires a firm understanding of the technology, bandwidth requirements versus numbers of channels per TX, and the fact it varies (protocol), effective range because the carrier frequency (to get the bandwidth) would have to be at least a magnitude higher than 72MHz. Does everyone "just turn on" ( actually impossible), or does your TX use blue-tooth (eg: 802.11), and a central hub takes care of the communications protocol (which one...FHS, DSS, how about FSK or FDM). More B/W means a higher carrier, which equates to less effective range.
Go talk to the inovators using spread spectrum, you will learn that not only is their particular application limited by range, but also bandwidth, and more importantly a protocol that is limited to real-time commands of a rate which is in the stone age compared to what we are used to right now.
The above are legitimate hurdles that will have to be worked out before any sense of safety can equal, let alone surpass what we use now...
Simon Van Leeuwen RADIUS SYSTEMS PnP SYSTEMS - The E-Harness of Choice Cogito Ergo Zooom
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