There is no way I can begin to explain intercept point without a lot of unsane math and graphics.
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The short answer, at some signal strength and amplifier fed two (or more) signals will start to mix with each other, IE intermod.
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A real world example. On the corner of Broadway and Main Street the 2M [140~160MHz] from a variety of sources will cause most amateur 2M transceivers front ends to 'collapse' and you'd hear the wildest mix of transmitters.
The only amateur transceiver I knew that began to withstand the intense RF at that location had "helical resonators" in the front end between the antenna port and RF stages.
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There are two types of intercept.
IP2
https://www.everythingrf.com/community/what-is-ip2
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IP3
https://www.everythingrf.com/community/what-is-ip3
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The PA0RDT, and other active antennas, can be modeled on a very short antenna coupled to a broadband RF amplifier.
The PA0RDT antenna works because the output transistor can withstand fairly high Ic, much more then most other affordable transistors.
There are other ways to get there but the PA0RDT is the simplest.
The FJET input device presents an extremely high input impedance with a very low gate capacitance.
MOSFETs have issues, many issues, that make them unsuitable for sane LF~HF active antenna designs.
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One of the nightmares of cellphone antenna systems is they have to deal with a lot of closely spaced RF signals with wildly varied signal strenghts.
If the cellphone companies had their way, there'd be no UHF TV, no 900MHz Part 15 license free devices, nothing but cellphones about 400MHz (and yea that'd wipe out the 70cm ham band and 400MHz business band).
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RF can get complicated really really fast.
It requires obscenely priced equipment to begin doing decent, repeatable, tests.
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We (local SW nuts) use a park that is located roughly equidistant between the 3 Lexington MW (AM BCB) transmitters.
590/630/1300 kHz.
By varying the length of the antenna element feeding the amp of the active antenna, one can achieve reasonably accurate IP comparisons for MW.
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Feel free to go to college, pick up and EE degree, become an amateur radio operator, be lucky enough to work in a 2way shop for a summer, then be taken under the wing of a retired Ph.D. who helped design some of the then state of the art RF systems.
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Like a point to point RF link at ~20 that required high gain, highly directional antennas, the system operated below the actual local RF noise floor.
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Another issue is a strong enough signal will cause any amplifier to go into gain compression. And that my friend is a story for another day. [though it is related to IP]
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The PA0RDT was the first practical, extremely short antenna, active antenna that actually worked. Of course there are a whole slew of "you gotta do this" to make it work right.
Simply put, you have to stop EMI, radio noise, from your home from reaching the antenna via the feedline [coax]. This is an extremely challenging theoretical exercise and will drive you crazy doing it in the real world.
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The site has an explanation of how these antennas work and what must be done to begin to achieve this.
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There probably need to be two new IO Groups, one for "Active antennas" [active E-Probe antennas] and EMI, what it is and mitigation.
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The last one is even more of a challenge then E-Probe antennas.
I've considered starting them but to do the job right would require about 10 times the time I can devote to it, and there is no point doing anything half assed.
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This is probably the best single resource into E-Probe Active Antennas. I suggest reading it and following all the links. Too many will return 'dead' but most will work.
The W0QE Active Monopole Preamplifier is one of the better designs out there.
And Chris Trask's designs are excellent but a challenge to find since his webpage went dark.
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In short, RF is a pain in the tush. If I had any sense I'd pick a different hobby. But then if I had any sense I'd be rich.?
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