This is what I gathered.  And what I didnt know before, thus the OP.  Thanks everyone, Im all straight.

Ya'll have lots of good ideas and starting points for me.  Thanks a lot.  Ill probably start off with a coax relay and use it HDX to begin with and later put up the vertical antenna and take some measurements.  Its a QS1R SDR.  Their webpage has 2 pieces of info:
The QS1R typically overloads at +9 to +10 dBm
You should not exceed ~3V pk-pk into 50 ohms. That is about 20 mW or 13 dBm.

Thanks again.  Hope to finally be on the air by the end of the year.

73, AK4YA

Lets say 18.1MHz at 54dBm/300watts.  I was hoping for the equation, that way any values could be applied.
Thanks!

If I have a radio whose front end can take up to 13dBm of input before overload/damage,  How much distance should there be between a dipole TX antenna and a vertical RX antenna, given no freq offset (both TX and RX are on the same frequency)?

Is there a rule of thumb or something?  I understand I should model both antennas relative to their position to each other, but as far as the distance between them, Im not sure at that point.

from http://www.w8ji.com/g5rv_facts.htm:

Thanks much.  I believe i understand.  So the relay in the amp must be able to provide a very high degree of separation between the rx and tx to prevent receiver damage?

Imagine a simple station, with a 100w HF half-duplex transceiver that has a single coax connector, (which means the transceiver has an internal TX/RX relay to switch the antenna between its TX and RX sections) a tuner, and an antenna.  Now say you want to add a section of power amps to reach 1500w output.  Do you simply stick the amps in between the transceiver and the tuner?  No TX/RX relays required?  If no relay required, then it looks like the amps will also be in the station's RX path from the antenna to the transceiver.  Is this how its done?

Now apply this to a full duplex radio that has 2 connectors, but you want to operate it as a HDX radio and use a single antenna.  So obviously, you need an external TX/RX switch.  With a few stages of amplification to reach, say, 1500w or 61.76dBm.  Where does the TX/RX relay go?  If the relay's 3 connections are between the radio's rx, the final PA stage's output, and the antenna, then the relay will require an insanely high amount separation.  But is this the recommended way?  Seems a bit dangerous.  Or can the relay go between the radios TX and RX connectors and the 1st stage amp's input, leaving the TX amplification in the staions RX path?

im having second thoughts on my antenna plan - glad I did this thread.  my cousin just graduated with a civileng degree, maybe he can point me to someone who can take a look at it and tell me its limits.

thanks all for the help.  maybe i will be on the air sometime this year.

OK, your work gives the following 2 equations:

sagdistance = (antennaweight*spanlength)/(4*counterweight)
counterweight = (antennaweight@spanlength)/(4@sagdistance)

Your example of 6ft of sag with a 50lb counterweight and a 120 ft span fits these numbers.
My chimney is actually 27ft.  Now, assuming I can make it by with 7ft of sag (which gives me a 20ft antenna at its lowest height) over 135ft span, the calculation says Ill need a 49lb weight.  Will a tree swaying with 49lbs rip a chimney off???

so I should use pulleys at both ends?  why wouldnt just at the chimney end be sufficient.  they chimney wont sway, its the tree that will sway.

Very interesting.  I suppose that "5" in the "5/50" is the weight of the antenna (10) divided by the number of ends supporting it (2)?

antenna/feedline is about 10lbs. distance from the tree/chimney is 130ft, since the antenna is 102ft I have a fairly decent range to pick from to go between the antenna end and the chimney.  Why is this important (not an engineering major)

Thanks!

Yes it will drop to a wood fence to run to qth.  I know its impractical to wire it straight as a razor, i was just speaking in engineering terms so as so start a conversation about weight requirements.  My main concerns are do I need a center support, and how much weight on the pulley end will I need to appropriately support the weight of the antenna as straight as needed for Ham radio requirements?

I definitely don't want to put much strain on a 30 ft bricked chimney

Got the MFJ model in this week.  Can I just string it between a 30ft tree and my 30ft chimney?  And drop the ladderline to the ground with no center support?  If so i plan to use a pulley on the chimney end with a weight to handle sway.  And dacron from the trees to insulators.

How much weight will be required on the pulley end?  I believe it requires infinite weight to get it 100% completely horizontal straight, but in our world, how much droop is acceptable?

Thanks everyone, hope to be on the air soon

Ive actually gotten far enough on http://www.fourier-series.com/rf-concepts/smithchart.html tutorials to understand what you've described.  But this all needs to happen at the antenna end of the transmission line, right?  So how does an antenna tuner do things?