As you are combining with an existing service (your CATV provider) they use professional grade modulators, you might not have to go that far ($2,500 and up) you will have to use commercial grade (rack mountable) units to band pass or block and to modulate, the price range of the project could be between $450 and $900
You might want to analyze first other options.
Among other options I would consider in running paralel wiring (wiring is cheapper) and use a remote control AB switch some TV sets bring two antenna inputs.
If you want to send (transmit) HDTV (Line A/V; composite, etc.) you could build up a CAT 5 or CAT 6 network and install a HDMI receiver to each TV set.
Other option could be install more IRD's from your satellite provider.
All these depend on the number of users you intend to feed with this service or signal.
If you still want to go for your first idea, in my personal opinion the best way to do it goes something like this:
This kind of combination need skills and some professional tools to make combination as good as possible, I will try to explain in the most simple way as possible.
Assuming your house is wired with RG-6 and you have a distribution center also called head end, this distribution center most likely is located near the first drop or division in your house, there you will feed, divide, split, combine, etc.
I would buy first a Agile CATV modulator, there are several manufacturers: Holland electronics, Blonder Tongue, Pico Macom, etc.
PLM860SAW : 860MHz Channelized-Agile PLL SAW Filtered A/V Modulator
This one from Pico Macom sells for over $210 plus S&H; the good thing from this one is it goes up to 860 MHz most cable operators will cover up to 750 MHz or even 850 MHz; don't know if they will go up to 1000 MHz; the tool needed for this is RF field strength meter (too expensive as you aren't going to use this so often) I have found used units at a good price in ebay (arround $250) of course there are units up to $5,000 that will cover even digital.....
The advantage you have of knowing the power you are receiving the signal from your cable operator and then you can combinate with your agile modulator evenly (balanced) so one wont fade or mask the other...
All this needs to be made in the first area the CATV enters the house (Your house headend), you will have to run a coax from your IRD to this headend (no splits), other coax cable should be run from this headend to your cable modem (directly, no splits) and then you have you old cable that serves all your users areas (tv's).
If you use Broadband (internet) from your cable operator, I personally would separate them first so you wouldn't affect the two way communication since you are TX from 5 - 50 MHz and Rx in any other frequency from 50 MHz to 850 MHz.
To separate Broadband from CATV I advice to use a Digital ready 2 way splitter:
Buy 2 Way Cable Splitter for Cable TV
The good thing from this unit is it has Blocking capacitors on each splitter port produce low intermodulation distortion figures that are essential in optimizing broadband network performance.
Connect broadband to the output labeled (S) it has red stipe.
The other port will be used to feed your house with images and combine with your private circuit (IRD).
This is the best splitter-combiner I would advice:
Channel Plus 2-Way Splitter/Combiner - 2512 - Smarthome
combine this with your agile modulator and your CATV provider.
you might need to power up signal levels from your CATV provider since most likely they will provide you with not more than +15 dBmV or at least +5 dBmV (this is when the meter is good) the agile modulator will give +55 dBmV differential is between 40 dBmV and 50 dBmV you have to buy at least a drop amplifier like this:
CDA-1P : 1 GHz 1 Out Bi-Directional Amp (Passive Reverse)
That unit covers 5 MHz to 1 GHz and has a max output of +23 dBmv.
I advice not to serve the house with more than +25 dBmv due to the fact it could create distored images on some TV sets, as a matter of fact all tv sets will be perfect with +0 dBmv but you have to consider insertion loss, cable run loss, etc.
To balance with +55 dBmv of the agile modulator, the unit has a potentiometer to adjust RF output levels, try not to adjust Aural carrier (hopefully it comes adjusted from factory to -15 dBmv from video carrier) the other potentiometer you can adjust is audio (for volume level) and video for clarity-darkness, I advice image to be set as dark as possible, clarity sometimes create audio distortion.
If after you combine you don't find free carriers (channels) that you can use with your agile modulator, then you might need a single channel eliminator like this:
CEF750 : 750 MHz Single Channel Elimination Filter
This must be requested due to the fact they manufacture them customized.
Here's a chart so you have an idea of frequency/channel:
http://200.78.236.213/catalog/Appendix.pdf
This is the only procedure I could state should give you a reliable combination.
Good Luck!!!!
