IF loop on modulator
 

I have got a number of agile modulators in use as short-range NTSC broadcast beacons. I have tried replacing the coaxial cable in the IF loop(s) with F-connector-to-phono-jack adapters & then using a high quality CVBS cable instead. It seems to have a better image quality on high contrast scenes this way.
Any reason this would be less than advisable? FWIW, the cord lengths are all under ~1 ft.

The video cable probably has a different impedance than the 75ohm RG6 coax it was designed for and it may be producing a favorable effect(there could be side effects though). Your not likely to blow anything, but there may be uneven response across the signal bandwidth as a result.

My theory is that F-connector to phono jack adapters probably wouldn't exist if using such cable would pose a danger to your equipment. I assume the cable you're using is shielded.

It is a shielded 75-ohm composite video cable.
What type of favorable effect could this result in? When I said it seemed to have better image quality, I meant the resolution looked a bit better...I suppose there could be deviations from the optimal across the channel bandwidth but I don't have the equipment to measure the video response.

If the bandwidth was not as even as it should be to start with and it is managing to balance that effect...Either in the mod or in the whole signal chain from video source to screen. That effect could be considered favorable.

Kind of a long shot I suppose.
I used a Leader LCG400 analog pattern generator to set up the modulators so that varying programme material wouldn't overmod the units. I need to get my hands on more sophisticated test gear like I used to have so that I can take the mystery out of questions like this.
For now the signals look good on my restored TVs so I'm happy

This may be technically incorrect (or not, if it's the same impedance as the original coax), but if it's boosting the high frequency detail and not to the point of overmodulating (buzz on white titles), go for it.

Not sure what's happening here, But I just did an A-B compar-o with RG-6 and the composite video cable, and the latter won hands-down on some test patterns for sharpness...it is hard to discern but it is a bit better with the video cable. These little improvements all add up in the end...