RG-8X vs. RG-8U coax?

[jazzsinger]

Nothing at all wrong with using 75 ohm cable with a "50 ohm radio". The feedpoint impedance of a properly installed dipole is a lot closer to 75 ohms than to 50.

That really depends on the antenna in use,

like you say it won't make any difference either way using 75 ohm or 50 ohm cable on a dipole as long as all the other cable characteristics are the same,either way you will have approx 1.5:1 mismatch, be it at the antenna end using 50 ohm coax or the radio/tuner end using 75 ohm coax.

the same could be said if you use a single run of 75 ohm coax of an odd 1/4 wave multiple to a quad with roughly 100 ohm feed impedance.As it will transform the 100 ohm impedance at one end to 50 ohm at the other.

there's probably other coax impedances that could be used too depending on the antenna in use and the required impedance transformation, but 75 ohm transformers are the most common.

 

[Beetle]

That really depends on the antenna in use,

like you say it won't make any difference either way using 75 ohm or 50 ohm cable on a dipole as long as all the other cable characteristics are the same,either way you will have approx 1.5:1 mismatch, be it at the antenna end using 50 ohm coax or the radio/tuner end using 75 ohm coax.

the same could be said if you use a single run of 75 ohm coax of an odd 1/4 wave multiple to a quad with roughly 100 ohm feed impedance.As it will transform the 100 ohm impedance at one end to 50 ohm at the other.

there's probably other coax impedances that could be used too depending on the antenna in use and the required impedance transformation, but 75 ohm transformers are the most common.

Yes indeed, and that's exactly how I feed my quad: one single run of RG-213 to a remote coax switch, and then 1/4-wave pieces of RG-11 for the 20M and 17M driven elements, and 3/4-wave pieces for 15/12/10. Reason for the 3/4-wavelengths is that 1/4-wavelength would have been too short and wouldn't have allowed the quad to rotate freely.

As for the possible 1.5:1 mismatch between 50 and 75 ohm coax - there's nothing at all wrong with that. I've used antennas/feedlines with a lot worse mismatches than that and gotten great results.

 

[CharlesS]

Years ago I used RG-8 on my base, but found out that the shielding and insulation were breaking down and the coax coating decomposing in the direct sun of SoCal.
Upgraded all my coax runs to a better quality coax that I wouldn't have to mess with for a long time.
All of the coax runs going up the hill are 100-130' so I decided on LMR 240 / LMR 400.
Discovered that the signal strength of RX/TX had increased since the coax had less loss and a higher velocity factor.
The link below shows the differences in dB loss or gain using different coax.


RF Parts Company ~ Coaxial Cable

 

[freecell]

TMS LMR400 has a line loss attenuation of .644 dB./100 feet (3.28 wl) @ 27.385 MHz.. With 100W. input to the line, under IDEAL load conditions, 13.8W. of power is dissipated leaving only 86.2W. to be delivered to the input of the antenna/load. If you're using RG8-X, RG8-M, RG8-U, RG-213 or RF-9914 your line losses via attenuation will be greater, more power will be lost. RF-9913 is compatible with LMR400 with regard to power / transmission line loss.