75 ohm cable to 50 ohm coax length calculator

[Retroradio]

To get the impedance to 50 ohms a 75 Ohm cable can be put inlin at the antenna. The net said predominately 1/4 wavelength, Some say 1/2 wavelength and following the formula (VF etc..) its a different length entirely.
So, the question is whats correct. If majority is correct that I have found its 1/4 wavelength.

This is part of winter experiments so any help and knowledge would be a appreciated.

 

[prcguy]

A length of 75 ohm cable can be used to match some things to 50 ohms but it depends entirely on the impedance you want to transform. For making a phasing harness to match two 50 ohms antennas to a 50 ohm its already figured out. If you want to match anything else it could take 1/8 wave or 1/4 wave or something else or the mismatch may not be able to be fixed using this method. Every situation will be different.

To get the impedance to 50 ohms a 75 Ohm cable can be put inlin at the antenna. The net said predominately 1/4 wavelength, Some say 1/2 wavelength and following the formula (VF etc..) its a different length entirely.
So, the question is whats correct. If majority is correct that I have found its 1/4 wavelength.

This is part of winter experiments so any help and knowledge would be a appreciated.

 

[Ubbe]

The voltage and current are at different phase in the coax. I don't remember how much and can be dependent of the coax type but perhaps its 45 degree, 1/8 wave. Where the voltage are at max the impedance are at max and where in the wave and phase angle the current are at max will the impedance be at its lowest. And the lenght of a wave are dependent of its frequency. So it will be frequency dependent, it will change impedance as soon as you start to move away from the frequency, and the lenght of the coax needed will also depend of the coax type.

/Ubbe

 

[Retroradio]

So basically the one formula I found ( which worked out just above 1/8) and the overall consensus of a 1/4 wave is the the way to go . Interesting that the majority go with the latter and there doesn't seem to be a consensus. Its all fun so I’m going to make up one of each and experiment. After all this hobby is all about experimenting....IMHO
Thanks!

 

[WA8ZTZ]

1/12 wave matching section may be what you are looking for

 

[prcguy]

I don't see where the OP mentions what he is trying to match to 50 ohms. Is it 75 ohms to 50 ohms or something else?

 

[Delta2_Coms]

Without knowing the impedance of the antenna at the end of the line, a formula won't do much good. If 50-ohm coax is being used for the transmission line, and the antenna is something other than 50-ohms, the impedance will vary along the entire length of the 50-ohm coax. In other words, the length of the the transmission line will have a part in determining the impedance the transceiver "sees".

It may not even be necessary to insert a piece of 75-ohm coax if the antenna match is already close. So another option is to experiment by shortening or lengthening the 50 ohm coax you already have. Experimenting with different lengths of transmission line will yield the best results for your setup...regardless of whether it's the 50-ohm already in place or a inserting a piece of 75-ohm.

If the antenna itself is not close to 50-ohms, then using the 75-ohm piece may be necessary anyway.

-All the Best

 

[R0am3r]

@Retroradio - I'm not sure what you are looking for, but I used a piece of RG-6 75 ohm coaxial cable as a matching section for my 30 Meter Delta Loop. The formula for the length of the matching section is as follows: Q-Match Coax (in feet) = 246 x [velocity factor] / Frequency (MHz). The RG-6 cable I used had a velocity factor of 66 percent.

For my loop, the length of the matching section worked out to: 16.04 feet = (246 x .66) / 10.125 MHz

I modeled the antenna with EZNec. Send me a message if you want to see the results on my website. This loop is a fun antenna to build and has decent performance. I have used this loop to finish WAS and DXCC (mixed CW and FT8) on 30M. Please note that I fed my loop on a corner of the triangle for a low take-off angle.