What size of Toroid

[KM5PS]

I have bee looking for several days on the web and need a little help with what size of tororid to use for a 75ohm to 50 ohm unun.  Needs to be able to handle legal power output.  I will only be using abt 600 watts for now.  I have some issues with my 75 ohm CATV hardline into 50 ohm rg213 coax.  So thought I would try to build a unun to see how that will help. A little history on my current setup.  I have abt 20 feet of 50 ohm coax going from radio to first side of 75 ohm hardline, then abt 110 feet of 75 ohm hardline to tower, then 80 feet of rg213 going up to tribander on top of tower.  Same setup for six meters.  I have been testing some this weekend during the vhf contest and with my bird meter at the base of the tower I show abt 60 watts out and abt 8 watts reflected, this run has 100 feet of rg8x coax to the beam (not the best to use, but all I had at the time).  I would like to improve this if I can.  Any suggestion would be great.  But thought I would try to build a unun to match the lines as close as possible. 
Thanks for the help
John
km5ps

[WX7G]

Your antenna is presenting a 2.15:1 VSWR at the base of the tower. The antenna VSWR is higher than this (line loss reduces the VSWR at the measurement point). That is the main issue and not that 75 ohm coax is used for part of the run.

[WX7G]

The VSWR at the 50 MHz antenna is 3.0:1.

[G4AON]

50 Ohm and 75 Ohm line connected together can give all sorts of problems, as at quarter wavelengths of 75 Ohm line it will transform 50 Ohms into 100 Ohms as used in the matching sections on some 2 element quad antennas. See: http://www.antennex.com/preview/New/quarter.htm

I would discard the 75 Ohm line and replace it with 50 Ohms throughout, unless needed for a specific match somewhere.

73 Dave

[N3OX]

I think something like a FT-240-43 core would suffice for the UNUN you want to build but I have not actually run any numbers on that.

A 75:50 ohm high power UNUN doesn't need a huge core.

[G3TXQ]

The only loss figures I have to hand are for a FT240-K core. That would need a minimum of 35 turns on it to handle 1500W continuous and stay within Fair-Rite's recommended temperature limits at 30MHz. Assuming you would be using a quintufilar winding, that would be 5x7 turns. There might be an issue whether the 240 size core could accommodate that many turns.

But as others have said, I don't think the UnUn is where you should be putting your efforts!

73,
Steve G3TXQ

[KM5PS]

Thanks to all so far who have responded.  I realized that on the testing that I had done,I did not chk the antenna set for some reason, but I did last night and it was 2.5.  I thought it was a lot lower than that when I put it up, so I will be taking it down and adjusting it.  The reason for the 75ohm hard line is for the long run to my tower.  The tower is any 120 away from house and then another 85 feet up tower to antenna.  I was just tryin to cut down on the loss of power in feed line.  I did test the hard line on 20 mtrs last night and was getting ant 580 watts base of tower with 600 watts out of amp. Tribander shows ant 1.5 sad on it.  For now that seems ok for now.  So for now I will be working on just the 6 meter line and only 100 watts thru it.
Just thought I better explain my setup a little better.
Thanks again
John

[G4AON]

The reason for the 75ohm hard line is for the long run to my tower.

John, the mixing of 50/75/50 Ohm feeder will cause you all manner of problems as there is a transformer effect as per the link in my previous post. The effect will vary by band too.

For what it's worth, I am currently QRV on 6m with a 2 element quad that uses a length of 75 Ohm cable to transform 100 Ohms at the antenna feed point down to 50 Ohms for the run to the radio. You can't just mix feeders of different impedance without problems.

73 Dave