Impedance step up

[AC0YW]

Do I want to use a balanced step up coil to transform 125 Ohm to 500 Ohm open wire, then use a 4:1 balun and Heliax at the transmit end to come back down, or just forget the mismatch at the antenna? I am thinking that the ratio is 10:1 for a transformer Impedance to transformer ratio-10:1 "internal Z" and 4:1 ratio. Anyone help me here? Band is 40 meters and antenna is a multi-
loop array.Should I also have a cap across the transformer at the antenna to reinforce the 125 Ohm input to the antenna? I just started to think! I will use a 1/4 wave open wire transformer to step between the 500 Ohm line and the 125 Ohm antenna. #Sorry for the drivel and wasting your time folks!!! It's late.....

[KA0GKT]

What is the reason for this operation?

The radiation resistance of a 1-wave loop where the diameter of the wire is small is around 100-Ohms. This is a 4:1 VSWR, and with 500-Ohm open wire. 4:1 is a perfectly decent VSWR for low loss open feed line. Since a transmatch will be necessary to get the impedance to a point where your transmitter will like it, feed the loop with ladder line all the way from the shack.

You didn't mention for what frequency your loop will be built; however, 1/2-wave loops also work. A 1/2 wave loop has a feedpoint impedance of around 320-Ohms. Build the loop for 80 and use it on 160 too! With 500-Ohm transmission line, that will be around a 1.6:1 VSWR. Again, a transmatch will be needed to match the output impedance of your transmitter.

Ladder line is a very low-loss transmission line. Additional VSWR losses therefore are quite small.

Don't bother changing over to coaxial transmission line, for this situation, just use ladderline.

73 DE KAØGKT/7

--Steve