[Friday: 3.4 miles; Today: 5 miles]
So…
I have this 75 Ohm CATV hardline already in place from my radio room to the top of the tower. I have been using it for my VHF (144 MHz) antenna.
I am thinking that I want to take that antenna down and use the CATV feed line for the WMFH radio station (95.5 MHz). The antenna is going to be 50 Ohms and the transmitter likes 50 Ohms, so the 75 Ohm line is not the best match.
So, I found this information on the web about 1/12th wavelength transformers and I’ve built two of those, or attempted to. Today I am testing my transformers using another length of CATV line (~ 80 ft?) and a 50 Ohm dummy load.
My transmitter has an indicator on the front for forward and reverse power in watts. I’m not sure I trust this reading completely, but that is what I have to work with.
| Condition | Foward Watts | Reverse Watts | VSWR (aiming for 1.0) |
| 50 Ohm line to dummy load | 77 | 1 | 1.26 |
| CATV to DL | 65 | 5 | 1.77 |
| Txfmr — CATV — DL | 93 | 7 | 1.76 |
| Txfmr — CATV — Txfmr — DL | 89 | 4 | 1.54 |
The transformers do help in comparison to using the CATV line directly but maybe not as good as using 50 Ohm feed line. But it is probably going to be good enough to start with, and it means I don’t have to spend $300 and run a new line up the tower.
The whole thing about feed line impedance is complicated by the fact that the length of the line becomes an issue if the VSWR is not 1.0. The VSWR figure for a single frequency will be different depending on the point of the line where the measurement is made. The CATV section I am testing with is a different length from what is on the tower so the numbers are going to look different there. I’m hoping it will be at least as good and maybe a bit better.