Balanced Feed Line and Antenna Tuner Impedance
A legal limit antenna tuner might still arc at much less power than the rating if the impedance of the antenna system is low. The wattage rating on the tuner is based on a specific impedance and reactance, which is ideal for the current rating of the capacitors, inductors and switches used in the tuner. The antenna and balanced feed line together provide the antenna system impedance specific to a frequency. Equal currents to not assure balance and low SWR does not necessarily mean everything is working well in the antenna system.
Modern antenna tuners use an internal 4:1 balun for balanced feed line. You can verify that your antenna tuner uses a 4:1 internal balun with an antenna analyzer. You need a 50-ohm coax jumper with no connector on one end to use with your antenna analyzer.
To determine the tuner's balun impedance:
1. Disconnect the wire jumper on the antenna tuner going between the balanced feed line and the random wire connector.
2. Connect the coax jumper to the analyzer and with the other end use the center conductor wire and connect it to the balanced line terminal of the antenna tuner where the wire jumper was previously originating when jumped to the random wire connector.
3. Connect the shielding of the jumper to the antenna tuner ground terminal. Do not connect it to the other balanced line terminal. The antenna tuner should be grounded along with the shielding of the coax jumper.
4. With the ladder line connected to the balanced feed line posts of the antenna tuner, and one of the ladder line conductors connected to the same post as the center conductor of the coax jumper on the specific post that was previously used with the wire jumper on the antenna tuner, you can now take an impedance reading using the antenna analyzer.
You can change the impedance by reducing or increasing the length of the ladder line. An impedance value that is too low can be a serious problem, greatly limiting the amount of power you can use when you transmit before arcing occurs within the matching system. The antenna tuner works best with between 50 and 600 ohms impedance.
The impedance will repeat the element to ladder line impedance every 1/2-wavelength times the line velocity factor. Changing the length of the ladder line will change the impedance reading where the ladder line terminates in the shack.
When you operate on a different frequency and band, the impedance value for the specific length of ladder line you are using will change. This goes back to the wavelength being the variable. Check the impedance match for the center of the portion of the band you will operate for each band. Check impedances across the band.
It will become necessary to use multiple lengths of ladder line if you wish to maintain optimal impedance values on each band. This relates to two specific concepts of operation, one being achieving better efficiency so that less of your signal is going to waste and secondly if you plan to operate using an amplifier and do not wish to fill the shack with RF or experience problems with arcing in the antenna tuner.