Matching 50 to 75Most signal generators have an output impedance of 50. To align an FM tuner or measure its performance, it's best to match this to the tuner's 75 input impedance. Mismatch loss is only 0.2 dB, but an unintended source impedance may alter the RF input circuit bandwidth or resonant frequency. This can degrade front-end tracking and affect intermod or desensitization measurements.

Minimum-Loss Pad

A simple minimum-loss pad provides a broadband match. Use chip resistors or the shortest possible lead lengths to minimize stray inductance and pickup of local broadcast signals. Loss is 5.6 dB for the 5% values shown. For calibrated voltage output, set the signal generator to 1.55 times the desired output level in microvolts. (For 1% resistors, use 43.2 and 86.6. Loss is 5.7 dB.)

L-Network

An L-network is nearly lossless. This circuit isn't broadband like a minimumloss pad, but it easily covers 88108 MHz.

This shows an L-network in a small metal box. The inductor is made of #14 wire and the capacitor is a small mica trimmer. I adjusted the turns spacing and the trimmer for best return loss over the FM band. Response must be checked with the box cover in place.

This shows the return loss of the L-network with a coaxial 75 load from 88 to 108 MHz. I used an HP 141T/8553B/8552B spectrum analyzer, 8443A tracking generator, and Anzac THV-50 power splitter. The return loss of the power splitter with a 50 load was a constant 30 dB across the FM band. This is the limit of the test setup. Results beyond this figure indicate cancellation of the residual return loss. The L-network loss measured 0.05 dB. For calibrated voltage output, set the generator to 0.83 times the desired output level in microvolts.

Twelfth-Wave TransformerA twelfth-wave matching transformer , a special case of a series-section transformer, will match 50 to 75 with negligible loss and no adjustment.

The transformer consists of a 75 coax section in series with a 50 section, each about one-twelfth wavelength long and connected to the opposite terminal impedance.

To splice the sections, cut the cable jackets back , strip the dielectric , overlap the center conductors, and solder them. Place a piece of slit

dielectric over the joint, put a dab of superglue in the slit, overlap the shields, and solder them. Cover the splice with heat-shrink tubing. The calculated section length for solid-dielectric cable with a velocity factor of 0.66 is 6. Install a BNC connector on the 75 cable and an F-connector on the 50 cable. The location within the connectors where the impedance changes isn't obvious. I assume from the end for the BNC and for the F-connector. When cutting the cables, account for this, for the centerconductor length each connector requires, and for the splice overlap, which effectively shortens the 75 section . Cables with higher velocity factor require a longer section length. For example, 0.83 is common for foam dielectric. Such cables require a length of 0.83 0.66 6 = 8. This set of transmission line utilities contains a series-section calculator you can use to design a transformer for any kind of cable.

This is SWR calculated for a twelfth-wave transformer made of solid-dielectric RG-58 and RG-59 and cut for 98 MHz.

This is calculated mismatch loss plus cable loss. Like the L-network, the twelfth-wave transformer loss measured 0.05 dB. For calibrated voltage output, set the generator to 0.83 times the desired output level in microvolts.

Matching 50 to 300Vacuum tube tuners provide 300 antenna terminals. They may connect to a balanced RF transformer, with a floating or grounded-centertap primary, or to an unbalanced input circuit with one terminal grounded. For a grounded centertap, connect a 50:75 matching network to one antenna terminal and nearby chassis ground. For a floating primary, ground one terminal and use the following matching network.

Loss is 13.3 dB. For calibrated voltage output, set the generator to 1.88 times the desired output level in microvolts.