matching50to75homs

8/2/2019 matching50to75homs

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Matching 50 to 75

Most signal generators have an output impedance of 50. To alignan FM tuner or measure its performance, it's best to match this tothe tuner's 75 input impedance. Mismatch loss is only 0.2 dB,

but an unintended source impedance may alter the RF inputcircuit bandwidth or resonant frequency. This can degrade front-end tracking and affect intermod or desensitizationmeasurements.

Minimum-Loss Pad

A simple minimum-loss pad provides a broadband match. Usechip resistors or the shortest possible lead lengths to minimizestray inductance and pickup of local broadcast signals. Loss is 5.6dB for the 5% values shown. For calibrated voltage output, set thesignal generator to 1.55 times the desired output level in

microvolts. (For 1% resistors, use 43.2 and 86.6. Loss is 5.7dB.)

L-Network

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

88108 MHz

Page 1 of 6Matching 50 to 75

23/02/2012http://www.ham-radio.com/k6sti/match.htm


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This shows an L-network in a small metal box. The inductor ismade of #14 wire and the capacitor is a small mica trimmer. Iadjusted the turns spacing and the trimmer for best return lossover the FM band. Response must be checked with the box coverin place.




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This shows the return loss of the L-network with a coaxial 75load from 88 to 108 MHz. I used an HP 141T/8553B/8552Bspectrum analyzer, 8443A tracking generator, and Anzac THV-50power splitter. The return loss of the power splitter with a 50load was a constant 30 dB across the FM band. This is the limit ofthe test setup. Results beyond this figure indicate cancellation ofthe residual return loss.

The L-network loss measured 0.05 dB. For calibrated voltageoutput, set the generator to 0.83 times the desired output level inmicrovolts.

Twelfth-Wave Transformer

A twelfth-wave matching transformer, a special case of a series-section transformer, will match 50 to 75 with negligible lossand no adjustment.




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The transformer consists of a 75 coax section in series with a50 section, each about one-twelfth wavelength long andconnected to the opposite terminal impedance.

To splice the sections, cut the cable jackets back , strip thedielectric , overlap the center conductors, and solder them.Place a piece of slit dielectric over the joint, put a dab ofsuperglue in the slit, overlap the shields, and solder them. Cover




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the splice with heat-shrink tubing.

The calculated section length for solid-dielectric cable with avelocityfactor of 0.66 is 6. Install a BNC connector on the 75cable and an F-connector on the 50 cable. The location withinthe 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 center-conductorlength each connector requires, and for the splice overlap, whicheffectively shortens the 75 section .

Cables with higher velocity factor require a longer section length.For example, 0.83 is common for foam dielectric. Such cablesrequire a length of 0.83 0.66 6 = 8. This set oftransmission line utilities contains a series-section calculator youcan use to design a transformer for any kind of cable.

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




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This is calculated mismatch loss plus cable loss.

Like the L-network, the twelfth-wave transformer loss measured0.05 dB. For calibrated voltage output, set the generator to 0.83times the desired output level in microvolts.

Matching 50 to 300

Vacuum tube tuners provide 300 antenna terminals. They mayconnect to a balanced RF transformer, with a floating or

grounded-centertap primary, or to an unbalanced input circuitwith one terminal grounded. For a grounded centertap, connect a50:75 matching network to one antenna terminal and nearbychassis ground. For a floating primary, ground one terminal anduse the following matching network.

Loss is 13.3 dB. For calibrated voltage output, set the generator to1.88 times the desired output level in microvolts.

January 28, 2011


matching50to75homs

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