Coaxial cables are measured by impedance. The most common impedance values are 50 ohms and 75 ohms.

different cables. 50 ohms is most often used for radio transmitting and receiving applications.

75 Ohm is primary used for video and audio. 75 Ohm cables are the standard coax cable you find everywhere inside your home and office from the back of the tv to cable & satellite tv boxes and cable internet router. They are commonly used and are often pre-wired in many homes and businesses.

BNC (Bayonet Neill-Concelman) RF connectors make it easy to connect coaxial cables with radio-frequency equipment like radios and TVs, composite video on commercial video devices, and ethernet networks. They are available in 50 and 75 ohms models. Physically, the main differences can be found in the center pins and dialectric insulators. 75-ohm BNC connectors feature Teflon as a dialectric, and surround the outer spring fingers with air. Its center pin maintains a consistent diameter in both the front and rear areas (this is important—read on to find out why). 50-ohm connectors, on the other hand, use Delrin to surround the spring fingers, and its center pin is larger in the crimp area. You’ll need different crimp tools for each type of center pin.

If the connectors are 50 ohm, you are introducing impedance matching problems on your instalaltions, as if the impedance of cameras, DVRs and cables is 75 ohms. But most of DVRs and cameras in the market are manufactured with BNC connectors of 50 ohm impedance.

Sometimes there is need to convert between 50 ohms and 75 ohms systems. The web page https://electronics.stackexchange.com/questions/36168/how-to-build-a-75-ohm-to-50-ohm-converter/36186 gives some useful tips for impedance conversion:

A twelfth-wave transformer can match 50Ω to 75Ω with negligible loss and no adjustment (but at limited frequency range). It is a special case of a series-section transformer.

The Twelfth-Wave Transformer is often a more convenient alternative to the more well-known quarter-wave transformer

Choose Coax and calculate 1/12 λ of 50Ω coax. and 1/12 λ of 75Ω coax. for chosen frequency.. This works from DC to 1.5 x center F chosen. These two cables will transform the impedance for maximal power transfer. Choose the lowest loss cable you have avail. and best connectors to achieve less than 0.1dB loss for these short lengths.

For calibrated voltage output, set the generator to 0.83 times the desired output level in microvolts.

Convenient calculator: http://vk1od.net/calc/tl/tllc.php#NoteModellingLoss

Choose F, coax , 50 Ω load 1/12 λ option enter 0.083333 [wavelength], then calculate. Repeat for both 50Ω & 75Ω coax choosing type of coax as 1st entry.

Web page https://electronics.stackexchange.com/questions/234914/diy-75ohm-to-50ohm-impedance-matching-circuit-to-work-at-uhf-frequencies-around gives the simplest solution is to accept some signal (power) loss by using a two resistor solution like this:

But a better solution (less signal power loss) would be to use a transformer based impedance adapter. However these are more difficult to make.

Web page http://ham-radio.com/k6sti/match.htm gives the following advice:

Matching 50Ω to 75Ω

Most signal generators have an output impedance of 50Ω. To align an FM tuner or measure its performance, it’s best to match this to the tuner’s 75Ω input impedance. Mismatch loss is only 0.2 dB, but a source impedance that differs from the design value 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 43.2Ω and 86.6Ω. Loss is 5.7 dB.)

L-Network

An L-network is nearly lossless. This circuit isn’t broadband like a minimum-loss pad

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.

## 0 Comments

Be the first to post a comment.