[Milsurplus] [Boatanchors] CU-872A/U antenna coupler FS near Boston

[Jim Whartenby]

Blame Bell Labs for the coax impedance compromise.  See: https://www.belden.com/blog/broadcast/50-Ohms-The-Forgotten-Impedance
Best impedance for power transmission: 30 ohms
Best impedance to withstand high voltage: 60 ohmsBest impedance for lowest loss: 75 ohmsThe BTL compromise impedance considering all of the above: 50 ohms
As already mentioned:Impedance of a vertical antenna is about 36 ohmsImpedance of horizontal antenna is about 72 ohmsHum, a two to one difference in impedance between these two antennas.....
Mismatch loss between 36 ohms and 50 ohms: 0.177 dB with a VSWR of 1.389 : 1Mismatch loss between 36 ohms and 75 ohms: 0.572 dB with a VSWR of 2.083 : 1Mismatch loss between 50 ohms and 75 ohms: 0.177 dB with a VSWR of 1.50 : 1Mismatch loss between 72 ohms and 75 ohms: 0.002 dB with a VSWR of 1.042 : 1So the compromise impedance holds for either a vertical or horizontal antenna impedance and a 50 ohm transmission line.  Depending on frequency and cable length, RG-8 attenuation loss could easily mask the mismatch losses of the above.

Can't calculate the Mismatch Loss and VSWR for the UHF connector since the impedance of the connector is not controlled; it could be anything.  Which makes the initial mismatch loss concern between 75 and 50 ohm cables connecting to the CU-872A/U antenna coupler meaningless if UHF connectors are used. 
BTW, the eBay price of UHF and N connectors is $2 v $4.44, not all that much of a difference IMHO.
Regards,Jim







  
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