<story><title>Why 50 Ohms?</title><url>https://www.microwaves101.com/encyclopedias/why-fifty-ohms</url></story><parent_chain></parent_chain><comment><author>phkamp</author><text>The choice of 75 Ohm is documented in ultimate detail in Bell Systems Technical Journal.&lt;p&gt;Here is a 1934 article as an example, long before anybody had even named microwaves:&lt;p&gt;&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;archive.org&amp;#x2F;details&amp;#x2F;bstj13-4-532&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;archive.org&amp;#x2F;details&amp;#x2F;bstj13-4-532&lt;/a&gt;&lt;p&gt;75Ohm gave the lowest loss and thus the longest distance between repeater-amplifiers on the trans-continental carrier-telephony coax-cables.&lt;p&gt;These same cables (and their brethern: microwave links) also carried television, thus enabling the first &amp;quot;television networks&amp;quot; as opposed to &amp;quot;television stations&amp;quot;.&lt;p&gt;In many cases even the stand-alone television stations paid AT&amp;amp;T to connect their down-town studios to the hill-top transmitter.&lt;p&gt;So television uses 75Ohm because AT&amp;amp;T did.&lt;p&gt;And AT&amp;amp;T did to minimize the number of repeater amplifiers across USA, because 75Ohm has the lowest loss.</text></comment>
<story><title>Why 50 Ohms?</title><url>https://www.microwaves101.com/encyclopedias/why-fifty-ohms</url></story><parent_chain></parent_chain><comment><author>IndrekR</author><text>There is also another story [1]. That standard air-core coax lines were built using regular plumbing pipes in US. As the impedance of a coax is proportional to &lt;i&gt;log(inner diameter of the shield&amp;#x2F;outer diameter of the core)&lt;/i&gt; this generated standard impedance around 50 ohms for many different tube configurations.&lt;p&gt;(10 minutes later)&lt;p&gt;However, I just calculated this for standard copper tubing size [2] and this probably is just a myth, except for few specific tube sizes like 1&amp;#x2F;4+5&amp;#x2F;8 or 1.5+3.5 inches.&lt;p&gt;For some reason, tubing is still mostly given in inches and neither its OD nor ID correspond to tube size. OD is always 0.125in larger than tube size.&lt;p&gt;[1] &lt;a href=&quot;http:&amp;#x2F;&amp;#x2F;www.rfcafe.com&amp;#x2F;references&amp;#x2F;electrical&amp;#x2F;history-of-50-ohms.htm&quot; rel=&quot;nofollow&quot;&gt;http:&amp;#x2F;&amp;#x2F;www.rfcafe.com&amp;#x2F;references&amp;#x2F;electrical&amp;#x2F;history-of-50-oh...&lt;/a&gt;&lt;p&gt;[2] &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;www.petersenproducts.com&amp;#x2F;Copper-Tubing-Sizes-s&amp;#x2F;1979.htm&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;www.petersenproducts.com&amp;#x2F;Copper-Tubing-Sizes-s&amp;#x2F;1979....&lt;/a&gt;</text></comment>