<story><title>Why 50 Ohms?</title><url>https://www.microwaves101.com/encyclopedias/why-fifty-ohms</url></story><parent_chain><item><author>XorNot</author><text>Trying to figure out what pipe to ask for when I have the physical measurements in hand is one of the ongoing mysteries of the world for me. &amp;quot;Oh that pipe is &amp;lt;some number you will not get with any type of measuring tool&amp;gt;&amp;quot; is one of the most bizarre conventions out there.&lt;p&gt;That said, this isn&amp;#x27;t even the only place it turns up - there are timber sizes which are specified as one dimension and just &amp;quot;commonly known&amp;quot; to actually be a different one: &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;www.thesprucecrafts.com&amp;#x2F;why-isnt-a-2x4-a-2x4-3970461&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;www.thesprucecrafts.com&amp;#x2F;why-isnt-a-2x4-a-2x4-3970461&lt;/a&gt;</text></item><item><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></item></parent_chain><comment><author>kube-system</author><text>Pipe was originally measured by inside diameter.&lt;p&gt;Anyway, over the past 100 years, metallurgy got better and they could make the same strength pipe with thinner walls.&lt;p&gt;So now, because of the thickness change, the old pipes that measured 1&amp;#x2F;2” inside and the fittings for those pipes were the right size for new pipes that measured about 3&amp;#x2F;5” inside.  They had the same outside diameter, but a larger inside diameter.&lt;p&gt;Instead of confusing everyone and requiring them to remember compatibility charts for every size pipe (and guessing about the age of the pipe), they just “solved” the problem by selling the new pipes under the name of the old size they were compatible with.&lt;p&gt;Tl;dr: Plumbers have technical debt that goes &lt;i&gt;way&lt;/i&gt; back.</text></comment>
<story><title>Why 50 Ohms?</title><url>https://www.microwaves101.com/encyclopedias/why-fifty-ohms</url></story><parent_chain><item><author>XorNot</author><text>Trying to figure out what pipe to ask for when I have the physical measurements in hand is one of the ongoing mysteries of the world for me. &amp;quot;Oh that pipe is &amp;lt;some number you will not get with any type of measuring tool&amp;gt;&amp;quot; is one of the most bizarre conventions out there.&lt;p&gt;That said, this isn&amp;#x27;t even the only place it turns up - there are timber sizes which are specified as one dimension and just &amp;quot;commonly known&amp;quot; to actually be a different one: &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;www.thesprucecrafts.com&amp;#x2F;why-isnt-a-2x4-a-2x4-3970461&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;www.thesprucecrafts.com&amp;#x2F;why-isnt-a-2x4-a-2x4-3970461&lt;/a&gt;</text></item><item><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></item></parent_chain><comment><author>jacquesm</author><text>The timber sizes are different because you are buying &lt;i&gt;planed&lt;/i&gt; timer, rough timber would have the exact dimensions you are looking for. So a 2x4 rough is really 2x4&amp;quot;</text></comment>