[TowerTalk] outdoor junction shelters

[Drew Vonada-Smith]

Really, no blasphemy at all.  Negligible effect, and easy to show with any modelling tool.

Lets say, for worst HF case, this is at 10 M.  Think of the splice as a section of transmission line that is the wrong impedance.  That's really all it is, resistively it is fine, the connection quality is good, it is just not constant 50 ohm s impedance.

If the center conductors wires are hanging in open air, let's take a wild guess and say the characteristic impedance of that section is 300 ohms.  I'm going to guess that the splice is 4 inches long.   Again, these are pretty much worst case figures.  A wavelength at 28 MHz is about 421 in, so let's call our splice 1/100 of a wavelength or 3.6 degrees.

Now, take your favorite transmission line tool, and design a "matching section" of a 300 ohm line, 3.6 degrees long in your 50 ohm system and calculate the effect seen by the source.  Answer - negligible.  

This is a bit oversimplified, but the takeaway is this.  There is no magic in coax or connectors, and the physics of transmission lines are scaled to wavelength.  If you do anything that to a line that is a small fraction of a wavelength, the effect is correspondingly small.  There is nothing at all wrong with splicing coax by wirenuts at HF, if you could make it waterproof and reliable.

This, by the way, is the same reason your "non-constant impedance" PL-259 connectors do no harm up into VHF, and you don't need to use "N".  Too short a "transmission line" to matter much.

73,
Drew K3PA
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Message: 5
Subject: Re: [TowerTalk] outdoor junction shelters
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Argh! Blasphemy!

Anybody TDR such a splice?


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