[TowerTalk] outdoor junction shelters

[Wayne Kline]

Drew,

  Tried to stay away from this thread, but you point if splceing @ HF  jolted my reaction.
My Mentor and I know  friend of yours  Gerry Mathis  W3GM  was the king of wire nuts. The all plastic ones.
He had a 10 meter stack 100/50 ft 5 element  and a separate  10  4 element @ 20 ft  all switched with coax  transformers.  The junctions  were all made with wire nuts  and electrical tape.  His  stations 10 meter numbers were always at or near the top in many contests !

I am not saying this is the way to do these things BUT in hindsight  it works !

Wayne W3EA  ,
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10

From: Drew Vonada-Smith<mailto:drew at whisperingwoods.org>
Sent: Sunday, January 24, 2021 12:18 PM
To: towertalk at contesting.com<mailto:towertalk at contesting.com>
Subject: Re: [TowerTalk] outdoor junction shelters

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
------------------------------

Message: 5
Subject: Re: [TowerTalk] outdoor junction shelters
Message-ID:
        <57aa3524-9f0f-e4be-8511-e1bcb996060e at audiosystemsgroup.com>
Content-Type: text/plain; charset=utf-8; format=flowed

Argh! Blasphemy!

Anybody TDR such a splice?


------------------------------
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk