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@whisperingwoods.org>
Sent: Sunday, January 24, 2021 12:18 PM
To: towertalk@contesting.com<mailto:towertalk@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@audiosystemsgroup.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Argh! Blasphemy!
Anybody TDR such a splice?
------------------------------
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