[Elecraft] OT: Feedline Question

[Guy Olinger K2AV]

The effect of spacers is a very complex equation. One large modifier to the
end result is the number of spacers per unit of length. Then there is the
issue that the relative permittivity in between spacers reverts to air away
from the spacer. Then there is the area on either side of a spacer where
the effective permittivity blends from that of the spacer to that of air.

Then there is the loss based on number of spacers and loss tangent of the
spacer material modifying the lesser air loss.

Beyond that there is this peculiar ham inclination to use indoor insulated
electrical wire (THHN - rated dry indoors inside conduit only, 600 volts at
60 Hz) for QRO outdoor RF purposes. E.g.
https://www.73cnc.com/product_p/ls31.htm

Figure the wildly variant composition of PVC insulation at manufacture
followed by years of deteriorating UV outdoors.

In the end it is far easier to construct a feedline with a scientific guess
to obtain a target Z zero and then measure and adjust design to hit it on
the head and/or reduce its loss.

In practice there is little difference between 400 and 450 that is not
soaked up by all the tuner or circuitry finaglement we must engage in to
convert a wild range of Z to the narrow, narrow range actually tolerated by
our transistor finals.

Wireman for years has been selling various window lines we all
euphemistically call "450 ohm" which in fact vary between 360 and 440 ohm Z
zero. The reason for the variance is the spacing on all those lines is
identical regardless of the variation in wire diameter.

The constant wire diameter reduces the manufacturing setup and run costs
passed on to a cheep penny pinching ham population. [I include myself as
afflicted by this penny-pinching malaise. I just try to keep this nearly
irresistible inclination from dragging me into stupidity, as it has already
done on some number of occasions.]

73, Guy K2AV


On Tue, Apr 18, 2017 at 11:00 AM, k3hx at juno.com <k3hx at juno.com> wrote:

>
>
> Message: 4
> Date: Tue, 18 Apr 2017 03:31:59 +0000 (UTC)
> From: Al Lorona <alorona at sbcglobal.net>
> To: "elecraft at mailman.qth.net" <elecraft at mailman.qth.net>
> Subject: Re: [Elecraft] OT; Feedline question
> Message-ID: <852881305.2998918.1492486319834 at mail.yahoo.com>
> Content-Type: text/plain; charset=UTF-8
>
> If instead of using the relative permittivity of air, 1.00059, for
> calculations of open-wire line, you use the effective permittivity
> including any spacers used to homebrew the line, this value will increase
> slightly.
>
>
>
> For instance, I am using spacers made of a material called "nylon 6,6 30%
> glass fiber-reinforced" which happens to have a relative permittivity of
> 3.9. This has the effect of increasing the effective permittivity between
> the wires from 1.00059 to 1.08656. Another way of saying this is that the
> velocity factor goes from 1 (for pure air) to 0.959, or about 96% with the
> spacers. The effect of this is to make the characteristic impedance of the
> line drop from around 480 to around 460 ohms.
>
> ( See http://www.emclab.cei.uec.ac.jp/xiao/Wire/index.html )
>
> This obsessive exercise on my part illustrates that spacers don't make a
> whole lot of difference, but they do make a difference. You may want to
> include their effect or not.
>
> Al  W6LX OM Al, Thank you for taking the time to share your results on
> this forum.
> "That which is not rigorous is meaningless.".......Attributed to Blaise
> Pascal The results may not be significant or even of minor interest to
> many, but how would we know had this work not be done and presented here.
> 72, Tim Colbert  K3HX
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