[SEDXC] Antenna and feedline (Re: is list very quiet or am I no longer subscribed?)
Frank T Brady
franktbrady at comcast.net
Sun Nov 9 16:54:29 EST 2014
Oy vey !!!
*Outstanding* response Tom, but the truth sure hurts at this point - I
have an awful lot of thinking to do now.
73 and thank you very much for the much needed information.
Frank W0ECS
On 11/9/2014 2:15 PM, Tom W8JI wrote:
> Hi Frank,
>
> I have a somewhat long answer, but I hope it is useful. :)
>
> Balanced lines radiate for five reasons:
>
> 1.) The line conductors are not balanced in current level at every
> point along the entire line length
> 2.) The line conductors are not exactly 180 degrees apart in phase at
> every point along the line
> 3.) The line conductors are unbalanced with voltage with respect to
> surroundings at any point
> 4.) The voltages are not exactly 180 out-of-phase with respect to
> surroundings at any point
> 5.) The conductor spacing is noticeable compared to the wavelength
>
> Of the above five, the only one anyone ever seems to pay attention to
> is the first one, current levels, and they look at that just at one
> point and generally as a scalar parameter without considering phase.
>
> If we disregard phase.... which almost everyone does.....a balanced
> line can have exactly equal currents on two conductors and be 100%
> out-of-balance. As a matter of fact MFJ sells a meter, and there have
> been many articles on building meters and testing feedlines in QST and
> other places, that only consider scalar currents (and only at one point).
>
> If you have a hand-held meter that samples the electric field, such as
> a system with a small whip antenna, primary sensitivity is to the
> electric field. The meter is just reading the voltage difference
> between the probe and the instrument case, and if you have a hand near
> it, to you and whatever you are around.
>
> The antenna system can easily have nearly equal and opposite currents
> and show a problem with e-field reading near the antenna. You probably
> have a completely uncalibrated probe without any quantitative
> measurements, so we don't have any idea if it is a problem or not.
>
> With all that in mind, that the measurement method is of very limited
> value, there are a few general truths:
>
> 1.) Higher impedance and/or wider spaced lines are more problematic
> for things around the line and for line balance. A 600 ohm line, all
> things equal, is worse than the typical 400-ohm window lines we use
> (they are virtually never 450-ohm lines, even though they are called
> 450-ohm)
>
> 2.) The input impedance of the line determines balun effectiveness,
> and that impedance varies greatly with antenna, line length, and line
> impedance.
>
> Lower impedance lines are generally better. The best line impedance
> for minimizing SWR extremes would be the geometric mean of the
> antenna's lowest and high impedances. If the antenna impedance ranges
> from 50 ohms to 4000 ohms, the mean impedance is the square root of
> 50*4000, or 447 ohms. At 50 ohm frequencies a 450 ohm feedline has a
> 9:1 SWR, and at 4000 ohm antenna frequencies the feedline has 9:1 SWR.
>
> This means, depending on line and antenna length and on frequency, the
> line input impedance is somewhere between 50 and 4000 ohms in that case.
>
> A line other than 450 ohms would expand the impedance range
> potentially seen at the transmitter end. Say we use a 600 ohm line
> with the same antenna impedance range. Now we have a line SWR of 12:1
> at 50 ohms, so the high impedance could be 12*600 or 7200 ohms
> (depending on line length) and the low impedance 600/12 = 50 ohms.
>
> We make things worse by using a higher impedance line or a line away
> from the geometric mean of the antenna impedances, not better. As a
> matter of fact in most installations, a 300 ohm line is better overall
> than 450 ohms. The general trend is something less than 450 and above
> 300 or so is best.
>
> A 600 ohm line is worse than a 300-400 ohm line for input impedance
> excursions in multiband applications, and it is generally more
> problematic for line radiation. You were trying to correct wet weather
> changes in the line. Unfortunately, a higher Z line makes the system
> more sensitive to line radiation and line length.
>
> That's life with antennas. We reduce one problem, and some other issue
> becomes worse.
>
> 73, Tom
>
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