[SEDXC] Antenna and feedline (Re: is list very quiet or am I no longer subscribed?)

[Tom W8JI]

Hi Frank,

I have a somewhat long answer, but I hope it is useful. :)

> I chose a 135 ft doublet fed with 600 ohm open line to eliminate the 
> weather variations I had with a doublet I fed with window line.  I also 
> wanted to try my variable feedline length relay system originally built 
> for my old window line fed doublet in order to select the optimum feedline 
> length for each of the 80 thru 10 bands.  My hope was that narrowing down 
> the width of my 600 ohm line as it passes through the relays would be such 
> a short distance, relatively speaking, that it wouldn't cause any serious 
> problems.  I also had to 'droop' 20' of wire at each end of the 135' 
> flattop due to space limitations (80m is very important to me as I only 
> need 11 countries to get 8bdxcc).
>
> The system (sorta) works - the relay system and control software finds and 
> switches in the correct feedline segment lengths to get good swr on all 
> bands.  *However* - I noticed wierd stray RF problems on some bands and 
> when I took a field strength meter out to the antenna area, I found that 
> the meter pegs when I get it within a few feet of the feedline.  I also 
> peg the meter when I put it near my TV cable on the outside of the house - 
> which is within a few feet of my feedline for a short distance. I don't 
> know whether or not such a measurement meaningfully indicates unwanted 
> feedline radiation but it sure doesn't seem right, and I do have lots of 
> RF everywhere.

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