Perhaps this should be another thread, but digression is the order of the
day here on Usenet... SO, Steve continues.
"Electrical vs. physical length of an antenna." I am going to pick on
this bone here and stick my neck out. Correct me if I'm wrong (Like I have
to say that here...) I think there are two posters, Alex and Cecil who
used the term "electrical Length" in reference to antennas (copied below)
and like its correctness. I haven't personally focused on this use of the
term until now because I knew what was _meant_ when it was used, but after
some thought I feel this is a very misleading use of the term. Particularly
for the beginner. Here goes...
In a previous post, I talked about a "physically short" antenna that was
then "tuned" by adding some kind of a "loading" device. Here, "physically
short" means that it is shorter than: some other length, which ,
naturally has no reactive drive impedance. A half wave dipole, that we are
all familiar with, being the simplest to understand. This is what I am
calling a "natural length" antenna which has no reactance in its drive
impedance.
This antenna _IS_ a half wave long (don't nit pick about end effects,
etc., please) It naturally resonates - by this we all (I hope) mean that it
looks all real with no reactance - or zero phase between V and I.
NOW, we cut some off the antenna and "shorten" it. It is no longer 1/2
lambda long.
Whoa! it has some reactive part in the drive Z. AND it ain't the usual
72 ohms, or whatever, for the real part either. !
Cripes ! now what do we do ? (obviously inserting the rhetorical
question). We all know that an antenna Z just has to be all real, right?
Well that's another thread, because it doesn't have to be, but let's shelve
that one for later. Let's say that we REALLY want an all resistive drive Z,
and that's ok. Well, being experts in using one reactance to cancel
another, we just put another with the opposite sign in series and
presto...all resistive. DONE.
OOPS The real part ain't 72 ohms anymore! WA-HAPPEN" ?? Well, most of you
know. This shortened antenna ain't 72-jx ohms when we cut it up, it went to
something like 40-jx ohms and all we sis was strip off the -jX to leave the
40 ohms real.
So let's look at this. We now have an antenna that is certainly not a 1/2
wavelength long any more RIGHT???
AND... to make matters worse...
The real part we wound up with ain't 72 ohms any more either, RIGHT ??
SO how in blazes can we say that this, in any way, shape or form resembles
a 1/2 wave antenna !huh? huh! tell me, huh?
Well, the only way is that we got it to be "resonant". and the 1/2 wave is
resonant.... Well, that doesn't work with the mental model I have in my
brain, buster.
Someone might like to say that we took the wire we cut off and wound it up
into a coil and stuck it back on, so we still have a 1/2 wave total. Except
my extremely educated and experienced (not to mention arrogant) gut tells me
that you're going to have quite a bit of wire left over when you get all the
X out. So there must be more to the story.
WAIT ! WAIT ! you say. Do we _NOT_ talk about electrical length and
physical length when on the subject of transmission lines, huh? huh? say it
now...we DO don't we? you can't deny it, can ya?....
Hold on there partner, I say. You're mixing your apple basket with the
bananas. There's a glitch in the nomenclature soup.
Chew on this for a while. Transmission line has a physical length, yes, but
measured in feet, inches and the like....right. This is a physical length
measured in the 3 dimensional space we all have grown to know and love.
HOWEVER, transmission line has electrical length measured in ...fanfare
please... DEGREES. Degrees ain't a length. We can equate it to some
ruler length, but that is not what we are after in this case. Well, we know
that we need to be talking about the wavelength of our signal when it is in
the transmission line. That is what matters, not the physical length,
because the wave length in the transmission line is shorter than in air. We
need to know where on the _wave_ we are, because we are concerned about
phase at this point. So we CAN relate a specific PLACE on the line which
gets us to the Phase we want.
Now for the final muddy water exercise...
If we desire to keep this term and use it consistently, then we must talk
about the electrical length in _degrees_, no?
BUT, BUT, BUT you say... half a wave length is 180 degrees, so therefore it
is ok to use the wavelengths measure for this thing called "electrical
length".
OK, lets go there... Lets talk about what we could call an "electrical
length" for this antenna. How many _degrees long_ is the aforementioned
shortened antenna?
Just so you are sure to know: Here's the final mud.
What two points are YOU going to use for this "Electrical length" In other
words; Just what is the antenna?...The radiator only? The radiator AND the
coil. Does the coil radiate, is it or is it not part? Take your pick.
In other words, this is a highly questionable and indefinite and confusing
way to talk about an antenna. I don't think "electrical length" should be
used for an antenna. I don't think it makes sense because you are really
talking about, in the first approximation, whether you have gotten back to
you favorite place called resonance --nothing more -- - which has nothing to
do with length, but rather impedance matching with (usually) lumped
elements. And besides, how do you KNOW with 100% certainty that this
"shortened 1/2 wave" [let's say a 32 ft dipole on 40M] has been
"lengthened" (by use of loading coils) to 1/2 wave length rather than, say
oh, I don't know...3/2 "electrically"??
[[yea, yea. I know there has been a never ending thread on here about what's
going on in the antenna loading coil---current at the ends, phase and
probably a zillion other things with words flying all about]]
For a transmission line it is all very clear. For antennas, there are miles
of room for confusion so my vote is to - cut that out with the beginners
My brain hurts--why do I do this....
--
73, Steve N, K,9;d, c. i My email has no u's.
The two posts in question
=============================
In article , Jimmy wrote:
Is an antenna that is resonant on 10 meters still a 1/4 wavelength antenna
if it is physically only 4ft long or would this be an 1/8 wavelength
If the antenna is an electrical 1/4 wave then it is a 1/4 wave antenna, even
if it is physically shortened by use of coils.
Alex / AB2RC
================================
"Cecil Moore" wrote in message
...
Jimmy wrote:
Is an antenna that is resonant on 10 meters still a 1/4 wavelength
antenna
if it is physically only 4ft long or would this be an 1/8 wavelength
antenna(more or less).I am saying this should be called an 1/8 wl
antenna
though I am arguing with those who generally know more more about this
than
I. Not all the old timers disagree with me, so I am betting this is a
pretty
common problem when discussing antennas.
What you have is a *physically* short antenna that is *electrically*
1/4WL long. This usually involves a loading coil.
Also, the electrical length of a piece of coax is longer than it's
physical length because of the velocity factor less than 1.0.
--
73, Cecil http://www.qsl.net/w5dxp