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102" Whip Antenna Analysis

C2

C2

Sr. Member
Aug 3, 2005
2,408
79
158
102 inch whip mounted top dead center of a 4dr sedan with a ~4 inch spring. Total antenna length from sheetmetal to tip is 105.9375 inches.

Measured with the highest dollar and mostest accurate antenna analyzers I could get my hands on.

(Actual vehicular with what I call a standard 1/4 wave, unloaded radiator, and real reflectomoter using FDR with highest RF interferance immunity)

Make of it what you want.

102c.gif


That one looked suspicious so I got better equipment (read higher resolution):

A1.gif


Hmm...same result!

I don't have a 6" spring or a ball mount handy, but the formula to calculate resonance is basically linear, so it's easy to figure out for your frequency.
 

the swr graphs are nice but the smith charts are meaningless
as long as they're referenced to a Zo of 1 + j0. i have already
told you that once before. until the chart is adjusted for Zo =
50 + j0 at center sphere the resistance and reactance figures
are meaningless.

The centre of the SMITH chart is at gamma = 0 which is
where the transmission line is "matched", and where the
normalised load impedance z=1+j0; that is, the resistive part
of the load impedance equals the transmission line impedance,
and the reactive part of the load impedance is zero.

THE "1" IN z=1+j0 IS NOT THE RESISTIVE PART OF THE
LOAD IMPEDANCE AND IT DOES NOT EQUAL THE
TRANSMISSION LINE IMPEDANCE for our purposes here
therefore your results and any conclusions drawn from them
are INCORRECT. on the one hand the swr graphs indicate
that the real resistive part of the impedance is somewhere
near 50 ohms while the smith charts represent something
closer to one and they are in total disagreement with one
another. you're obvioously a newbie to smith charts and
you're using them incorrectly.


i think most of us here know that the resistive part of the
impedance present at the feedpoint is much higher that 1
ohm. in your smith charts the only way the load is matched
to the line is if the line Zo = 1, Zo in this case representing
the characteristic impedance of the line. i don't think so.

here is an example of a smith chart based on a Zo of Z =
50 +j0, or 50 ohm transmission line. note that the "50" is in
the center of the chart. do you get it yet?

loop-1-801MHz-suppressed-040331.jpg


50 Ohm Smith Chart

using your figures in the second smith chart: R = 1.13 X = 0,
if we perform a little correction for Zo = 50 + j0:

1.13 X 50 = 56.5 ohms resistive. 56.5 / 50 = 1.13 VSWR.
the chart is not yielding the resistance but instead is
providing the reflection coefficient of the circuit. notice
the similarity of the (r) reflection coefficient in the smith
chart and the stated swr in the graph gui........the (r) is
NOT RESISTANCE.
 
Interesting post.
Is this data based on readings taken while stilling still? I would be interested to see results of data taken while in motion at different speeds to see if this has an impact on the results. This is a mobile antenna and I'm wondering if the statements I have heard regarding change in SWR while moving are true.

regards,

Wayne C.
 
how much of a change is there due to movement? I currently run a wilson 5000, which I am happy with. I am about to set up my second car and was considering a 102" whip, but I will be in motion.
 
The smith plots are de-normalized plots, meaning that you need to take the value and multiply by the characteristic impedance.

It does not matter that the plot is de-normalized and if you ploted the same thing on a normalized plot, the trace would be in exactly the same place.

For the first plot, r=1.18 x=0.00 (unitless). So just multiply by 50 ohms to get the normalized impedance R=59 X=0 ohms.

You may not be a noob, but your narrow minded. Obviously the center is not 1 ohm, it is simply 1 (unitless). The plots are in exact agreement, your just too blind to see it.

This system allows you to measure systems with any characteristic impedance, whether it be 50 ohms 75 ohms, 300 ohms, 450 ohms, 3 ohms or whatever.
 
"The smith plots are de-normalized plots, meaning that
you need to take the value and multiply by the
characteristic impedance."

that's correct and i believe that i demonstrated that in
more than one previous post.

the (r) value in your swr graphs and smith charts is not
resistance, it's the reflection coefficient. it would be a
lot less confusing to plot the chart around a 50 ohm Zo
since that's what we're all used to working with or simply
point out that since you insist on using the de-normalized
plot that the value for (r) is the reflection coefficient and
not the resistance as some seeing it here may wrongly
interpret.

i'm not being narrow minded, just practical. if the chart is
capable of producing direct results without any further
calculations required then it just makes sense to use the
chart in the proper mode, particularly for those not familiar
with smith charts and their functions.

of course the 1 = unity (or does it) but now you can see
how that without certain information available how that
could be construed (and misrepresented the figures justify it)
to mean exactly what i alluded to earlier. just for future
reference if you're going to continue to use the chart
in that fashion then you might want to specify the Zo
on a per case basis or use Zo = 50 + j0 by default and
revert when other characteristic impedances are called
for. that will be a lot less confusing for everyone.

the smith chart is just a plot of complex reflection overlaid
with an impedance and/or admittance grid referenced to
a 1-ohm characteristic impedance. how 'bout that?

anyway, getting back to the purpose of this thread it's
easy for everyone using your graphs to see that if your
<106" antenna was lengthened further that resonance
would occur closer towards 27.185. i think that it's pretty
clear already that resonance is going to occur closer to
108" than the 234 formula would indicate. thanks for the
input. if you're interested maybe later i'll explain to you
why the 234 formula didn't work here, regardless of the
VF of the metal being used. if you stand there and look
at the antenna in the mounting location you described it
should be pretty easy to figure out.
 
freecell said:
since you insist on using the de-normalized
plot...
Click to expand...

I'll take it up with the multi-billion dollar industry leader, Agilent (HP), and their equally expensive "engineers."

But I don't think they will change the design of their $20K reflectometer to accomadate a freecell on a CB/ham forum.


freecell said:
...the value for (r) is the reflection coefficient...
Click to expand...

NO!

For the first plot, the VSWR at M1 = 1.134:1 @ 27.50 MHz, at the same marker on the smith plot, r = 1.13, which IS the de-normalized resistance (normalized to 56.5 ohms).

It is NOT the reflection coefficient!

At the same marker:
The return loss is 24.04 dB
The reflection coefficient is ~ 0.063
The transmitted power is 99.61%

granted, I did cut off the bottom of the graph where is says Z0 = 50ohms

but I really dont see why it makes a difference and the plot is, like I said, in agreement with the VSWR plot.

Your just making an issue of something for the sake of making an issue.

Big whoop, I fixed it. Same plot!

102c2.gif
 
yeah, that's another point of confusion because the
reflection coefficient can only be derived from the swr
dependent on impedance at the particular plot points.
to refer to the (r) as reflection coefficient was my mistake.
i was merely pointing out the similarity between the VSWR
and what the smith chart was reporting as (r). that won't
happen again. have fun with the antenna.
 
freecell said:
anyway, getting back to the purpose of this thread it's easy for everyone using your graphs to see that if your <106" antenna was lengthened further that resonance
would occur closer towards 27.185. i think that it's pretty
clear already that resonance is going to occur closer to
108" than the 234 formula would indicate...should be pretty easy to figure out.
Click to expand...

It is interesting that the 246 constant seems to be supported, but my measurement has many uncontrolled variables.

I read the web site where you got that smith plot long ago, and took opportunity to revisit the measurements there. You choose the loop plot, but if you look at the dipole plot, the result is even more interesting, and possibly related to our topic.

In that plot (which looks a bit too perfect...series capacitor?), resonance occurs at 546.8 MHz with a dipole length of 257 mm.

I solved for the constant "C" that we are mainly discussing here:

C/F(in MHz) = L (in feet)

C = (546.8 MHz)*(257 mm)/(304.8 mm/foot)

C = 461 for 1/2 wave dipole.

So 1/2 of that is 230!

I guess it's not easy to figure out, for me anyway :(
 
I was not going to test the whip further.

I think the task now is to understand the results. The results are what they are, that is why I looked at the other (hopefully) real world results on the other site, especially considering that they have addressed many more variables than I...

With that in mind, I feel further away from drawing any reasonable conclusions since the two sets of results I have available are contradictary.

I'm looking at my setup once again and wonder why 234 din't work? I mean the wave is traveling through metal, so it should slow down and be shorter than in free space.

One other point that seems to have been glossed over...the Smith plots and data (r and x) that I am able to work with, graphically anyway, are normalized, not de-normalized.
 
Just for the heck of it, take a look at the antenna section of the amateur Handbook. Someplace in there it'll give the reasoning behind the '234'' number. Deals with the relative size of the conductor used. Then, when you take into account that all of the "magic" numbers for finding wave lengths (or fractions of them) are for practical use, you can see why there are, or can be, differences in results. They are 'ball-park' numbers, as in pencil and paper results, not computer computations to the 27th decimal place. Always 'fudge factors' involved when translating between mathimatical computations and 'real world' results, unless you're willing to do some really fantastic definitions of environmental variables...
- 'Doc
 

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