1/2 wave vertical Impedance ???
 

I heard that a 1/2 wave vertical has a Z of about 2000 ohms at the base.
Is this the Z between the base of the vertical and ground that RF at the
resonant frequency
'sees' ?

Doesn't the height that the vertical base is above ground affect this
2000 ohm valve?
Or does then metal car roof nullify any height above ground affects on
the Z?

I've noticed that measuring the return loss of a 1/4 vertical is very low.
But measuring the return loss of a 1/2 wave or 5/8 wave antenna is
higher.??????????????

Yes, if the base is above ground, it will affect the impedance.
Consider that the feedpoint impedance of any vertical (any length, any
diameter) fed against a perfect ground plane in freespace will be
exactly half the impedance of the same vertical fed against a mirror
image of itself in freespace (in other words, a symmetrical dipole).
The closer the base is to being a ground plane, the less effect there
will be because of its height. How big is that car roof compared to a
wavelength of the frequency you're interested in? Since the impedance
of a halfwave fed against whatever counterpoise you have will depend a
lot on the diameter of the halfwave element, you'd probably better just
tune your matching network to get acceptable match to whatever you're
using to feed the antenna.

If there's no matching network between your "return loss measurer" and
the antenna feedpoint, I'd expect a better match for the quarter wave
than for the half wave, assuming an instrument calibrated for 50 ohms
(or even 75 ohms). A better match would be a higher return loss. Are
you interpreting the numbers correctly? What do you get when you put a
50 ohm load on your instrument? What do you get when you put a 1000 or
2000 ohm load on it?

Cheers,
Tom

"bbnn" wrote -
..
I heard that a 1/2 wave vertical has a Z of about 2000 ohms at the
base.
Is this the Z between the base of the vertical and ground that RF at
the
resonant frequency
'sees' ?

Doesn't the height that the vertical base is above ground affect
this
2000 ohm valve?
Or does then metal car roof nullify any height above ground affects
on
the Z?

I've noticed that measuring the return loss of a 1/4 vertical is
very low.
But measuring the return loss of a 1/2 wave or 5/8 wave antenna is
higher.??????????????
===============================

It is important to ask the right questions. The right questions are
those to which there are answers and practical measurements can be
made to confirm them.

For example, there is no answer to the question what is the impedance
between the lower end of a half-wave dipole and ground when the lower
end is at a height above ground.

It is impossible to make such a measurement directly because the
impedance meter MUST be associated with a length of wire which reaches
between the bottom end of the antenna and ground. This length of wire
MUST form part of the antenna and the antenna length is then no longer
a 1/2-wave dipole.

If the impedance meter is connected directly to the bottom of the
antenna then the measurement is no longer with respect to ground but
with respect to ground PLUS the impedance of that length of wire
whatever impedance it may have.

I hope, but I doubt it, whether I have made myself clear.

To understand what is going on it is necessary to resort to
theoretical arguments. Only the arguments are real - not the
measurements.

Thre is a 1/2-wave dipole isolated in space. It doesn't matter whether
it is horizontal or vertical. It is possible theoretically to
calculate from its length, diameter and frequency its input impedance
at one end.

But relative to what? To measure it is impossible. To where is the
other side of the meter to be connected? Nevertheless we mustn't
allow this stumbling block to get in our way. The theoretical, perhaps
approximate, calculated value can be used in further calculations to
answer real and very practical questions.

The input resistance of a half-wave vertical near to ground is 2 or 3
thousand ohms. It is uncertain because it depends on wire/conductor
diameter and on the conductivity and permittivity of the soil.
Because of its high value it is also sensitive to the antenna's
environment, trees, etc.

When fed at its base, near to ground, its input resistance can be
considered to be its radiation resistance. Because of its high value
relative to the ground connection resistance, very high power
efficiency can be expected.

Incidentally, half-wave or 5/8ths-wave verticals (not critical) have
the lowest radiation angles and are therefore good DX antennas
preferable to 1/2-wave horizontal dipoles at HF.
----
Reg, G4FGQ

This is pretty much what I have run into from hands on experience, when a
half wave is elevated above ground a 50 to 800 ohm stepup autotransformer
constructed on a toroid usually is sufficient to get the SWR to a resonable
level, infrequently a 1600 ohm tap works better (sometimes a match is had
with a 4:1 balun, go figure.)
Anyway, I find impedance can be greatly affected by relatively small changes
in antenna height...

Regards

"Reg Edwards" wrote in message
...

"bbnn" wrote -
.
I heard that a 1/2 wave vertical has a Z of about 2000 ohms at the
base.
Is this the Z between the base of the vertical and ground that RF at
the
resonant frequency
'sees' ?

Doesn't the height that the vertical base is above ground affect
this
2000 ohm valve?
Or does then metal car roof nullify any height above ground affects
on
the Z?

I've noticed that measuring the return loss of a 1/4 vertical is
very low.
But measuring the return loss of a 1/2 wave or 5/8 wave antenna is
higher.??????????????
===============================

It is important to ask the right questions. The right questions are
those to which there are answers and practical measurements can be
made to confirm them.

For example, there is no answer to the question what is the impedance
between the lower end of a half-wave dipole and ground when the lower
end is at a height above ground.

It is impossible to make such a measurement directly because the
impedance meter MUST be associated with a length of wire which reaches
between the bottom end of the antenna and ground. This length of wire
MUST form part of the antenna and the antenna length is then no longer
a 1/2-wave dipole.

If the impedance meter is connected directly to the bottom of the
antenna then the measurement is no longer with respect to ground but
with respect to ground PLUS the impedance of that length of wire
whatever impedance it may have.

I hope, but I doubt it, whether I have made myself clear.

To understand what is going on it is necessary to resort to
theoretical arguments. Only the arguments are real - not the
measurements.

Thre is a 1/2-wave dipole isolated in space. It doesn't matter whether
it is horizontal or vertical. It is possible theoretically to
calculate from its length, diameter and frequency its input impedance
at one end.

But relative to what? To measure it is impossible. To where is the
other side of the meter to be connected? Nevertheless we mustn't
allow this stumbling block to get in our way. The theoretical, perhaps
approximate, calculated value can be used in further calculations to
answer real and very practical questions.

The input resistance of a half-wave vertical near to ground is 2 or 3
thousand ohms. It is uncertain because it depends on wire/conductor
diameter and on the conductivity and permittivity of the soil.
Because of its high value it is also sensitive to the antenna's
environment, trees, etc.

When fed at its base, near to ground, its input resistance can be
considered to be its radiation resistance. Because of its high value
relative to the ground connection resistance, very high power
efficiency can be expected.

Incidentally, half-wave or 5/8ths-wave verticals (not critical) have
the lowest radiation angles and are therefore good DX antennas
preferable to 1/2-wave horizontal dipoles at HF.
----
Reg, G4FGQ

"Reg Edwards" wrote
The input resistance of a half-wave vertical near to ground is 2 or 3
thousand ohms. It is uncertain because it depends on wire/conductor
diameter and on the conductivity and permittivity of the soil.
Because of its high value it is also sensitive to the antenna's
environment, trees, etc.
________________

Terman (Radio Engineer's Handbook, p 846) shows a chart for the radiation
resistance and reactance ranges for five, guyed MW broadcast towers, as
measured against a good radial ground system--an ohm or two above true
earth. For electrically 1/2-wave radiators, R ranged from 300 ohms to 450
ohms and Z ranged from about -100 ohms to -350 ohms.

RF

Check for the works by the late Dr. Branko Popovic,
who was a consultant to European broadcasters
on tall ( 1/4 wave length) vertical antenna array design.

He was my PhD research advisor at Virginia Tech
in 1968 and 1969. He was on an exchange plan
from Belgrade. I was his first PhD candidate.
He worked from physical reality on antennas
and was an inspiration to me.

Cheers,

Ron McConnell


Ronald C. McConnell, PhD

WGS-84: N 40º 46' 57.9" +/-0.1"
W 74º 41' 21.3" +/-0.1"
FN20ps.77GU96 +/-
V +5058 H +1504

http://home.earthlink.net/~rcmcc

Imagine if we had only Roman numerals for mathematics...
- RCMcC

"Richard Fry" wrote -
"Reg Edwards" wrote
The input resistance of a half-wave vertical near to ground is 2
or 3
thousand ohms. It is uncertain because it depends on
wire/conductor
diameter and on the conductivity and permittivity of the soil.
Because of its high value it is also sensitive to the antenna's
environment, trees, etc.
________________

Terman (Radio Engineer's Handbook, p 846) shows a chart for the
radiation
resistance and reactance ranges for five, guyed MW broadcast towers,
as
measured against a good radial ground system--an ohm or two above
true
earth. For electrically 1/2-wave radiators, R ranged from 300 ohms
to 450
ohms and Z ranged from about -100 ohms to -350 ohms.

RF

=====================================

As I said, 1/2-wave antenna input impedance is a function of conductor
diameter. It is not a relatively fixed value (around 36 ohms) as is a
1/4-wave vertical.

Impedance decreases as diameter increases. Or as the ratio of
height/diameter decreases.

Terman's LF and MF towers (not wires) have extrordinary thick
conductors (probably latttice masts) compared with a low antenna
height in terms of wavelengths. Hence the low input impedances in his
examples.

If I remember correctly, Terman makes no reference to height/diameter
ratio and is therefore misleading by using tower antennas as typical
examples. He tells only half the story. Terman should not always be
used as the Bible.

EZNEC can be used to estimate input impedances of the more common
1/2-wave wire antennas at HF when very near to ground. Which is what
amateurs are interested in. They usually fall in the range 1500 to
3500 ohms.

At VHF and UHF with short, fat, thick rods for conductors, (similar in
proportion to LF lattice towers) input impedances can fall to several
hundred ohms or smaller.

Input impedance values of isolated dipoles are theoretically
calculable but are impossible to measure. Nevertheless, calculated
values are very useful in antenna system design. Couldn't do without
them!

Maths takes precedence over measurements. Without maths, experimenters
flounder about in the dark.
----
Reg, G4FGQ

"Reg Edwards" wrote:
As I said, 1/2-wave antenna input impedance is a function of conductor
diameter. It is not a relatively fixed value (around 36 ohms) as is a
1/4-wave vertical.
_____________________

Terman's chart, which I scanned and emailed to you, shows a 1/4-wave
vertical with an ~R of 31 ohms to 48 ohms, and ~j of -15 ohms to -40 ohms
for the five MW towers. This range actually is about the same by % change
as shown for the 1/2-wave radiator (~R of 300 ohms to 450 ohms, ~j of -100
ohms to -350 ohms).

Maths takes precedence over measurements. Without maths,
experimenters flounder about in the dark.

Both techniques should give about the same results, when both are done
correctly.

RF