Good morning,

This question is: Taking measurements at the antenna and what they mean?

The antenna is a loaded vertical over a set of untuned radials. The feed is 50
Ohm coax with a 1:1 isolation transformer at the antenna. I intend to run a
separate coax for measurement purposes to the feed point, stimulate the antenna
with a 10 mw signal at the operating frequency, and measure the voltage across
the antenna feed and the ground connection.

Will the voltages indicate the relative RF impedance of antenna (including
loading coil) versus the ground? If the ground were near perfect the voltage at
the feedpoint should be close to zero. These voltages should indicate the power
disapation difference between the antenna and the ground.

Thanks,
Dan

In article , dansawyeror
wrote:

Good morning,

This question is: Taking measurements at the antenna and what they mean?

The antenna is a loaded vertical over a set of untuned radials. The feed
is 50
Ohm coax with a 1:1 isolation transformer at the antenna. I intend to run a
separate coax for measurement purposes to the feed point, stimulate the
antenna
with a 10 mw signal at the operating frequency, and measure the voltage
across
the antenna feed and the ground connection.

Will the voltages indicate the relative RF impedance of antenna (including
loading coil) versus the ground? If the ground were near perfect the
voltage at
the feedpoint should be close to zero. These voltages should indicate
the power
disapation difference between the antenna and the ground.

Thanks,
Dan

Hello, Dan. If we have a quarter wavelength monopole the voltage at the
feedpoint is at a minimum but definitely not zero even if we had a
perfectly electrically conducting (PEC) ground. If we had a PEC ground
and a monopole with no resistive (ohmic) losses then the impedance seen at
the feedpoint at resonance would be about 37 ohms resistive. The 37 ohms
is the radiation resistance of the antenna. This is where the RF power
gets "dissipated" when that power is radiated into free space.

If we use an electrically short antenna and put in say, a loading coil at
the feedpoint to resonate the monopole at the desired frequency, we are
looking at a radiation resistance of less than 37 ohms. It could be quite
small depending on how short the antenna is. If in addition we have
finite losses in the monopole structure, loading coil and in the
ground/radials, we are measuring a resistance that is the sum of all ohmic
losses and radiation resistance. And that's a classic problem - how to
separate out all these losses. Without making radiation pattern
measurements (to obtain total power radiated) about all you can do is
calculate the radiation resistance for a PEC monopole fed against a PEC
ground (or use an electromagnetics computation program like NEC to model
the structure.). The measured value less the calculated radiation
resistance would reflect the ohmic losses. And you still don't know how
much of this is ground loss without knowledge of the other ohmic losses.

There exits another direct measurement technique (Wheeler Cap) for
determing antenna efficiency that can be readily applied to antennas in
the GHz range but is not practical at HF because of the scaling factor
(compare the size of 2.4 GHz yagi or log-periodic with the same type at 14
MHz to put things in perspective). Sincerely,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

............some "seat-of-the-pants" observations (to use an old aviator expression):
To judge the performances of two similar-sized mobile installations, note the value
of the shunt-C required across the feed-coax to provide good SWR 50 ohm match (or
shunt-L if that option is used).
The more C required (less capacitive reactance), the better [or similarly, the
less L (less inductive reactance), the better].
This method of match is actually a version of the classic "L-match" where the
mobile whip with its loading coil represents either R in series with Inductance, OR
R in series with Capacitance at the feedpoint....depending on whether the loading
coil is tuned slightly Below, or Above the resonant frequency. The shunt C (or L) is
the third element in the L-Match Equation. (Many readers will already know this)
Since standard antenna equations calculate the actual radiation resistance to be
very small in electrically- short mobile antennas, we know that with a perfect
ground, and zero-loss mast and coil, the feed-point resistance would be only the
radiation resistance of a few ohms, perhaps 2 to 5 (W8JI's web page calculates a
15 ft. antenna on 1.8 MHz to be only 1.22 ohms).
Therefore, if the mobile feedpoint was found to have low SWR 50 ohm match with
no shunt element, that would be very bad news........it would mean that perhaps 2
parts out of 50 were radiated, and the other 48 parts were loss! And of course,
here no shunt C (or L) would be required which is the worst news.
Conversely, back to "seat-of-the-pants thinking", the more C (or smaller L)
required, the smaller the feed-point impedance had to have been....meaning less
loss resistance included at the feedpoint.

On Thu, 08 Jun 2006 07:03:05 -0700, dansawyeror
wrote:

Ohm coax with a 1:1 isolation transformer at the antenna. I intend to run a
separate coax for measurement purposes to the feed point, stimulate the antenna
with a 10 mw signal at the operating frequency, and measure the voltage across
the antenna feed and the ground connection.

You seem to be implying more than one measurement, and that with those
measurements you can separately determine (radiation resistance + loss
resistance in the vertical and coil) and loss resistance in the ground
system.

Can you elaborate?

Perhaps a block diagram of your measurement setup might reveal what
you are thinking. (If you need a tool for drawing schematics in ascii
text, Google for AACircuit. Alternatively, sketch it, scan it and
publish it on a web site somewhere... DON'T attach it to your news
post.)

Owen
--

Thank you all for your replys, they were all interesting.

The first set of test measurements were taken on a 2-meter loaded vertical. Two
voltage readings were taken via a high impedance scope probe, one at the
feedpoint of the coax center connection, and the other from the connection of
the coax shield to the antenna ground. In this case the ground is a simple
raised metal 1/4 inch screen mesh about 10 square feet.

The antenna was tuned to resonance. The relative voltage readings were the
center conductor was 2x the ground reading. I do not have a way to measure
current at the base at this time.

However looking ahead, with power = iv would this imply about 2 times the energy
is being radiated from the vertical element as through the base?

Thanks Dan - kb0qil

dansawyeror wrote:
Good morning,

This question is: Taking measurements at the antenna and what they mean?

The antenna is a loaded vertical over a set of untuned radials. The feed
is 50 Ohm coax with a 1:1 isolation transformer at the antenna. I intend
to run a separate coax for measurement purposes to the feed point,
stimulate the antenna with a 10 mw signal at the operating frequency,
and measure the voltage across the antenna feed and the ground connection.

Will the voltages indicate the relative RF impedance of antenna
(including loading coil) versus the ground? If the ground were near
perfect the voltage at the feedpoint should be close to zero. These
voltages should indicate the power disapation difference between the
antenna and the ground.

Thanks,
Dan

On Sun, 11 Jun 2006 09:35:31 -0700, dansawyeror
wrote:

The first set of test measurements were taken on a 2-meter loaded vertical.

What frequency?

Two
voltage readings were taken via a high impedance scope probe,

This sounds like a set up to imagining better results than can be
obtained. Scope probes are not magic detectors and what you describe,
following, sounds like wishing:

one at the
feedpoint of the coax center connection, and the other from the connection of
the coax shield to the antenna ground.

A Scope probe has two (2) connections, and at any frequency in HF and
above, their separation is going to have a significant impact on
measurement. You never say just where the second lead goes for each
voltage reading. If you made NO second connection (and just left that
at up to the far end of the probe's connection to a mystery box) then
you have definitely confirmed that dream is in Technicolor and
surround-sound.

In this case the ground is a simple
raised metal 1/4 inch screen mesh about 10 square feet.

This is rather less than fully descriptive as 10 square feet can take
many forms.

The antenna was tuned to resonance. The relative voltage readings were the
center conductor was 2x the ground reading. I do not have a way to measure
current at the base at this time.

And this raises one particularly pointed question:
Ground was at what potential with respect to ground?

However looking ahead, with power = iv would this imply about 2 times the energy
is being radiated from the vertical element as through the base?

Hi Dan,

There is far too much ambiguity in what you post. What is posted is
fraught with conceptual problems. Basically, you've managed to obtain
two voltages whose relationship is meaningless at best. I'm sure the
potentials exist, but they define a circuit that has no practical
application (and that circuit is NOT the antenna NOR its ground
screen).

In short, just what do you think you are achieving (another
ambiguity)?

73's
Richard Clark, KB7QHC

All,

There have been replies with some good comments and recommendations for not
taking meaningless measurements.

First, the objective: Modeling software predicts a loaded vertical with a 'good'
ground will have a low input impedance. This is a step on the trail to measure
that impedance.

Second, a vector voltmeter, when feed by directional couplers can be used to
measure impedance angle and therefore resonance. It can also be use to measure
the forward and re-flected power.

The setup is a calibrated signal generator, connected to a 7904 scope (A plug
in) for reference purposes only, connected to a pair of couplers connected to
the vector voltmeter, connected to test antenna. The frequency is 147 MHz. The
system is near resonance. The voltage measurements are taken with a pair of 10x
probes, connected to a dual channel plug in (B plug in).

There is no significant change to the signal or the vector voltmeter readings
when the probe leads are attached. The phase changes by less the 10 degrees and
none of the amplitudes change noticeably. I do not believe the measurement
system is materially influencing the measurements.

The next step is to make a current probe to attach to the base of the antenna.
The result is the two 'independent' systems, the couplers and the vector
voltmeter, and the voltage and current probe, should yield similar antenna
impedances.

- Dan

dansawyeror wrote:
Thank you all for your replys, they were all interesting.

The first set of test measurements were taken on a 2-meter loaded
vertical. Two voltage readings were taken via a high impedance scope
probe, one at the feedpoint of the coax center connection, and the other
from the connection of the coax shield to the antenna ground. In this
case the ground is a simple raised metal 1/4 inch screen mesh about 10
square feet.

The antenna was tuned to resonance. The relative voltage readings were
the center conductor was 2x the ground reading. I do not have a way to
measure current at the base at this time.

However looking ahead, with power = iv would this imply about 2 times
the energy is being radiated from the vertical element as through the base?

Thanks Dan - kb0qil

dansawyeror wrote:

Good morning,

This question is: Taking measurements at the antenna and what they mean?

The antenna is a loaded vertical over a set of untuned radials. The
feed is 50 Ohm coax with a 1:1 isolation transformer at the antenna. I
intend to run a separate coax for measurement purposes to the feed
point, stimulate the antenna with a 10 mw signal at the operating
frequency, and measure the voltage across the antenna feed and the
ground connection.

Will the voltages indicate the relative RF impedance of antenna
(including loading coil) versus the ground? If the ground were near
perfect the voltage at the feedpoint should be close to zero. These
voltages should indicate the power disapation difference between the
antenna and the ground.

Thanks,
Dan

On Sun, 11 Jun 2006 17:53:00 -0700, dansawyeror
wrote:

The frequency is 147 MHz. The
system is near resonance. The voltage measurements are taken with a pair of 10x
probes, connected to a dual channel plug in (B plug in).

Hi Dan,

The use of 10x scope probes is an illusion at best, and certainly not
the way the "pros" do it by a long stretch.

There is no significant change to the signal or the vector voltmeter readings
when the probe leads are attached.

Compared to what? This is again an example of ambiguity that offers
nothing tangible except a "feel-good" kind of experience. Do you know
why there is "no significant change?" If you replace a 1x feed with a
10x probe, wouldn't you expect change? Your faith is based on what
would normally set off red lights and have warning flags being waved.

The phase changes by less the 10 degrees and
none of the amplitudes change noticeably. I do not believe the measurement
system is materially influencing the measurements.

This is a faith-based feeling instead of rational expectation. It is
overwhelmingly obvious by your statement above that the system is
wholly out of whack with expectations.

A simple review of the actual probe can be instructive in this case to
explain why you see little change, and why you wouldn't use a scope
probe in the first place. Most 10x scope probes are not rated into
the VHF. The 10x probe is generally a 10 MOhm resistor with a
parallel 10pf variable capacitor. Simple math would reveal that at
147 MHz that cap presents about 100 Ohms reactance - hardly worth the
effort and certainly no where near a 10x function. The equipment 10x
probe (not a 10x scope probe) has only 2pF of capacitance (5 times the
reactance of a scope probe) and a tenth of the 10x scope probe
resistance. What you are reading as scaled voltage is not what is
present to be measured, but a complex, phase shifted value.

You have absolutely no rational basis to use a 10x probe of any kind.
Skip this unnecessary elaboration.

73's
Richard Clark, KB7QHC

In addition to the weaknesses of scope probes mentioned by Richard is
the ground lead. Even a short one has considerable reactance at VHF, and
its use will promote common mode current on the outside of the probe
cable. Any decent scope measurement at very high frequencies is done
with an adapter which connects directly to the ground ring near the end
of the probe, through a very short conductor. An example is the one sold
by Tektronix, P/N 013-0085-00 the last time I checked. Even it must be
used with some care.

I don't see any hope in your making meaningful measurement with the
setup you describe -- there are too many potential sources of serious error.

Roy Lewallen, W7EL

"dansawyeror" wrote in message
. ..
Thank you all for your replys, they were all interesting.

The first set of test measurements were taken on a 2-meter loaded
vertical. Two
voltage readings were taken via a high impedance scope probe, one at the
feedpoint of the coax center connection, and the other from the connection
of
the coax shield to the antenna ground. In this case the ground is a simple
raised metal 1/4 inch screen mesh about 10 square feet.

The antenna was tuned to resonance. The relative voltage readings were the
center conductor was 2x the ground reading. I do not have a way to measure
current at the base at this time.

However looking ahead, with power = iv would this imply about 2 times the
energy
is being radiated from the vertical element as through the base?

Thanks Dan - kb0qil

dansawyeror wrote:
Good morning,

This question is: Taking measurements at the antenna and what they mean?

The antenna is a loaded vertical over a set of untuned radials. The feed
is 50 Ohm coax with a 1:1 isolation transformer at the antenna. I intend
to run a separate coax for measurement purposes to the feed point,
stimulate the antenna with a 10 mw signal at the operating frequency,
and measure the voltage across the antenna feed and the ground
connection.

Will the voltages indicate the relative RF impedance of antenna
(including loading coil) versus the ground? If the ground were near
perfect the voltage at the feedpoint should be close to zero. These
voltages should indicate the power disapation difference between the
antenna and the ground.

Thanks,
Dan


Dan,
I'm echoing the others on this. Using scope probes for this kind of
measurenet is a very touchy undertaking at the very best, even with the best
quality equipment and much experience. Other methods of measuting RF are
required and used by those who know. That's why they were developed. If
you do this at 10M or below, then you have more chance of getting correct
results.
That being said, it is not clear what you are trying to measure since you
say you are, in essence, measuring "ground". It is not clear just what you
are doing. Unfortunately, from your description, the way you describe this,
I feel you do not have enough technical background to understand the
technical concepts well enough. You appear to have some knowledge, but not
enough, in what appear to be basics, to do this.
The way you describe your technique, you are not connecting the probe ground
to anything. This makes no sense. RF measurements may seem like "black
magic" but it takes nothing more than *really* understanding all the basics
and all the stray effects really, really well.

At least you seem to understand that "ground" is not the absolute zero
reference that others seem to believe it is / can be / needs to be.

A few comments in general:
You *MUST* connect *BOTH* parts of the probe tip if you hope to get anything
close to true results.
Both these connections must have no effect on the measurement.
The probe must be capable of measuring what you are attempting.
There are high frequency 10x probes, but all the other sonsiderations are
even more important because they will be even more susceptable to the side
effects just because they *are* capable of making the measurements.
The Shield / ground part of the scope probe and cable leading back to the
scope is something which can have a significant effect on what you see on
the scope face. The antenna can be inducing current into this and giving
false readings. This is a major problem in measuting any high frequency
signals. There must be no possibility of the scope cable picking up any
signal. I have seen very misleading results when this technique is tried on
digital circuits which have fast rise times and therefore high frequency
components.

Good luck & 73, Steve, K9DCI