From: Michael Coslo
Date: Fri, 21 Aug 2009 Time: 09:51:05

I tend to make my SWR readings right at the antenna, whenever possible.
That length of coax to the xceiver will only make it look better in
most cases.

- 73 de Mike N3LI -

Mike,

With the 4170 I can calibrate the instrument to compensate for the
feeder impedance. After calibration, the indicated impedance at the TX
end of the feeder is actually the antenna feedpoint impedance. This
makes life a *lot* easier.

--
73
Ian, G3NRW

Ian Wade G3NRW wrote:
From: Michael Coslo
Date: Fri, 21 Aug 2009 Time: 09:51:05

I tend to make my SWR readings right at the antenna, whenever
possible. That length of coax to the xceiver will only make it look
better in most cases.

- 73 de Mike N3LI -

Mike,

With the 4170 I can calibrate the instrument to compensate for the
feeder impedance. After calibration, the indicated impedance at the TX
end of the feeder is actually the antenna feedpoint impedance. This
makes life a *lot* easier.

Oaky, well good enough. My vertical uses a stub on the feedline, so I
have to be out there anyhow.with a connector in the line. Seems like 6
of one, half dozen of the other situation.

- 73 De Mike N3LI -

Ian Wade G3NRW wrote:

Mike,

With the 4170 I can calibrate the instrument to compensate for the
feeder impedance. After calibration, the indicated impedance at the TX
end of the feeder is actually the antenna feedpoint impedance. This
makes life a *lot* easier.

Easy, yes. But If you're not careful, this can be a great example of
garbage in, garbage out.

I frequently calculate out the feedline transformation when making
antenna measurements. But it's essential that you realize a small error
in estimating the feedline loss(*) or length can sometimes result in a
very large error in calculated impedance. This is particularly true if
there's a large impedance mismatch between the line and antenna.
Transmission line impedance, which can vary a lot from the specified
nominal value (I've seen +/-20% with coax, more with ladder line), also
has an effect on the result. So whenever I need accurate results or
whenever the line Z0 is quite different from the antenna impedance, I
start by carefully measuring the properties of the actual transmission
line I'll be using.

If you're not convinced, spend a few minutes playing with something like
N6BV's TLW calculator that comes with the ARRL Antenna Book.

(*) Some simplified techniques ignore transmission line loss altogether.
This can lead to very inaccurate results in some situations. And loss is
often quite different than the specified value, so it really has to be
measured if it makes a significant difference.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Ian Wade G3NRW wrote:

Mike,

With the 4170 I can calibrate the instrument to compensate for the
feeder impedance. After calibration, the indicated impedance at the TX
end of the feeder is actually the antenna feedpoint impedance. This
makes life a *lot* easier.

Easy, yes. But If you're not careful, this can be a great example of
garbage in, garbage out.

I frequently calculate out the feedline transformation when making
antenna measurements. But it's essential that you realize a small error
in estimating the feedline loss(*) or length can sometimes result in a
very large error in calculated impedance. This is particularly true if
there's a large impedance mismatch between the line and antenna.
Transmission line impedance, which can vary a lot from the specified
nominal value (I've seen +/-20% with coax, more with ladder line), also
has an effect on the result. So whenever I need accurate results or
whenever the line Z0 is quite different from the antenna impedance, I
start by carefully measuring the properties of the actual transmission
line I'll be using.

If you're not convinced, spend a few minutes playing with something like
N6BV's TLW calculator that comes with the ARRL Antenna Book.

(*) Some simplified techniques ignore transmission line loss altogether.
This can lead to very inaccurate results in some situations. And loss is
often quite different than the specified value, so it really has to be
measured if it makes a significant difference.

Roy Lewallen, W7EL

The 4170 makes this a lot easier as you can measure the feedline actual
parameters as well as calibrate out their effects.


--
Jim Pennino

Remove .spam.sux to reply.

Roy Lewallen wrote:
Ian Wade G3NRW wrote:

Mike,

With the 4170 I can calibrate the instrument to compensate for the
feeder impedance. After calibration, the indicated impedance at the TX
end of the feeder is actually the antenna feedpoint impedance. This
makes life a *lot* easier.

Easy, yes. But If you're not careful, this can be a great example of
garbage in, garbage out.

I frequently calculate out the feedline transformation when making
antenna measurements. But it's essential that you realize a small error
in estimating the feedline loss(*) or length can sometimes result in a
very large error in calculated impedance.


Most of the new analyzers that do "fixture cal" actually measure the
line, so you're not doing any estimating.

Somewhere in the future, as hams get more familiar with this kind of
thing, I see folks installing a little relay box at the feedpoint of the
antenna that has a short, open, and load, as well as the antenna.

I've done this with an RCS-8V, modifying it to add a noninductive 50 ohm
load and to short one of the relays. I then use my VNA to calibrate the
relay box (or, more accurately, to just confirm that the load and short
is good enough) with a short test cable.

After I went through the hassle of modifying, I realized that I would
have actually been better off to just get 50 ohm loads with PL-259s on
them and PL-259 shorts. oh well, it's done now.

So anyway, the RCS-8V has 5 ports and a common. I hook the the feedline
on port 1, the antenna on port 5, the short on port 2 and the load on
port 3. (Sometime in the future, I'm going to hook a phase reversing
transformer on port 4). By turning on and off the various ports, I can
connect loads and shorts to antennas and/or feedlines or leave them open.

In a real fancy application, I'd put a remote controlled antenna tuner
with a few extra relays in it at the feedpoint. Hmm.. another project to
languish half completed in my garage for years.

From: Michael Coslo
Date: Fri, 21 Aug 2009 Time: 15:05:13


Oaky, well good enough. My vertical uses a stub on the feedline, so I
have to be out there anyhow.with a connector in the line. Seems like 6
of one, half dozen of the other situation.

- 73 De Mike N3LI -

Agreed. But I guess your antenna feedpoint is more-or-less at ground
level. That's a little different from peering through binoculars at an
MFJ suspended at the feedpoint 50ft above ground .... grin

--
73
Ian, G3NRW

wrote:


The 4170 makes this a lot easier as you can measure the feedline actual
parameters as well as calibrate out their effects.


This is a dumb question on my part, but what you are saying is that the
mitigating effects that the cable has on the VSWR, making it look better
in general, can not only be calculated and "calibrated out", but that
the actual SWR of your antenna at the feedpoint is then given?

As you get closer to 1.1:1 at the actual antenna, would accuracy then
suffer? If feedline loss can bring an antenna that is not near that to a
level approaching that, wouldn't it mean that teh calibration is
somewhere in the noise?

Like I say, this could be a really stoopid question.

- 73 de Mike N3LI -

Michael Coslo wrote:
wrote:


The 4170 makes this a lot easier as you can measure the feedline actual
parameters as well as calibrate out their effects.


This is a dumb question on my part, but what you are saying is that the
mitigating effects that the cable has on the VSWR, making it look better
in general, can not only be calculated and "calibrated out", but that
the actual SWR of your antenna at the feedpoint is then given?

As you get closer to 1.1:1 at the actual antenna, would accuracy then
suffer? If feedline loss can bring an antenna that is not near that to a
level approaching that, wouldn't it mean that teh calibration is
somewhere in the noise?

Like I say, this could be a really stoopid question.

- 73 de Mike N3LI -

Basically what you do is calibrate the instrument at the measurement
point, whether that point is the instrument connector or at the end of
a length of coax.

You attach an open, a short and a known resistance; 50 ohms by default
but it is user definable.

The instrument than frequency sweeps and stores the results in a user
definable calibration file.

When you make a measurement of an unknown, you define which calibration
file to use and the instrument corrects the readings to display the
characteristics at the measurement point.

Given that this is a $500 insturment and not a $20,000 labratory instrument
there are going to be limits to how accurate all this is.

After having used the AIM for a while, my opinion is that it far execeeds
what is required for practical amateur usage.

If you want to see some actual numbers, you can find a comparison of the
results of an AIM 4170 compared to HP lab equiment at:

http://www.bnk.com/w0qe/AIM4170_page1.html


--
Jim Pennino

Remove .spam.sux to reply.

Michael Coslo wrote:
wrote:


The 4170 makes this a lot easier as you can measure the feedline actual
parameters as well as calibrate out their effects.


This is a dumb question on my part, but what you are saying is that the
mitigating effects that the cable has on the VSWR, making it look better
in general, can not only be calculated and "calibrated out", but that
the actual SWR of your antenna at the feedpoint is then given?

As you get closer to 1.1:1 at the actual antenna, would accuracy then
suffer? If feedline loss can bring an antenna that is not near that to a
level approaching that, wouldn't it mean that teh calibration is
somewhere in the noise?

Like I say, this could be a really stoopid question.

- 73 de Mike N3LI -

Not at all. Imagine that you have a very lossy line. You'll read very
nearly the cable Z0 regardless of what's at the other end. Extreme
changes in far-end impedance will make very little difference at the
input end, so it's impossible to tell with any accuracy what's at the
far end by looking at the near-end impedance. Another case to consider
is one where the Z0 of the cable is very different than the Z of the
load. In that case, a tiny change in line Z0, length, or loss changes
the input Z for a given load Z. It can be impossible to measure the line
length, impedance, or loss with sufficient accuracy to find the far end
impedance with decent accuracy.

This doesn't mean you can't get measurements good enough for amateur or
even professional use. But on the other hand, your measurements can be
total garbage in spite of your cable measurements if you fail to realize
just how sensitive the result can be to small errors. A careful
experimenter will do a sensitivity analysis which tells how large an
error in results is caused by an error in measuring the feedline or in
the input impedance measurement, then the probable measurement errors
are estimated to determine the probable error in the calculated result.
While a mathematical sensitivity analysis is the rigorous way to do
this, something like N6BV's TLW program is just fine for most amateur
purposes. Or, if you're using one of the instruments that does the
calculation for you, try telling it the line is a few percent longer or
shorter, or has a Z0 or loss a few percent different from what it said
or you measured. See how much it changes the result. If the change is
small, no problem. But if it's large, it means that extreme care and
maybe some other techniques have to be used to get a good measurement.

Let me give an example, done with TLW. Suppose we're measuring the
impedance of an antenna at 30 MHz through 100 feet of RG-8x. TLW gives
these nominal values for RG-8x:

Z0 - 50.2 - j0.47
Velocity factor - 0.8
Loss - 1.926 dB/100'

And suppose that these are exactly what our measurement of the cable
said. We measure 21 + j20 at the input end, and conclude that the
impedance of the antenna is 374 - j84 ohms.

But suppose the measurement at the input end was inaccurate by about 5%,
and that the actual input end Z was 21 + j20. Then the load Z is 322 -
j105, about 15% off in R, 25% in Z. Or maybe the cable measurement was
off by just 1%, and the cable is really 101 and not 100 feet long. In
that case, the antenna Z is really 129 + j166 ohms. We're even on the
other side of resonance from where we thought. Or maybe the velocity
factor was rounded a bit and it's really closer to 0.81 than 0.8. How
much difference would that small error make? Well, the antenna Z would
be 53 - j120 ohms with our input measurement of 21 + j20!

So, what's the real antenna impedance? 374 - j84, 322 - j105, 129 +
j166, or 53 - j120? You're fooling yourself if you think you really know.

It's easy to get lulled into believing that just because we read a value
to six decimal places, it's accurate. But you're usually doing very well
to get within a few percent in spite of all those digits. And when that
few percent results in a much bigger error in calculated results, it's
even more important to realize the limitations of your accuracy.

Roy Lewallen, W7EL

Corrections:

I apologize, and misinterpreted my scribbled notes. The conclusion is
the same, but some of the quoted numbers are a little off. Here are the
correct ones.

------------

And suppose that these are exactly what our measurement of the cable
said. We measure 21 + j20 at the input end, and conclude that the
impedance of the antenna is 322 - j105 ohms.

But suppose the measurement at the input end was inaccurate by about 5%,
and that the actual input end Z was 22 + j21. Then the load Z is 273 -
j125, about 15% off in R, 20% in X. . .

-------------

Roy Lewallen wrote:

And suppose that these are exactly what our measurement of the cable
said. We measure 21 + j20 at the input end, and conclude that the
impedance of the antenna is 374 - j84 ohms.

But suppose the measurement at the input end was inaccurate by about 5%,
and that the actual input end Z was 21 + j20. Then the load Z is 322 -
j105, about 15% off in R, 25% in Z. . .