Ian Wade G3NRW wrote:

The other nice thing about the 4170 is the ability to control it
remotely. With the 4170 connected to the antenna system in the shack and
controlled by a local PC, it's then possible to remotely access the PC
from a cheapy notebook in the backyard. You can make antenna adjustments
out in the yard and see the effects of the changes immediately, without
having to repeatedly walk back indoors to look at the 4170 PC. This
feature alone has saved me a *lot* of time:


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 -

Michael Coslo wrote:
Ian Wade G3NRW wrote:

The other nice thing about the 4170 is the ability to control it
remotely. With the 4170 connected to the antenna system in the shack and
controlled by a local PC, it's then possible to remotely access the PC
from a cheapy notebook in the backyard. You can make antenna adjustments
out in the yard and see the effects of the changes immediately, without
having to repeatedly walk back indoors to look at the 4170 PC. This
feature alone has saved me a *lot* of time:


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 -

One of the advantages of the 4170 is the ability to calibrate out the
effects of the feedline.


--
Jim Pennino

Remove .spam.sux to reply.

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 -

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

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.

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.

..
snip

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.

When I inspected antennas, we had two multi-kilobuck "Site Master"
instruments from Anritsu, mentioned here, that had a set of calibrated
terminations. IIRC, to calibrate the unit(s), we had to connect the
terminations, a short, a 50-ohm resistor and a shielded open circuit, one at
a time, to the instrument and tell it which one was connected. It swept the
frequencies of interest and stored its own baseline behavior over that band
of interest. Then, anything connected to it was referenced to that
baseline. We could also store a range of sweep frequencies (usually by the
name or type of antenna we intended to sweep) and it would recall all the
parameters. Automated, repeatable sweep testing is not available (yet) in
lower cost instruments.

I presume we could have calibrated any given cable, too. (Never required by
our test memos.)

Sal