UHF antenna measurements
 

My question

What is the simplest method for me to measure the antenna resonant
frequency (X=0) and antenna resistance (at resonance) of a small 70 cm
yagi, to a reasonable level of accuracy.

I have a counter and GDO which work at 70 cm, and can homebrew
anything else required.

Thanks in anticipation

MikeN ZL1BNB

Dear MikeN:

I have an antique Omega pocket watch which needs adjustment. I have a 24"
pipe wrench, an 8 pound sledge hammer and a blowtorch. What's the simplest
method for me to adjust the movement for best accuracy?


See the problem? Mismatch between job and tools. You need a UHF bridge,
and a means to calibrate it. Or a network analyzer. And some reference
standards. Unfortunately, you do not have the tools to build the tools you
need to do the job. Any universities or electronics labs with Ham employees
nearby? Might be your best bet.

--
Crazy George
Remove N O and S P A M imbedded in return address
"MikeN" wrote in message
...
My question

What is the simplest method for me to measure the antenna resonant
frequency (X=0) and antenna resistance (at resonance) of a small 70 cm
yagi, to a reasonable level of accuracy.

I have a counter and GDO which work at 70 cm, and can homebrew
anything else required.

Thanks in anticipation

MikeN ZL1BNB

Thanks George

I can make a UHF bridge and a dummy load rfom SMD resistors which I
think should be good enough for 70 cm. Any comments?

Mike N

On Thu, 4 Dec 2003 17:37:50 -0600, "Crazy George"
wrote:

Dear MikeN:

I have an antique Omega pocket watch which needs adjustment. I have a 24"
pipe wrench, an 8 pound sledge hammer and a blowtorch. What's the simplest
method for me to adjust the movement for best accuracy?


See the problem? Mismatch between job and tools. You need a UHF bridge,
and a means to calibrate it. Or a network analyzer. And some reference
standards. Unfortunately, you do not have the tools to build the tools you
need to do the job. Any universities or electronics labs with Ham employees
nearby? Might be your best bet.

On Fri, 05 Dec 2003 17:11:53 +1300, MikeN
wrote:

Thanks George

I can make a UHF bridge and a dummy load rfom SMD resistors which I
think should be good enough for 70 cm. Any comments?

Mike N


Hi Mike,

George has a point. Building a bridge begs the question: which one?

In the collection of works under bridges, there are many, most of
which are entirely unsuited to the job.

The first problems one would encounter is balance and isolation. At
UHF this actually is simple because visualization lends to revealing
those flaws in the implementation. IF, of course, you look at it with
that perspective. Another problem is scale. Your bridge might
present a significant intrusion into the variable being measured.

How do you put a 6X3X9 project box holding your bridge into practice
with an antenna that is the same size? The problem of isolation. SMD
components are a good first pass, but a bridge has its variables to
perform the measurement. How big is the read-out scale to the knob in
relation to that same antenna? The smaller you make it, the closer
you get your face to the antenna (isolation and balance again). How
do you adjust the balance without literally putting your hand into the
equation?

OK, so you make the bridge a remote device, you measure through a
line. Then the question becomes, are you measuring the antenna, or
the antenna AND the line? You need to isolate the line, and you do
that through a reference. Where did you get the reference?
Presumably another SMD, but was the line choked?

Another, classic solution is to scale your problems away. Take what
instrumentation you have, that exhibits the precision you need, that
contains as much error as you can tolerate, and scale a model of your
antenna to suit IT. Take a measure and reverse scale the model
antenna physical design to the target frequency. What you have to
watch out for here are those variables that are scale/frequency
variant like ground loss, dissipation factors and such. Again the
problem of perspective IF you don't know what to look for.

73's
Richard Clark, KB7QHC

Thanks Jacques, George and Richard for you replies.

I've got in mind building an HA8ET return loss bridge
http://www.pollak.sulinet.hu/elektro/hidak/hidak.htm, if necessary
separating it from the antenna by a couple of wavelengths of coax,
expecting the antenna load to repeat at half-multiples of a
wavelength.

This resistive bridge is physically small and specified at 40 dB
return loss at 70 cm, so why shouldn't it give some meaningful
although not necessarily exact results for the impedance?

Would the separating cable add to the imprecision.of the results.

Any views?

MikeN ZL1BNB


On Fri, 05 Dec 2003 06:28:44 GMT, Richard Clark
wrote:

On Fri, 05 Dec 2003 17:11:53 +1300, MikeN
wrote:

Thanks George

I can make a UHF bridge and a dummy load rfom SMD resistors which I
think should be good enough for 70 cm. Any comments?

Mike N


Hi Mike,

George has a point. Building a bridge begs the question: which one?

In the collection of works under bridges, there are many, most of
which are entirely unsuited to the job.

The first problems one would encounter is balance and isolation. At
UHF this actually is simple because visualization lends to revealing
those flaws in the implementation. IF, of course, you look at it with
that perspective. Another problem is scale. Your bridge might
present a significant intrusion into the variable being measured.

How do you put a 6X3X9 project box holding your bridge into practice
with an antenna that is the same size? The problem of isolation. SMD
components are a good first pass, but a bridge has its variables to
perform the measurement. How big is the read-out scale to the knob in
relation to that same antenna? The smaller you make it, the closer
you get your face to the antenna (isolation and balance again). How
do you adjust the balance without literally putting your hand into the
equation?

OK, so you make the bridge a remote device, you measure through a
line. Then the question becomes, are you measuring the antenna, or
the antenna AND the line? You need to isolate the line, and you do
that through a reference. Where did you get the reference?
Presumably another SMD, but was the line choked?

Another, classic solution is to scale your problems away. Take what
instrumentation you have, that exhibits the precision you need, that
contains as much error as you can tolerate, and scale a model of your
antenna to suit IT. Take a measure and reverse scale the model
antenna physical design to the target frequency. What you have to
watch out for here are those variables that are scale/frequency
variant like ground loss, dissipation factors and such. Again the
problem of perspective IF you don't know what to look for.

73's
Richard Clark, KB7QHC

On Mon, 08 Dec 2003 17:52:27 +1300, MikeN
wrote:

Thanks Jacques, George and Richard for you replies.

I've got in mind building an HA8ET return loss bridge
http://www.pollak.sulinet.hu/elektro/hidak/hidak.htm, if necessary
separating it from the antenna by a couple of wavelengths of coax,
expecting the antenna load to repeat at half-multiples of a
wavelength.

This resistive bridge is physically small and specified at 40 dB
return loss at 70 cm, so why shouldn't it give some meaningful
although not necessarily exact results for the impedance?

Would the separating cable add to the imprecision.of the results.

Any views?

MikeN ZL1BNB

Hi Mike,

Resistive Bridges (in other words, those not using directional
coupling through ferrites) are pretty robust up to this region. You
definitely want to keep the power down because it will cause the
reference load to change resistance if it heats up appreciably.

Also, use odd 1/8th wave cable lengths so that the reactance change
that is being transformed is not on the edge of a steep slope
(resonance causes things to change fast making accurate measurements
difficult). Reference Walt Maxwell's "Reflections" to delve deeper
into this matter. Half waves for casting back a repeated Z only if
you can insure you can measure a short at the connector (not as easy
as it may seem - hence the 1/8th wave recommendation).

73's
Richard Clark, KB7QHC