Hello Tom

My responses**

"K7ITM" wrote in message
...
On Jan 30, 2:08 pm, "Suzy" not@valid wrote:
"K7ITM" wrote in message

...

On Jan 30, 9:57 am, "Suzy" not@valid wrote:
...

I'd like to thank all you kind fellows for your assistance, but
unfortunately a lot of it is rather above my head. I need to be
pointed
at
an article that spells out exactly what dimensions, how to build etc.
(eg
is
there one in the ARRL handbook?) And Owen, no in this case Suzy if
not
short for Susan, so I'm not the person you knew.

OK, back to the first posting I made here. How about if I build one,
test it, and post the design and results somewhere? As I noted in one
of my postings, if I were to make one, I'd first consider how much
power I wanted to read, full scale, so if my offer is appealing, let
me know how much power you want to measure, max. Don't tell me a
kilowatt if you really are going to use it at 10 watts, because if I
design for a kilowatt, 10 watts will be low enough that you won't be
able to read it very well. In fact, I'd propose 10 watts as a
reasonable full scale for a lot of ham uses.

Cheers,
Tom

Hi Tom

That's a kind offer, but you needn't go to all the trouble of building
it.
Just a pointer at the practical design will do (but no complex theory --
over my head!).

I want to investigate various 70 cm antennas (central frequency in
Australia
435 MHz). TX is switchable 5 10 20 watts. I want to standardise on BNC,
and
have readouts on analogue meters (probably 1 mA movements)

OK, I gave this some thought last night. I see a couple problems...

Though you could use 1mA meter movements, that puts you at a detected
power level high enough that the meter scale won't be linear in power,
assuming Schottky or germanium diode detectors.

**Pardon my ignorance, but isn't it just a case of using an op amp or
whatever to suit whatever meter movement I have? ANyway, I have now sourced
a 100 microamp meter (MU65) with a 3.9K resistance (sounds strange as the 1
mA one has a 210 ohm resistance.

BTW, I'n not bothered about the linear issue. I will be having two meters to
show forward and relected powers simultaneously. I don't want to clculate
actual SWR.

To me, having a
linear power scale is a big advantage, because then you can reasonably
accurately figure SWR without having to worry about temperature
compensation of the detectors. There's a reason that Bird power
meters use a sensitive microammeter movement. (I think I've heard
30uA full scale, but I'm not sure about that.) Anyway, that's why I
suggested using a DVM for readout.

The second problem is, if you want to implement a microstrip design,
how do you get the trace width right? If you're afraid of surface
mount parts, how will you control the trace width to +/- a fraction of
a millimeter? On 1.6mm thick PC board, assuming FR4 with a relative
dielectric constant of 4.75, you'd like to have a trace width about
2.78mm to get a 50 ohm line. If your trace is 3.5mm wide, you get a
bit under 44 ohms, and if your trace comes out 2.0mm wide, you get a
line that's almost 60 ohms. If you can do the PC board
photographically and have confidence that you can control the trace
width to within 0.1mm, that would work. If you're doing it by
scribing the copper and pulling up unwanted copper, I think you'll
have to be working under a pretty good microscope to get to much
closer than a mm of the desired width-- or maybe cut it on a milling
machine.

I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

But there's still the problem of making the two
(or three) all the same width. Not knowing how you might be able to
do this, I'm rather discouraged about how this would come out. Maybe
there's a better way to make the coupled lines that's easier for a
typical ham with minimal shop facilities to handle. Or maybe if there
was enough interest, someone could make some boards with guaranteed
performance.

On the positive side, I did find BNC jacks that edge-mount on PC
boards, so that part of it becomes easy at least.

Cheers,
Tom

Cheers

On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote:

I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

This is called residual SWR in a reflectometer. You load it with a
known good load, and what SWR you find (or what is exhibited by the
two meters) inhabits the reflectometer itself. Then you flip it over
and apply your source into the goesoutta with the known load on the
comesinna. You then proceed to reduce the residual SWR in both
directions.

Finding a good load is another matter, and I reported one (a precision
RF resistor) with specific characteristics here last week. Consult
the thread "RF Power Resistors from Caddock."

All of $10-$20 to accomplish.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote:

I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

This is called residual SWR in a reflectometer. You load it with a
known good load, and what SWR you find (or what is exhibited by the
two meters) inhabits the reflectometer itself. Then you flip it over
and apply your source into the goesoutta with the known load on the
comesinna. You then proceed to reduce the residual SWR in both
directions.

Finding a good load is another matter, and I reported one (a precision
RF resistor) with specific characteristics here last week. Consult
the thread "RF Power Resistors from Caddock."

All of $10-$20 to accomplish.

73's
Richard Clark, KB7QHC

I'm in Australia!

Suzy wrote:
"Richard Clark" wrote in message
...

On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote:


I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

This is called residual SWR in a reflectometer. You load it with a
known good load, and what SWR you find (or what is exhibited by the
two meters) inhabits the reflectometer itself. Then you flip it over
and apply your source into the goesoutta with the known load on the
comesinna. You then proceed to reduce the residual SWR in both
directions.

Finding a good load is another matter, and I reported one (a precision
RF resistor) with specific characteristics here last week. Consult
the thread "RF Power Resistors from Caddock."

All of $10-$20 to accomplish.

73's
Richard Clark, KB7QHC


I'm in Australia!

http://www.caddock.com/ has a link to their Australian distributor
(granted, they may have some punitive minimum order).

A begging letter to Caddock for a sample might work?

Or, do the big mailorder companies ship to Australia (the parts are in
the few bucks range.. postage and shipping could be more)

"Jim Lux" wrote in message
...
Suzy wrote:
"Richard Clark" wrote in message
...

On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote:


I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

This is called residual SWR in a reflectometer. You load it with a
known good load, and what SWR you find (or what is exhibited by the
two meters) inhabits the reflectometer itself. Then you flip it over
and apply your source into the goesoutta with the known load on the
comesinna. You then proceed to reduce the residual SWR in both
directions.

Finding a good load is another matter, and I reported one (a precision
RF resistor) with specific characteristics here last week. Consult
the thread "RF Power Resistors from Caddock."

All of $10-$20 to accomplish.

73's
Richard Clark, KB7QHC


I'm in Australia!

http://www.caddock.com/ has a link to their Australian distributor
(granted, they may have some punitive minimum order).

A begging letter to Caddock for a sample might work?

Or, do the big mailorder companies ship to Australia (the parts are in the
few bucks range.. postage and shipping could be more)

Thanks very much for that, but I am hoping to get a Bird Termaline 50W dummy
load, which I hope will enable me to progress. Thank you all for holding
this floury (not flowery!) hand.

Jim Lux wrote in news:fntlce$h96$1
@news.jpl.nasa.gov:

http://www.caddock.com/ has a link to their Australian distributor
(granted, they may have some punitive minimum order).

A begging letter to Caddock for a sample might work?

Or, do the big mailorder companies ship to Australia (the parts are in
the few bucks range.. postage and shipping could be more)




Suzy,

I had a look at Farnell (au) for these type of resistors recently
(Caddock are not the only maker).

My recollection is that a 100W resistor is likely to be good for 50W -
60W in a practical application, and they were about $28 ea + $12
shipping, and IIRC minimum order quantity was one.

That is not to recommend them for a simple accurate load for 70cm.

Owen

"Owen Duffy" wrote in message
...
Jim Lux wrote in news:fntlce$h96$1
@news.jpl.nasa.gov:

http://www.caddock.com/ has a link to their Australian distributor
(granted, they may have some punitive minimum order).

A begging letter to Caddock for a sample might work?

Or, do the big mailorder companies ship to Australia (the parts are in
the few bucks range.. postage and shipping could be more)




Suzy,

I had a look at Farnell (au) for these type of resistors recently
(Caddock are not the only maker).

My recollection is that a 100W resistor is likely to be good for 50W -
60W in a practical application, and they were about $28 ea + $12
shipping, and IIRC minimum order quantity was one.

That is not to recommend them for a simple accurate load for 70cm.

Owen

But hopefully the Bird Termaline 50W model 8085 will do the job?

"Suzy" not@valid wrote in :


But hopefully the Bird Termaline 50W model 8085 will do the job?

I think it is rated to 3GHz, look after it and it will be very useful.

People tend to overlook the value of a known dummy load, it is a very
useful piece of test equipment, and one of the first you should own.

If it is used, take the opportunity to test it some time.

With that in you kit, you have a chance of calibrating a VSWR meter
sampler.

You might find the following article of interest, skip the maths if it
gives you a headache, but the discussion towards the end is relevant to
setting up / testing a VSWR meter: http://www.vk1od.net/VSWR/VSWRMeter.htm
..

Perhaps you have some trusted ham neighbours who can help you out.

Owen

Suzy wrote:
"Owen Duffy" wrote in message
...

Jim Lux wrote in news:fntlce$h96$1
:


http://www.caddock.com/ has a link to their Australian distributor
(granted, they may have some punitive minimum order).

A begging letter to Caddock for a sample might work?

Or, do the big mailorder companies ship to Australia (the parts are in
the few bucks range.. postage and shipping could be more)


Suzy,

I had a look at Farnell (au) for these type of resistors recently
(Caddock are not the only maker).

My recollection is that a 100W resistor is likely to be good for 50W -
60W in a practical application, and they were about $28 ea + $12
shipping, and IIRC minimum order quantity was one.

That is not to recommend them for a simple accurate load for 70cm.

Owen


But hopefully the Bird Termaline 50W model 8085 will do the job?




in AU, have you checked surplus places like
http://www.users.bigpond.com/alandevlin/forsale.html

Indeed, it shows a HP 774D as being sold, but perhaps he'd have other stuff.

On Jan 31, 11:12 am, "Suzy" not@valid wrote:
Hello Tom

My responses**

"K7ITM" wrote in message
....
OK, I gave this some thought last night. I see a couple problems...

Though you could use 1mA meter movements, that puts you at a detected
power level high enough that the meter scale won't be linear in power,
assuming Schottky or germanium diode detectors.

**Pardon my ignorance, but isn't it just a case of using an op amp or
whatever to suit whatever meter movement I have? ANyway, I have now sourced
a 100 microamp meter (MU65) with a 3.9K resistance (sounds strange as the 1
mA one has a 210 ohm resistance.

Ah, OK. I had assumed you wanted to use just the meters, with no
amplifier. With amplifiers, the 1mA meters will be fine. But we need
op amps that have very low offset voltage and drift--I would prefer to
set the meter full scale to correspond to around a millivolt or two of
detected DC. I suppose if you have a way to zero the offset and it
doesn't drift, that'll be OK. Then we need to make sure the amplifier
is reasonably immune to 450MHz signals floating around... This is all
"do-able" but there are some details you'll have to pay attention to.
I'd suggest using a couple of the RF power detector ICs available from
Analog Devices or Linear Technology, but we're back to surface mount
stuff again at that point.


BTW, I'n not bothered about the linear issue. I will be having two meters to
show forward and relected powers simultaneously. I don't want to clculate
actual SWR.

Well, yes, but wouldn't you want to know whether "0.1" on the meter
represented 1/10 the power of "1.0" on the meter, rather than 1/100 of
the power?? If you don't pay a little attention to the level of RF
the detector is actually detecting, you're liable to have that
problem.


....
I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

What do you mean "no" test gear? You'll have the directional coupler
with meters itself, and a power source. The only other thing you need
is a 50 ohm load to put on the coupler output. Is it not worth having
at least a load you can trust? With a known good load, you feed some
power through the coupler and adjust for zero indicated return; turn
the coupler around and make sure the other port also reads zero. With
an open or short load you should get equal readings on the forward and
reverse meters. (I suppose you need two couplers and a good load to
insure that the through line of the coupler is also the same impedance
as the load...) I have a good network analyzer on my bench at work,
but without a calibration load to test and calibrate it with, I don't
know how good its readings really are.

Cheers,
Tom