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.