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