"Richard Clark" wrote in message
...
On Wed, 30 Mar 2005 00:48:17 GMT, Jaggy Taggy
wrote:

I recently got myself a noise bridge to do some quantitative measurements
on
my antennas.
Lets say I am a bit disappointed. The small dials of the unit make all my
antennas to appear very similar indeed and I am wondering if the unit is
broken (not according to the test I did with a known load) or if I am
expecting way too much.

Hi Uwe,

The Cadillac of Bridges is the General Radio 1606-A. However, it
requires you to have a source and detector. It is only a Bridge, but
it is a precision Bridge. The circuit is quite simple, but its triple
shielding and isolation are old-world craftsmanship. These things
sold at a price equivalent to 6 months wage for a bench tech. 6
months wages currently will buy you equipment that could do what was
only possible in a standards lab back then - but this is still in the
region of 5 figures.

You will need a stable source capable of 1 to 10mW power. You will
need a well shielded communications receiver to act as a detector.
You will also need a steady hand to balance the bridge (which may be
part of your problem with the noise bridge). Nulls are often more
than 100 dB deep. If you lack sufficient shielding for the detector,
this will degrade the nulls considerably. If you lack stability, you
will never find the null.

The MFJs of the world are quick and dirty, but when they reach their
limits you can only guess because they will as easily feed you a bogus
reading when they are out to lunch. The GR 1606 has limits too, but
they are obvious by dial indication and a top end of 60MHz usage (a
more practical top end, however, is 30MHz).

73's
Richard Clark, KB7QHC


Richard

As you know, I built that slotted line that works fairly well for load
impedance measurements at 2 meters, and can be used at 100 MHz for FM
antenna work. But, it is big and ugly. I do like the concept MFJ uses
even though the results are surely less accurate than a good bridge. If I
get an urge to include a HF antenna or other device whose impedance I want
to determine, I might buy a MFJ. But, if a decent bridge ever becomes
available at one of the HAM swap meets I'll sure pick up anything thats
affordable.
I am impressed that computer programs seem to have made impedance
*measurement* unnecessary to most HAMS. I sure wish I was enough smarter
to be able to manipulate the computer so I could get confidant about
computer program results. That really impresses me when I read about how
accurately antenna impedance can be predicted for various changes in
parameters.

When you guys arent argueing about stuff I cant understand, I sure learn
alot from this Antenna Group.

Thanks
Jerry