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[Amps] hp 8405a mostly working - yea!!

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Subject: [Amps] hp 8405a mostly working - yea!!
From: Dan Sawyer <>
Date: Sat, 17 Dec 2005 16:43:07 -0800
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I want to thank many of you for a great deal of help. After more the 6 
months, most of which was spent working around the 'white elephant' 
project in the middle of the bench the 8405a is mostly working. The 
issue was a broken drive transistor in the pulse circuit and many bad 
diodes on the probes. I ended up getting another unit to get a base 
probe board.

I am using 1N5711 diodes as replacements. They seem to work very well. 
The only issue remaining is probably an alignment issue. The unit shows 
about a plus to minus 2 degree phase shift over 120 to 160 MHz, as an 
example. The phase is VERY sensitive to tuning so it is reasonable that 
minor difference in the sensitivity between the two probes can cause 
phase shifts.

Now - how is this thing used? The objective that started this adventure 
was to measure the impedance of an 80 meter mid-loaded dipole over a 
medium to poor ground. I gave up on a direct reading and shifted to a 
'representative example". Currently, a scale model on 2 meters in the 
shack. (The shack is closer and it is winter here so it is much easier 
to work there.)

The first objective is to actually measure the performance of this 2 
meter model. It is constructed of # 10 solid copper wire, 100 cm base, a 
5 turn, loose wound coil, about 25 mm in diameter, and about 20 mm long, 
and a 100 cm tip. Using a 2 meter rig on 5 watts an SWR meter is very 
happy at about 144 MHz and reads about 1.1.

Now introduce the 8405a into this. I took a commercial bi-directional 
coupler, terminated it in 50 Ohms and adjusted the 8405a to read 0 phase 
at 144 MHz. (This was about plus 30 degrees) I then connected the 
coupler to the base of the antenna with the 8405a and signal still 
attached. After some adjusting the 8405 reads close to 0 at about 143 
MHz.  (This was also very sensitive).

My immediate question is: Is the setup actually reading the resonance 
(or close to it) of this antenna setup? If not what did I do wrong? How 
can this procedure be fixed to read the impedance.
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