Hi all, I was quite interested in Brian's measuremnets on his aerial as have
have seen some similar 'effects'. Making these measurements is not easy and
some precautions are necessary.
Brian does not say what his noise bridge is, but there are several very
different versions of this useful device. The simplest has only resistive
balance. This will only yield the correct value if the aerial impedance is
exactly resistive. Any residual reactance even due to the bridge connections
can cause errors. The connection of ground or earth connections can be one
problem area, as it was with my measurements.
I have several different wires in the grounding region and I originally
wanted to check whether the losses varied with different earthing regimes
(mains earth only, a single 1.5metre by 20mm diam copper pipe, a 70m
counterpoise earthed at the remote end.) I used a Hatfield LE300/A RF
bridge. The bridge was zeroed and the balance was achieved with the aerial,
then the different 'earthing regimes were connected in turn. To my surprise
the plain mains earth gave the best result with other connections increasing
the loss component. After a little head scratching I went back to try to
repeat the measurements very carefully. I found that were I had gone wrong
was I should have zeroed the bridge for each different earthing regime,
before measuring the aerial.
When I did this I found I got the expected result with the losses decreasing
with each extra component of the earthing regime connected. The value
dropping from 120ohms to 70ohms with all connected. (after making the
calculation to convert the parallel arrangement of the LE300 to the normal
series R-C connection) I also found that I needed to run a considerable
voltage into the bridge to get a good relative null depth, and
stability.(2.5 to 4v rms from the Wayne Kerr SD286) I hope I didn't make
too much QRM.... Hi. I did not listen on the detector but I expect that big
commercial signals being picked could distort the null. The noise bridge
uses a receiver, so has selective detector, but I still wonder what the
effect of 'received power' is on the position of the null. I suppose one way
would be to plot a series of measurements across the band.
The null on the LE300 was quite deep and narrow and very sensitive to the
reactance setting.
There is no doubt that Brian's aerial works well and he is putting out a
good signal.
One the same topic, Rik muses as to whether ground losses are frequency
dependent, I think from the data from John on the Earth Wireless web site,
it almost certainly is.
As a final 'contentious point' I believe that the final article (Appendix 1,
from 1922!) in the new LF handbook supports Laurie's contentions about the
effect of the geometry of extra top-load on losses over poor ground.
Cheers de Alan G3NYK
[email protected]
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