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VK5ZVS |
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Projects 'n' Stuff |
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Broadcast Rejection Filter. |
The good thing about flying a kite with an antenna wire attached, is
not only the excellent transmit signal but also the receive signals. The bad news is, that this also means
the received noise level increase too! This filter was built to help minimise the out-of-band signals
from overloading the front end of the radio with the various signal mixing combinations that may
be present at the receiver input.
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The original circuit is from the ARRL Handbook and has been
around for many years. Obviously, a testament to it being a good filter.
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A little further theoretical analysis shows that the rejection of broadcast radio
signals near 1600kHz to be not that well attenuated at all and could lead to receiver performance problems,
especially when flying a kite! The program I used for the filter analysis is called ELSIE from TonneSoftware. The student version
is avaiable as a free download from http://www.tonnesoftware.com/elsie.html.
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Here's the schematic for the filter after using the program's
optimisation feature. The values have been rounded to the nearest integer value and the LC meter
I've built, has been used to find suitable capacitor combinations from standard component values.
The inductors were designed using a program called mini Ring Core Calculator V1.2 by DL5SWB and
available as a free download from
http://www.dl5swb.de/html/mini_ring_core_calculator.htm. Mind you, this analysis is theoretical. Let's see
what happens in real life when we build this circuit...
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Here we can see that the Transmission loss (attenuation) is in the order
of -45dB as compared with -6.9dB at 1.6MHz in the original filter design. The loss at 1.8MHz has only changed
by about 0.1 to 0.2db.
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Here we can see the differences between the original filter
and the version I've created. The dashed line -1- indicates the optimised design (see above display).
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This is where theory meets practical. We can see, we are actually
quite close to the theoretical values with our built filter. The vswr values are better.
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The measured return loss is a little higher
than expected, but has typical values in the pass band as that of other
similar practical filter designs. Some "fine tuning" may help further increase the
transmission loss at 1.6MHz. This is a good starting point for evaluation that
the filter actually works in the way I've intended.
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I've included the partslist of the components I've used
in this project, so comparison can be made to those used in the original article.  
L2 = 3.25uH, T50-2 core, 25T, 0.4mm
L4 = 3.94uH, T50-2 core, 28T, 0.4mm
L6 = 5.11uH, T50-2 core, 32T, 0.4mm
C1 = 1155pF, 1nF // 150pF styroseal
C2 = 0.0127uF, 1 x 0.012uF polyester
C3 = 1520pF, 1n2 // 330pF styroseal
C4 = 2600pF, 1n2 // 560pF // 830pF styroseal
C5 = 1767pf, 1n2 // 560pF styroseal
C6 = 2590pF, 1n2 // 1n2 // 120pF styroseal
C7 = 1454pF, 1n2 // 270pF styroseal
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