Header image  
Projects 'n' Stuff  
  Home  | Back  |
Broadcast Rejection Filter.
The finished project.

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.
BC Filter
Original circuit.

The original circuit is from the ARRL Handbook and has been around for many years. Obviously, a testament to it being a good filter.
Original published schematic
Original circuit filter response.

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.
Original filter response
Optimising the filter for 160m.

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...
Optimised filter for 160m
Optimised filter response.

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.
Optimised filter response
Comparison between the two filters.

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).
Compared filter response
Measured filter response.

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.
Measured vswr response

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.
Measured return loss response
Optimised filter partslist.

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

    2006 - 2009 S.Spirat 
Another SPIRAT initiative