Author Topic: Band Pass Filter for 27Mhz  (Read 246 times)


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Band Pass Filter for 27Mhz
« on: September 24, 2021, 09:22:01 pm »
Here's a quick look at a "BPF" for the 11mtr band. This particular Band Pass Filter was built out of necessity for the current WebSDR I have running at my QTH.

This "BPF" has a total of 10 Elements (sometimes called poles) in the system, which has a very good rejection for unwanted signals appearing on the SDR waterfall. When I originally setup the WebSDR, there was not only "Birdies" but "Harmonic" images appearing, along with 2 very powerful FM Broadcast stations around 26.500 and 27.415 Mhz. This wasn't going to make a very popular SDR with those there, so I had to make a filter.

Photo of Home-Made BPF Below


As you can see from the photo, the filter is constructed in a very typical fashion, with each section being isolated from the next "well almost". Anyhow this is a very important thing to do, as if we was to omit the ground screening from each stage, we would have a very poorly working filter with unwanted break-thru likely to occur from the very signals we are trying to filter out.

There are many types of band pass filters we can use eg. "Butterworth" or "Bessel" to name a couple, but this particular filter is based on the "Chebyshev" design, which has one of the steepest cutoffs.

In basic terms the filter is merely nothing more than individual tuned stages, both in "Series and Parallel" configurations at the precise pass band center frequency. Which in this case has a center frequency of "27.250" Mhz.

Below showing the cutoff range of the Chebyshev BPF.

As we can see from the above graph, this filter not only cuts off the FM Broadcast band, but also removes all unwanted signals from AM Broadcast Stations too. From 520Khz to 1.8 MHz the dB loss is approximately -130dB and for FM Broadcast 87.MHz to 108 MHz the cut of is approximately -65dB to -80dB respectively.   

What we have effectively is to different pass filters working in unison. "RTL-SDR" do sell filters for removing unwanted Broadcast stations for both AM and FM, but they are packaged and sold as individual items. Whereas in this case we've achieved both in one simple Band Pass Filter.

Schematic Layout


Above shows the circuitry layout for the "BPF" should you wish to construct your own. You can either choose one or both methods of tuning to calibrate your filter. The method I decided on was to make the inductors variable, as opposed to variable capacitors.  The type of tuning is essentially "Permeability Tuning". I prefer this way to calibrate the filter more so than variable capacitors. But you can easily use both methods for extra adjustment if you wish.

Below is the listed component values required to build the filter. Note that some of the calculated values are not going to be easy to obtain as in fixed disc capacitors, or of the peg pre-wound inductors. So your going to need to add capacitors in either series or parallel to make-up the required values (or near as possible) to the calculated values.

Components Values

L1   0.3133 uH   
C1   147.93 pF   
L2   0.4422 uH   
C2   104.81 pF   
L3   0.1819 uH   
C3   254.77 pF   
L4   0.4422 uH   
C4   104.81 pF   
L5   0.3133 uH   
C5   147.93 pF


To calibrate (or align) the filter you can either use a "Vector Network Analyzer" or "VNA" for short with a signal injected at the input end, and the VNA connected to filters output. You will be able to have a visual picture of the filters cutoff, and fine tune these with the adjustments methods you decided to use when constructing the filter.

The other option if a VNA is not available to you, is to just simply place the filter inline between a normal CB radio that's tuned to the CF of the filter, with a power meter and dummy load connected to its output. Tune the filter until maximum forward power is achieved. Set the radio to no more than 4watts or lower when making the adjustments, as the lower the input power, the easier it be to see on the power meter.
« Last Edit: September 29, 2021, 05:53:20 pm by 26AM081 »

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Re: Band Pass Filter for 27Mhz
« Reply #1 on: September 29, 2021, 11:32:03 pm »
Good stuff, Will. Certainly seems to be working v well.  8)