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Junk Box NDB Low Pass Filter

Introduction

With winter approaching, many HAM and SWL

hobbyists find intrigue in tuning in NDB or

Non-Directional Beacons. Although the tunable NDB

band depends on your location, in Canada it may be

found in a band ranging from about 190 to 535 KHz.

Canadian beacons either have just a carrier (no

offset)or are tuned using the USB with about a 400

Hertz offset, however, different offset frequencies and

certainly LSB are used when receiving DX from other

countries.

Less than 10 Km away from my QTH is a 10 KW AM

radio station at 1150 KHz. On my test receiver, the S-

meter reads off the scale (> 60 dB over S-9) when

tuned to this frequency. This local radio station causes

spurious, second-order intermodulation products

(direct mixing) that all but wipes out some weaker NDB stations that I am trying to tune in. Certainly, having a 500 Hertz

crystal IF filter is useful, but attenuating this local QRM is also desirable and is the topic of this web page. Many general

coverage receivers offer limited or in some cases no filtering of the NDB band, however an outboard filter is an easy

project to build in one afternoon.

Update Oct 11, 2010: Here is a link to a version of this project built by Robert, K5TD

Project Schematic

Topics 1998 - 2002 Topics 2003 - 2005 Topics 2006 - 2009 Topics 2010 - 2011

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To the left is the project schematic. It seems

odd to build a low pass filter to reduce BCB

interference (as usually a high pass filter is

required for this purpose at HF) however for

NDB, an aggressive low pass filter is required.

For simplicity sake, a 7 element Chebychev low

pass filter was chosen. Since it is easy to wind

reasonably high-Q inductors for 10 uH and

greater inductance using number 61 material o

a ferrite torroid, the FT50-61 core was chosen

for all of the inductors. Number 22 gauge wire

was used for the coils to keep the unloaded Q

as high as possible. The FT37-61 ferrite is als

suitable, but will have less Q and require

smaller gauge wire. Use 19 turns instead of 17

for the 20.2 uH and 21 turns instead of 19 for

the 24.1 uH coil. Do not use number 43 materia

ferrite cores.

Components

I do not stock RF capacitors greater than 2200 pF,

so junk box ceramic capacitors were used to build

this filter. In fact, this design specifically uses more

common, standard value capacitors to reduce cost

and to not have to order in parts. Certainly, the

astute builder could use higher quality capacitors

or even large powdered iron torroids instead of the

ferrite cores for inductors if higher performance is

desired. Try to use high Q capacitors if you can

find or are purchasing them. Poly or silver micacaps would be great choices. You can substitute a

5000 pF capacitor for the 4700 pF called for in the

schematic.

To the right is a photograph of the components I

used in the project breadboard.

Breadboard

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To the left is the completed

project. Ugly construction as

always, was used. The

inductors were spaced at leas

2.5 cm (1 inch) apart at right

angles to try to minimize

unwanted coupling.

GPLA Simulation

Above is the plot of the filter during simulation with GPLA. The simulation calculated an attenuation of ~46dB at 1150 KHz

At 1000 KHz the signal was 40 dB down, at 800 KHz it was ~24 dB down and at 630 KHz, the attenuation was only ~5.7

dB! Clearly this filter is not suitable if the offending BCB interference is from a station significantly less than 0.8 MHz. For

my situation, this filter is acceptable. A 5 element Chebychev filter was also designed and plotted but was discarded as

there was only 32 dB attenuation at 1150 KHz.

Since I wanted to tune as high as 535 KHz, the 533 KHz cutoff frequency was chosen. Additional work to help those with

strong BCB interference at the lower BCB will be attempted in the future and presented on another web page.

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Shown above a photo of the Skookum beacon SX. It is on 389 KHz. This NDB is located in Skookumchuck BC, Canada

Links

My friend and fellow NDB enthusiast, Ken, VE7KPB has a posting on his web site showing some of the beacons he haslogged from his QTH. Consider trying some of these frequencies from your own QTH to get used to finding beacons. Noteyou must temporarily allow pop ups to see his excellent log.

We recommend this non-directional beacon search and log utility program called WWSU from VE3GOP It must be registereand is a wonderful low cost tool.I was near beacon L in Balti, Moldova (, ) in 2006. Below is a snippet from the VE3GOP program showingbeacon L and also some nearby beacons. ( ). !

Martin Francis has an excellent NDB web site including the free program called NDB WEBLOG for a number of platforms

Some beacons may be located using this NavAid web site

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Clint, KA7OEI has an informative web site regarding NDB listening including using digital computer processing to dig out weaksignals. This is also a great overview site for newcomers to NDB.

Conclusion

To the right is the outcome when I connect

my frequency counter directly to my antennacoax cable feed line. 1150 KHz is mynemesis frequency! Happily it can be tamedwith a little filtering to allow NDB listening andlogging.

To the right is the outcome with my SWLantenna coaxial feed line connected directly

to the scope. The scope was on the 0.5 voltsper cm scale, so the peak to peak voltage is0.2 volts. In just about any high-gain audioamplifier I build, if I touch my finger to theinput, I can hear AM 1150 loud and clear - nowonder!

Good luck with your own NDB adventures

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