AM Broadcast Rejection Filter Testing

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I have a 270 foot off-center fed dipole which works pretty well for my 160m and other band needs (with an external tuner). My HF rig is a Yaesu FT-450D which appears to have a great front end to suppress unwanted signals. Occasionally I like to look at the spectrum in Gqrx with my RTL-SDR dongle. But take a look at the image below!

That’s a lot of annoying interference

The RTL-SDR does very little to clear this up, but given the reasonable price and utility of the device, it is not unexpected.

I have the RTL2832U model SDR which works on HF

I have a number of very strong AM broadcast stations nearby so I thought I would give a couple filters a go at it. First, I tested with a cheap RTL-SDR brand “Broadcast AM Reject Filter” which attenuates below 2.6MHz.

RTL-SDR Broadcast AM Reject Filter

Now let’s take a look at the Gqrx spectrum waterfall

40 meters in Gqrx with SDR filter

That’s a great amount of cleanup! Now I know my FT-450D does a better job than the SDR dongle, but I figured I would check out a legitimate filter which can be used with a transmitter. I found the M-402x by Morgan Systems which attenuates below 1.8MHz and can tolerate 200 watts of power (they have another version if you don’t use 160m). I ordered this from KF7P Metalwerks which came quickly and excellently packaged.

M-402x Filter

Now let’s take a look at this filter in the Gqrx spectrum waterfall like we did with the other. Obviously my attenuation is a little bit less because I wanted to use 160m which is close to the AM broadcast band. Also, these pictures were taken at night when the signal was hitting me the hardest (in the day it looked even better).

Gqrx with the M-402x

I am very happy with the results and the M-402x is now installed with the FT-450D and I am currently listening to a great signal on 1.900MHz!

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