AM broadcast band filter

For the W5TSU.net:8030 receiver I build a broadcast band filter. This should front end overload and intermod created by these stations and allow for better Ham band receiption. I also wanted to learn how to build my own coils.

I found a design at the vk3il blog. This filter was modeled with the ELSIE (free version). It is a 7-pole Chebychev filter with cutoff frequencies of 1.7MHz.

Scmatic

I started by winding some coils from 22, 26 and 30 awg magnet wire. I use a MingHe model LC100-A meter to masure these coils. I found holding the coils with my fingers didn’t make a sizeable different in the coil. What did make a difference was how tight the coile was wound and the lingth of the wire used. The size of the winding did make some difference but not as much as the amount of wire and how stable the winds where.

To make my coils I used a thick wall plastic tube. I put a hole with a T pin in the tube and put the 22awg wire through the hole. The wound the wire around the tube. Testing told me less then 20 turns would produce a coil with less then 2uh.

As I wound the wire I would use the LC meter to learn the value. When the value reached the 1.6 or 1.4uh needed I would put another hole in the tube and push the other end of the wire through it. I then secured the coil in place with some heat scrink.

Here are some pictures of the process and the results on the radio.

Coil tube. Whole made with T pin

Coil Measure.

AM broadcast bandfilter

Band before filter

Band with filter

I used 470pf instead of the 560pf caps. The result is a higher cut off frequency. It is 2.9MHz not 1.7MHz. I’ll correct this when I get the correct caps.