High Pass Filter

In the quest to cut the cord and be able to watch OTA TV from the Atlanta area, I started experimenting with antennas just to see what was feasible. The desire is to be able to receive the major networks, ABC, NBC, CBS, Fox and PBS. I live about 60 miles ENE of the transmitters and they are all within about a 6 degree azimuth. (see https://www.fcc.gov/media/engineering/dtvmaps)

I have a 25 year old "35 mile" antenna and just for grins, I plugged it in to the left over Dish Network cable and propped the antenna on top of a nearby bush about 5 feet off the ground. I had no hope that this would provide optimum results, but to my surprise, I received all the desired channels at full strength as reported by my HAUPPAUGE WinTV-quadHD PCI Express TV Tuner Card about 60 feed away from the antenna. The results were very interesting indeed, but I wanted to get rid of the old Dish cable and satellite dish mounting hardware.

The solution seemed obvious. I bought the hardware and mast necessary to mount the antenna on the roof about 25 feet up. Everyone knows, 25 feet elevation is much better than 5 feet on top of a bush. A quick channel scan and lo and behold, I received all the same channels except for WXIA-NBC transmitting on RF channel 10. Say what? I spent hours getting on and off the roof rotating the antenna in small increments with the same result.

The new digital TV is kind of funny. Generally speaking, you either get perfect reception or no reception. If the tuner can't lock on to the channel, it moves on looking for the next channel higher up the band. For some reason, the tuner couldn't lock on to channel 10, even though I suspected the signal strength was more than sufficient.

In my mind, the next logical step was to upgrade my antenna to a super duper "100 mile" antenna designed for High VHF and UHF (where all my desired stations resided). I went from an old VHF/UHF antenna with a boom length of maybe 5 feet to a more appropriate brand new High VHF / UHF antenna with a boom length of nearly 8 feet. The results were exactly the same except I had to be more particular about how I aimed the antenna because of the much narrower beam pattern.

Time to regroup. I'm an old Ham who grew up in the analog age and I really wanted to get a look at the actual signal to try and see what was going on. After a little research, I came across the RTL-SDR software defined radio dongle and figured I could use it to take a peek at the signal. Here it is in all its glory:


This thing was $22 from Amazon and I'm so sad that I didn't buy one a long time ago. It can tune signals from 24 Mhz to 1.7 Ghz and with the SDR# (SDR Sharp) software package, it can decode Wide Band FM, Narrow Band FM, AM, CW, LSB, USB and DSB. With an external program, I was able to decode APRS packets on 144.390 Mhz.

I plugged my TV cable into the dongle and tuned to 192 Mhz and I see a beautiful pilot signal at 192.310 Mhz. The bandwidth of the dongle is only about 2 Mhz, so I had to scan up the band to see the remainder of the 6 Mhz TV signal. This is when I discovered a 15db anomaly as seen below:


By chance, I had Wide Band FM demodulation selected and when I placed the red line where you see it, from the speakers I hear a very clear station from the FM broadcast band. That told me all I needed to know. Intermod interference from the FM broadcast band was killing channel 10. There were several other lumps with sometimes multiple stations heard, but this one was the loudest. It seems that when the antenna was mounted 5 feet off the ground, the house was probably blocking the FM signals.

More research and I discovered that interference from the FM broadcast band is fairly common on channels in the High VHF TV band, especially in fringe areas. FM trap filters are available, but I decided to try my hand at building my own. Since I have no local channels in the Low VHF portion, instead of a trap filter, I thought it might be easier to build a High Pass Filter with a cut-off frequency around 150 Mhz. I just need to start passing signals higher than 108 to 180 Mhz (my PBS station on channel 8) so my target was pretty broad.

Referring to https://www.changpuak.ch/electronics/chebyshev_highpass.php for the filter design and

http://hamwaves.com/inductance/en/index.html#input for help with building the inductors I was able to come up with a design using 2 inductors and 3 capacitors.

Here is my very sophisticated jig I used to wind the coils using wire from an old toroid wrapped around the threads of a small lag screw.


I built the following from some scrap sheet metal and piece of PCB material:


It ain't pretty, but placed in line with the TV cable, there is no trace of the interference and the tuner now sees channel 10 along with all the others. I could not measure any insertion loss, so it must be minimal.

11 February 2019

There is an interesting article starting on page 62 in the March 2019 QST titled "Finding Interference With SDR".