Filters for Low VHF DTV DXing

[WHJ]

When listening for distant low band DTV signals, relatively close FM broadcast transmitters can overload many DTV tuners. I have a variety of filters for that use. I recently saw a low price one made by Televes which might be useful. It was made for CATV to separate return signals below 75 MHz from the regulator cable channels above there. Because of that, it can only be used to separate channels 2, 3 and 4 from the FM and above channels. Channels 5 and 6 need to be dealt with via other methods.

I am referring to the Televes 769220 filter.

I obviously removed the back of the filter.

The frequency response of the low frequency path is below.


The frequency response of the high frequency path is below.


Due to only passing channels 2, 3 and 4, purchasing one for yourself might be dismissed. Its price might make you consider it--$12 plus under $6 postage! I do not have anything to do with selling them, but I purchased my one from eBay. there are two sources:
Televes CoaxData Diplexer Filter 1Gbps TV-Data 2-68 Mhz/87-2150 Mhz 8424450167205 | eBay
Televes CoaxData Diplexer Filter 1Gbps TV-Data 2-68 Mhz/87-2150 Mhz 8424450167205 | eBay

Below is the Mini-Circuits filter that I currently use for eliminating frequencies below channels 2 and 3. Due to using that with my 9-element antenna with a 23-foot boom made for just those two channels, that's perfect.


At first thought, single frequency filters for your strongest FM station seem like a better idea. But they only cover one station. It is the cumulative signal level of all of the FM stations and the others passing through your tuner's filters which is important. Removing just the strongest station often does not solve the problem. If I had a strong signal on 88.1 MHz, I might still use the above filter along with a single frequency one tuned to 88.1. It is very difficult to pass channel 6 while eliminating everything at 88.1 MHz and all above there.

[WHJ]

I modified the above information so that I could better organize similar filters for easy comparison.

For my eight-element channel 4 and 5 antenna with a 13-foot boom I use a Mini-Circuits BPF-A69 low pass filter to minimize problems from strong FM signals.

Channel 6 reception is not limited too much.

For channel 6 reception I use my 10-element antenna with a 20-foot boom and have a CirQTel FBT filter to limit FM signals.

My strongest FM stations start at 96.9 MHz so this filter is adequate.

I use a vertical dipole antenna for more vertically polarized signals and to alert me if my directional antennas are pointed the wrong way. I use a K & L 8B114-70 filter with it.

I have not had problems using this filter.

[WHJ]

Win using one of my Winegard HD7698P antennas I want to eliminate any contaminating low VHF TV and FM signals. I use either Mini-Circuits RHP147 or SHP-200 filters to do that. The response of the latter is shown.


To send just the UHF TV signals to a tuner I use a Mini-Circuits BPF-A535 filter.


If I want to concentrate on reception of just one channel, I have Microwave Filter Company filters for channels 9, 21, 23, 25, 30, 32, 33, 34, and 36. An example of the 3278(4) filter for channel 25 is shown.


That has been just a small sample of the large number of filters which I have. Let me know if you are interested in seeing more and what type of filter which you are most interested in seeing my measurements on.

[Crazy Monkey]

I need to spend some more time and carefully review the spectrum using my Icom 8600. I'm thinking I haven't eliminated all the intermod within the low-VHF TV frequencies.

I'm currently using a Tin Lee custom trap with five notches for 88.1, 89.7, 91.3, 94.9, and 97.5. Those are all the local frequencies that overload the receivers here, with towers 1.59 miles and 2.0 miles away. I don't think that applying this 5-notch trap is enough. It attenuates each frequency 20 dB, and the overall insertion loss is low. I think I'll turn the preamp's FM trap on. That should attenuate everything above 92 Mhz (protecting channel 6), but I'll apply another notch to 89.7. I suspect that 94.9 and 89.7 are mixing and creating intermod on 84.5 (right in the middle of channel 6). I should be okay leaving 88.1 at the level it's currently at. If I can attenuate everything else above that, there shouldn't be any more mixing of 88.1 with other frequencies.

[WHJ]

With all of my changes this year, I don't have things perfect yet either. After receiving WCES on channel 6 from Georgia today, I rechecked things. I received WCES better on my channel 4 and 5 Yagi than my one for channel 6. That was due to the IMD that I still had from FM stations while using my channel 6 antenna. I swapped the filters that I had to the two antennas which helped the 6 antenna without hurting the 4 & 5 antenna.

I just ordered two more of the better filter--a Mini-Circuits BPF-A69. See: https://www.minicircuits.com/pdfs/BPF-A69+.pdf That is a relatively large SMD IC. My complete filter assembly is below.


Based on my use of it, I would redefine it as a channels 2 - 6 band pass filter. It improves things on channels 2 and 6 more than its minor losses hurts things. I purchased them from an eBay seller that I do not have any experience with. https://www.ebay.com/itm/193499033307 That price is a bit lower than that from Mini-Circuits or Mouser.

I found a lot of other towers near the one which WONE uses. There could be some non-broadcast transmitters on them using frequencies which are adding to your problem. Plus, your lower frequency FM transmitters make things more difficult.

[Crazy Monkey]

I have an FM trap that attenuates the entire FM band... 88 MHz and up. It has some attenuation at channel 5, but some channel 5's still get through. Channel 6 is a no-go with this filter. But I've done testing with it. I used to have a local analog "pretend FM" (Franken FM) on 87.7. They transmitted a test pattern video on 6. Distance was 22 miles. The FM trap would actually deliver good quality black and white video. Without the filter, channel 6 looked like a disaster with FM interference lines. If I recall, I believe I had WMDE 5 at 326 miles via Es using this filter.

It's extremely difficult here. My local 88.1 is 2 miles away and 7500 watts ERP. So it's hard to attenuate 88.1 and protect channel 6. I have a semi-local WOUC on DT 6 at 69 miles south of here. I've only seen it decode a few times with my tower-mounted VHF TV antenna. But yet I ran an autologger at my brother's house, 7 miles away, and he could occasionally get WOUC 6 to decode with a UHF-only antenna!! That was quite disappointing. So it made me give up on my pursuits realizing there's only so much that can be done to solve my problems here.

[WHJ]

I would use your single frequency notch filters, one of the BPF-A69 filters and then slowly add attenuation to the input of your tuner until the IMD noise went away.

Yesterday, after swapping my filters I added a 7 dB F attenuator to my channel 4 & 5 antenna and a 12 dB F attenuator to my channel 6 antenna. Those eliminated my IMD generated noise. I use homemade antenna mounted preamplifiers which are hard to overload, intending to add attenuation to the tuners to prevent their overload.

High gain transmit FM antennas will send most of their energy over your house 2 miles away. However, I noticed that at least WONE just had a two-bay antenna which will not have that narrow of an elevation lobe.

I suspect that WOUC could be received at your brother's house is because his UHF-only antenna provided weak FM signals which did not overload his tuner while still having just enough sensitivity to receive WOUC. That's how my first sentence plan would allow for reception of channel 5 and 6 signals. It may or may not be best to remove the attenuator when receiving the lower channels.

[Crazy Monkey]

I would use your single frequency notch filters, one of the BPF-A69 filters and then slowly add attenuation to the input of your tuner until the IMD noise went away.

Yesterday, after swapping my filters I added a 7 dB F attenuator to my channel 4 & 5 antenna and a 12 dB F attenuator to my channel 6 antenna. Those eliminated my IMD generated noise. I use homemade antenna mounted preamplifiers which are hard to overload, intending to add attenuation to the tuners to prevent their overload.

High gain transmit FM antennas will send most of their energy over your house 2 miles away. However, I noticed that at least WONE just had a two-bay antenna which will not have that narrow of an elevation lobe.

I suspect that WOUC could be received at your brother's house is because his UHF-only antenna provided weak FM signals which did not overload his tuner while still having just enough sensitivity to receive WOUC. That's how my first sentence plan would allow for reception of channel 5 and 6 signals. It may or may not be best to remove the attenuator when receiving the lower channels.

With the help of Bill Hepburn, he gave me advice for identifying overload and intermodulation. I removed all amps and filters from my FM roof-mounted antenna, and I hooked the coax directly to my Icom 8600. This is a pricey receiver that is very good at resisting overload and intermod. I noticed several frequencies that fall within the channel 6 spectrum where intermod was occurring… 82.9, 83.9, 84.5 were probably the strongest. I applied frequency-specific notch filters, and then I applied overall attenuation, and I also tried a broad 88-108 full band trap. None of these removed the intermod on channel 6. I’ve determined that the intermod is coming from the towers. It’s not from my radio. There’s nothing I can do about it. I’m sure there are certain out-of-band emission limits to this intermod, although I’m not sure if it’s considered acceptable or not. I also noticed that the nearby 97.5 transmitter is messy. They’re spewing noise from 96.5 to 98.5, and it’s not a local source. I took my SDR a few more miles down the road, and I could still see this elevated noise floor. Being 1 to 2 miles from the transmitters can be a problem if the broadcasters are not doing their best to suppress these out of band emissions. Locally, it seems that getting 5 miles out from the transmitters is enough to avoid this.

[WHJ]

I also noticed that the nearby 97.5 transmitter is messy. They’re spewing noise from 96.5 to 98.5, and it’s not a local source. I took my SDR a few more miles down the road, and I could still see this elevated noise floor. Being 1 to 2 miles from the transmitters can be a problem if the broadcasters are not doing their best to suppress these out of band emissions. Locally, it seems that getting 5 miles out from the transmitters is enough to avoid this.

You could be correct; you have done the work. But I am still suspicious. That distance increase should not eliminate products coming from the broadcaster's antenna. It would be enough if it is coming from poorly shielded equipment.

Perhaps Bill Hepburn suggested something similar, but an inexpensive way to confirm that the problem does not come from your preamplifier (you have not said that you have one or not) or receiver is to add an inexpensive fixed 10 dB attenuator before your preamplifier (if you have one) or receiver. If the IMD goes down 10 dB it is being received. If it goes down by 20 dB or more, the preamplifier or receiver is creating the IMD.

I have replaced some of my BPF-A69 filters with Mini-Circuits BPF-C45 filters.

[WHJ]

I forgot about my MFC 15690 DTV channel deletion filter. It has ten null filters in series to delete the 6 MHz DTV bandwidth. I previously aligned it below its lower frequency limit to eliminate channel 6. It worked fine even at that frequency.


I decided to realign it to see how well it could delete all of the FM frequencies. My goal was not to reduce any part of channel 6 any more than 5 dB while reducing the FM frequencies as much as possible. Most of the FM frequencies are down at least 40 dB. Another way of dealing with it would be to align each of the ten filters to my problem frequencies. If one station was really bad, I could double the attenuation by aligning two filters to the same frequency.


I've found that even very strong high VHF signals can increase the low VHF noise level. So, signals other than FM can cause noise problems.