RF Filters - What do you use / Why aren't you?

[vagrant]

Another members' post has me thinking about filters and how many I use for my main scanners. The antennas I use are a wideband discone and another that covers 800-925 MHz. A K&L (5IZ10-20/X1300-0/0) diplexer joins the two coaxial cables and then comes the filter fun.

Filters in no particular order:
A. 1-60 MHz ( I transmit on HF so this helps to some degree )
B. HF, AM & FM Broadcast bands ( Crazy strong RFI at my location )
C. NOAA Weather around 162 MHz ( The signal booms in here )
D. Two 152 MHz paging filters (It's really bad here and I need two)
E. Another FM broadcast 88-108 filter

A is made by Palomar Engineers
B is from ScannerMaster. HPN-30118 combined notch filter
C, D, and E are made by Dale Parr ( Par Filters)

I also monitor military aircraft and use the regular discone as well as an AT-197A/GR military air discone and a mil air 225-400 MHz filter made by Dale Parr too. They're connected to separate scanners/receivers and not both feeding together into one line.

I don't really monitor civilian air, so filter B works for my use. A more friendly filter for civil aviation that attenuates FM broadcast would be the Mini Circuits ZBSF-95+. It costs more, but if you enjoy monitoring civil air it should be in inline on your system.

* * * * * How did I discover I needed filters * * * * *

It started with garbled noise I would receive coming in with other signals, or wiping them out. Using an inexpensive USB dongle I initially took a look around 440 MHz and watched the waterfall on the computer screen. When the noise started and stopped on a receiver, I noticed the noise floor would shift simultaneously. It turned out to be a simulcast paging system in my area way down near 152 MHz. There are four transmitters and they're using 100 and or 300 watts per my FCC search to find the culprit. Their intended targets are pagers that have compromised antennas, so they really blast the watts to reach them. Also, if you think you don't need a filter because you only monitor digital modes, well if they're really strong signals, you may be fine.

While the paging system is a nasty offender, I also have a NOAA weather station nearby around 3500' above me in the Sierra's that uses 300 watts. Then we get to the unfriendly RFI from AM and FM broadcast stations using infinitely more watts. Some of those transmitters are less than five miles away with most others less than 25 miles from me. Additionally, I use similar filters inline on my HF and VHF/UHF amateur radios as well to help block the RFI. Those filters are specially made to use with transmitters.

* NOTE: Not everyone needs a filter. If you monitor a strong nearby transmitter or systems that have clear signals you are fortunate.

* EXTRA NOTE: Due to the losses from the filters and adapters, I use a pre-amp inline between the filters and scanners/receivers. You may or may not need to add a pre-amp. Everyone's needs are different. I do not recommend a pre-amp more than 10dB. Your noise floor will become unfriendly.

 

[KevinC]

I have notch filters for TV ch 8,11 and 13, FM broadcast and a band stop that kills everything above 470. I live about 10 miles from the huge antenna farms where all the TV stations are and with the repack I had too much energy below about 550 and had to kill everything above 470. For the 7/800 stuff I have diplexers that split above/below around 550 MHz.

Antenna is a R/S Sputnik on a Mini-Circuits ZFL1000VH amp into a Mini-Circuits 8-way splitter. For 7/800 I just have a 3 dB gain mobile mounted to a piece of 18”x18” metal.

 

[vagrant]

I initially addressed my 152 MHz RFI issue by making a simple stub filter using some coax and connector I had laying around. Even if I had to purchase the parts, it is an inexpensive method of filtering out unwanted noise. While it worked, it was quite wide, attenuating where I did not want and not enough on the interfering frequency. Additionally, the filter is unfriendly on the odd harmonics 456, 760 MHz. The attenuation reduces the higher it goes, but the 456 MHz is right in there with signals I want to receive. Another example is a stub filter for FM broadcast station on 90 MHz that may cause RFI. The odd harmonics there are 270 and 450 MHz which mil air and various public service use respectively. Still, a simple stub filter is a great way to test before spending money on a quality filter if needed.

There is also the unrealized RFI issue when a person improves their coaxial cable and antenna to something outdoors above the roofline. Suddenly there may be constant, or intermittent noise/static when monitoring particular frequencies that were not there before. I doubt their receiver, scanner, SDR dongle suddenly "failed" when in reality it is unrealized RFI that was introduced with the improved coax and or antenna. Overall the signal or signals may have improved, but the RFI comes right along with it in through the improved antenna, through the improved coax and into the scanner. An inline filter helps attenuate the unwanted "noise". If one uses a pre-amp, ensure the filter comes first on the line, then the pre-amp, then the scanner. No need to amplify RFI.

The mantra of upgrading to "...improved coax and antenna..." should include an FM broadcast filter and consideration of additional filters, especially the 152 MHz paging signals.

While I have quite a few filters made by Dale Parr, perhaps a third of them were purchased used. I enjoy saving money, but the $70 or $80 each for a quality filter is worth the money. First, find out the offending frequency using an inexpensive SDR dongle and then tell Dale the frequency when you order. He will tune it and seal the plugs before sending it.

* Even if there is a particular frequency you want to monitor, the signal may be causing you RFI. As previously noted I use a filter on 162.400 a known NOAA weather frequency. The filter definitely does not eliminate the signal, but reduces the RFI it causes. No need to shout when a whisper will get through just fine.

Whatever one enjoys monitoring, I hope this thread provides some help and encourages people to consider if they may need a filter or several. Not all filters are the same in quality, so do your research. I have a feeling KevinC and I aren't the only wise ones using filters. Still, we weren't born knowing we needed to use filters. We live we learn. Have fun!

 

[merlin]

I have a rather large assortment of low pass, hi pass, band pass,and broadcast traps(notch) for AM and FM.
many are from K&L, one tunable(200-380 MHz) Several are homebrew, even CB.
They help reduce noise a lot and a step attenuator that helps with front end overload.
The traps are in front of my LNA with an attenuator that gives me 6 Db gain, enough to make up for coax losses, insertion losses plus a little.

 

[kb5udf]

First, for the dedicated 700/800 antenna, I have a custom, active filter prototype (I guess) made by the late proprietor of Bluetail, given to me for testing. I think it is an amp, followed by saw filter(s)?. Works well, and that feeds a minicircuits amp into a splitter. I also have, for portable/as needed use, a dedicated 700 filter from the same person, I think it is a simple passive SAW.

For my main tower mounted discone (sirio, spaced for 100mhz plus), I feed a stridsburg FM broadcast filter->mini circuits amp->8port splitter. I think I have 2 or 3 of the ports off of this splitter going to their own coaxial dynamics 225-400 filters for mil-air.

I recently took one of the above mentioned 8 ports and fed it into another mini circuits amp to another 8 ports splitter, for more local/stronger signals, but in practice, I’m not loosing much weak signal ability.

With regard to 450mhz or so stuff, my location doesn’t really have problems with strong signals, but a bit of RFI in places; I probably need to do some more snooping and deploying ferries.

I’m not running any filters for 150mhz or so stuff. We don’t have much VHF paging left around here that I can see and while the somewhat near and strong NOAA WX can mess with a bit of nearby spectrum, there generally is not much in that area to hear, since it is all encrypted federal. The one exception is a few helicopters that inspect oil rigs on 166mhz NFM, but that’s far enough away from NOAA. In any event, I don’t have trouble with marine radio either, so I can let that be for now, and if I need to listen to something weak near the one pager of NOAA WX, I fire up the Airspy discovery HF+, which does a great job with nearby strong signals.

I do have a small rockwell/collins Civil air and filter I can use as well, but don’t always do so, b/c if scanning civil air with the HF discover plus it’s not needed. Also, oddly enough the airspy with the coaxial dynamics 225-400 filters still does fine on air and and vhf hi. I probably need to sweep the things and see where exactly they attenuate, but they definitely help with noise floor on 225-400 as shown by my ability to receive the UFO sats.

In summary, I’m going to guard that active 700/800 unit (same case design as a p25rx) Todd gave me with my life, because it was a game changer at my location, especially with cellular BAND 14, which is very strong at my location.

73’

 

[vagrant]

Thanks for the info on the Coaxial Dynamics filter. I will purchase one and compare it with my PAR 225-400. As to cellular problems, a tower was installed around 30 meters away. Yeah, good times

 

[mtindor]

Over the years I have bought numerous filters (and antennas) from Par Electronics (Dale Parfitt - W4OP). Sure, they are not cheap, but they do an amazing job. Where I am at now, I need no filters. There just isn't anything useful to pick up in this rural area. But back in the day my worst enemies were 152 Mhz and 454 Mhz paging.

One should check out the QSO Today interview of Dale on the QSO Today podcasts ( or read the transcription at W4OP ). Check out his QRZ page at W4OP Callsign Page for some interesting radios that he has either built or restored.

I really enjoyed Eric's interview with Dale on the QSO Today podcast.

m

 

[dlwtrunked]

I have notch filters for TV ch 8,11 and 13, FM broadcast and a band stop that kills everything above 470. I live about 10 miles from the huge antenna farms where all the TV stations are and with the repack I had too much energy below about 550 and had to kill everything above 470. For the 7/800 stuff I have diplexers that split above/below around 550 MHz.

Antenna is a R/S Sputnik on a Mini-Circuits ZFL1000VH amp into a Mini-Circuits 8-way splitter. For 7/800 I just have a 3 dB gain mobile mounted to a piece of 18”x18” metal.

As the real goal is to lower raise signal-to-noise ratio, and the ZFL1000VH has a fairly high NF (noise factor) near 5 dB, I would chose for a better amp.

 

[KevinC]

As the real goal is to lower raise signal-to-noise ratio, and the ZFL1000VH has a fairly high NF (noise factor) near 5 dB, I would chose for a better amp.

The amp is to overcome the loss of my splitter. At the time I got it (used) it was the cheapest I could find that had good IP1/IP3 specs. Due to the "repack" I have a ton of energy right above 470 MHz, but even with the IP1/IP3 of that amp I still had issues so I use a band stop at 470 now. As funds become available I may revisit replacing the amp, but for me it seems to be working fine.

 

[jwt873]

The only filter I have is a home brew low pass filter for listening to non directional air beacons as well as digital transmissions on the 630 and 2200 meter ham bands.

It's home brew. It's supposed to attenuate everything over 500 kHz, but I swept it and the cutoff is around 800 kHz. Still... It works well. Gets red of most of the AM broadcast hash in the VLF range.

 

[kb5udf]

I did try out one of the more “modern” much lower noise mini circuits amps, but it didn’t last much more than 2 weeks before it some how stopped functioning. I can only guess it was the transmitters on my tower or static from nearby lightning, but nothing else was damaged. In any event I couldn’t really notice much practical improvement with that unit and at 200 bucks a pop vs a 35 to 50buck used z fl it was pricey.