2 Meter Band-Pass Filters for Front-End Overload

Discussion in 'Amateur Radio News' started by NJ7V, May 23, 2021.

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  1. NJ7V

    NJ7V Premium Subscriber QRZ Page

    Have you ever had problems operating your radio near commercial communications towers? Maybe you were a victim of front-end overload? Find out what transceiver front-end overload is and ways you can mitigate it in this video. If you participate in Summits on the Air, it's very likely you've seen this. To demonstrate, I activate a SOTA peak with a large group of communication towers on it, which is notorious for causing problems on 2 meters FM.
     
    N6EY, BA7LFI, KD5BVX and 4 others like this.
  2. NI4Y

    NI4Y XML Subscriber QRZ Page

    You need to shield the hobby PCB. The RF is getting into the radio because it is acting like an antenna. Put it in a Hammond box and ground it to the case.
     
    K4YNZ, K7IRC and NJ7V like this.
  3. NJ7V

    NJ7V Premium Subscriber QRZ Page

    Ah, I see. I'll give that a whirl. I really would like to see it work well. Thanks.
     
  4. KC8UD

    KC8UD Ham Member QRZ Page

    Nice demonstration which is convincing of the usefulness of front end filtering to protect from overload. Just a few comments about what I observed in your video.
    1. The peak signal on your spectrum analyzer was around 101 MHz which is of course commercial FM broadcast.
    2. A possible reason the signal strength decreased slightly as you walked could be that you moved slightly to the side of a directional pattern from the transmit antenna. FM broadcasters will often use a directional pattern to direct more signal to a targeted population and sometimes to reduce interference to another station some hundred or so miles away on the same frequency.
    3. The second filter (50 dB) you used is not nearly as effective as it could be if you were to enclose it in an RF tight box. As it is now it's fully exposed to strong stray RF and can't possibly do it's job well. This may possibly turn out to be your best filter with lower insertion loss. Put it in a box and mount the connectors into the ends of the box.
     
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  5. NJ7V

    NJ7V Premium Subscriber QRZ Page

    Your comments are really appreciated. Thank you! I would really like to do a follow-up on that HobbyPCB filter after putting it in a box. Admittedly I am inexperienced with stuff like that. I'm sure I can go to the internet and find a box and enclose it, but if you or anybody else could offer more detail about a good box it would be much appreciated. Maybe it could be any old box? Maybe there are huge differences and it really maters what I select. I just don't know due to inexperience.
     
  6. K9EZ

    K9EZ Platinum Subscriber Platinum Subscriber QRZ Page

    Broadcast engineer here.....

    The BEST place to be is UNDER the tower (well as best as you can). There is little power transmitted down. Especially for FM stations that more Multiple FM Bays. The more Bays, the more power transmitted out to the horizon. If the FM station is very high in elevation they may use "downtilt" on the antennas to send the signal to the horizon instead of shooting over it. The further you go out from the tower the more signal you will get as you are getting into the aperture of the FM antenna.

    As long as you dont cross the fence there is not a thing to worry about. No need to be afraid of any commercial facility EXCEPT for a radar site. Do NOT look into an operating radar antenna.

    The stations on the mountain close to 101.1 is KALV 101.5 at 100kW and omni (NON directional) and KSLX 100.7 100kW omni.

    Here is a good representation of "under the tower power". Most of these stations are running 9 dB or better gain.

    [​IMG]

    Downtilt... still very little transmitted under the tower.

    upload_2021-5-25_12-35-26.png
     
    KO4LZ, K8AI, N3DAW and 8 others like this.
  7. KC8UD

    KC8UD Ham Member QRZ Page

    You can use something like this https://www.amazon.com/Support-92x3...rds=metal+project+boxes&qid=1621960497&sr=8-2.
    Insert one connector through a hole in one end and then you may need to use short jumpers or short length of coax to extend to the other connector mounted in the other end of the box. Of course choose a box with suitable dimensions to enclose both the filter board and allow proper spacing to mount the connectors. It's possible that places like DX Engineering and Giga Parts may have a selection of metal project boxes.
     
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  8. NJ7V

    NJ7V Premium Subscriber QRZ Page

    T
    Thanks. I did reach out to HobbyPCB and they gave me the info on the box it was made for. I appreciate your efforts in providing me another option. Regarding the box, they did say, however, " I don't think that's what the problem is. The EM simulation of the filter has the E and H fields at virtually 0 at any distance more than 10mm from the front of the PCB and 1mm from the back of the board so the hypothesis that it's acting like an antenna is probably not valid"

    I'll keep this thread updated as I investigate further and let everybody know what I find.
     
    Last edited: May 25, 2021
  9. NJ7V

    NJ7V Premium Subscriber QRZ Page

    More really good info! I'm learning a bunch for these posts and hope others are as well. Thanks. I suspect, based on where I was parked, that I was more outside the aperture when I moved closer, down, and to the left on the road.
     
    K9EZ likes this.
  10. NJ7V

    NJ7V Premium Subscriber QRZ Page

    HobbyPCB says their filter is not designed for what I was trying to use it for. On the 2m band it "lets through any signal from 130-180 MHz so if the communications site has lots of pagers and other services in the 150-174 MHz range this filter won't help at all."
     

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