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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. BA7LFI

    BA7LFI Ham Member QRZ Page

    My HF transreceiver often meet this situation.I can listen some broadcast HF radio of other frequency in 20m and 15m,these terrible signals make me annoyed!
    NJ7V likes this.
  2. W7MDN

    W7MDN Ham Member QRZ Page

    Charlie: Nice video. I learned something too.
  3. W1YW

    W1YW Ham Member QRZ Page

    That is correct. the radio is essentially a counterpoise part of the antenna, and this needs to be shielded in order to isolate the actual antenna from the radio.
  4. NC5P

    NC5P Ham Member QRZ Page

    I found that commercial (Part 90) single band radios seem to do better in high RF environments. You can get a decent Kenwood or Motorola analog HT pretty reasonable just to use for such situations.
    NJ7V likes this.
  5. K7NM

    K7NM Ham Member QRZ Page

    One approach since operation is "Portable" is to build a cavity resonator for the gain antenna feed. This can be done with solid printed circuit board, a piece of copper tubing and threaded stock with a couple of nuts. SO239s are put in one end for connecting coax in and out to the internal coupling loops which are adjusted in position and size for minimum insertion loss. The threaded stock on the other end adjusts a capacitor plate made of solid plated circuit board to adjust the resonant BPF frequency. Take it with you and your gain antenna and put it in line after tuned at home on the bench. The Q of this filter is very high. Q = F/BW (Frequency and Band Width). With an 8" on a side cavity (square) with a copper pipe center conductor (coaxial section) of 1/4 wave open on the tuned end, I have achieved Qs of nearly 900. Used similar ones made from Aluminum irrigation pipe on an FM commercial site in Montana with super performance. They are BIG relative to the ones in the video and would lay on the ground below the gain antenna. Just a suggestion. 73, Lee - K7NM
    NJ7V likes this.
  6. M7BLC

    M7BLC Ham Member QRZ Page

    So invest time and money in filtering on a less than $20 Baofeng ? Somehow my motivational curve sticks on zero :) .
    WD8CRT likes this.
  7. W1YW

    W1YW Ham Member QRZ Page

    I disagree.

    Its a learning experience to solve a real world problem. Granted, some other HT's would not have the problem to this degree.

    The notion of 'overload' and even intermod are fairly foreign to most hams. Always a good idea to have fun understanding second order effects:)

    Chip W1YW
    NJ7V likes this.
  8. W1YW

    W1YW Ham Member QRZ Page

    This is an extremely useful post. Keep it handy.

    Chip W1YW
    NJ7V likes this.
  9. M7BLC

    M7BLC Ham Member QRZ Page

    For those with the expertise and inclination to experiment I would agree although I would just buy a better piece of equipment (as it is I will stick with my UV5R as for various reasons I do not go near big antennas) . Would not modification invalidate the certification on such units ?
  10. W1YW

    W1YW Ham Member QRZ Page

    Did you yourself say its just a ($20) Baofeng...?


    Heck yeh I modify my radios! Bet you do too:)
    Last edited: May 28, 2021

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