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VHF Notch filter with Coax???

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by GJ6WRI, Oct 12, 2013.

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  1. GJ6WRI

    GJ6WRI XML Subscriber QRZ Page

    Icom in some of their more recent radios with WFM, have put an Intermediate Frequency in the Air Band!!!!
    Not a problem if there is no aviation activity in your area, but around my QTH, there is a low power Xmtr close to the IF frequency.
    The result is, if you want to listen to broadcast FM, then you will be hard pushed as the airband tx freq will swamp all but the very strongest of signals.
    So, what I need (I think) is a notch filter to nul-out the airband frequency. In my case, 134.675Mhz
    I recall many years back using a coax stub to null an unwanted UHF frequency, and hope to do the same here at VHF.
    My search so far hasn't resulted in any practical examples, so can you help?
    Any stub must be able to handle up to 100w of SSB, and less for FM operation.
    Your consideration and ideas/help would be greatly appreciated!
    Steve, GJ6WRI
  2. KA9JLM

    KA9JLM Platinum Subscriber QRZ Page

    I have built very sharp notch filters using hand made high q coils using 12 gauge wire and air variable caps, You can make as many notch filter stages as needed.

    The coax will work but is hard to trim to the exact frequency. If the coax is cut close enough you can use a 15pf variable cap on its end to fine tune it.

    Good Luck.
  3. GJ6WRI

    GJ6WRI XML Subscriber QRZ Page

    Is it literally just a 1/4 wavelength stub (open circuit)? :rolleyes: :confused:
    Last edited: Oct 12, 2013
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    For coax to work at this close a frequency (~7% frequency difference) you need a pretty high-Q trap, which means very low-loss coax. Offhand, having done this before, I don't think "small" coax will work well; 7/8" Heliax or so will work better. 1-5/8" Heliax would work even better than that.

    You can make the trap 1/4-wavelength with the end "open," or 1/2-wavelength with the end "shorted." Since 1/4-WL is shorter and will have less loss I'd try the open-end stub first.

    Formula would be 246/f(MHz) * VF = stub length in feet. For 134.675 MHz, and assuming the use of very low loss cellular dielectric cable with a VF of 0.84, this would be 246/134.675 * 0.84 = 1.53436 feet, or 18.4123 inches (46.76 cm). The stub length must include the length of the connector used to place it in parallel with the antenna line via a coaxial "tee" adapter. The "open end" should be as close to perfectly open as possible while maintaining coaxial design; I just use a hacksaw or equivalent on the Heliax to cut through everything (jacket, solid copper outer conductor, foam dielectric, copper center conductor) all in one cut to make the end "square," and then clean the end of any materials (shavings). Once the "magic" dimension has been found for maximum notching at the desired frequency, I put a piece of heat shrink tubing over the end and shrink it.

    A good high-Q stub can likely work; in my experience, a lower-Q stub, like one made of small flexible coax (.195" or .240" stuff) usually doesn't because the notch isn't deep enough.
  5. GJ6WRI

    GJ6WRI XML Subscriber QRZ Page

    Thanks. That's great.
    I had just cast my mind back to my RF theory, and came up with an equivalent result in Eu Metric!!!
    Strangely enough, I have all sorts of Heliax offcuts somewhere. I rescued long lengths from a job some years back where I decommissioned an analogue mobile cell station on a power station chimney! Think i'll try experimenting with bendy coax first though!!! :eek:
    Thanks again.
  6. G0HZU

    G0HZU QRZ Member

    Can I ask which is the closest ham band you want to use via the notch filter?If it is the 2m band then the quarter wave coax stub will be way too wide and will cause a lot of loss and mismatch on the 2m band. So you ideally need to state which aerial feed it is going into and what other ham bands share this aerial. A quarter wave stub using a bit of 50R coax is a classic way to get a notch response but in my opinion it is a poor choice unless you want to operate a long way from the notch and you also need to avoid any other higher order notches it also causes.
  7. G0HZU

    G0HZU QRZ Member

    My own recommendation would be to simply buy a decent stereo FM radio if you want to listen to broadcast FM. It's bound to outclass the Icom receiver in terms of audio quality and wouldn't cost much or take up much space.A 50R coaxial quarter wave stub for 134MHz would have the potential to degrade the VSWR of a typical ham aerial on 10m, 6m and 4m as well as totally wrecking performance on the nearby 2m band. You could try asking someone like PAR if they would make you a custom filter for your needs or you could try making a lumped filter yourself. But getting it to work reliably when passing 100W on the 2m band into a typical 2m aerial will be a 'challenge'
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Good luck, but I don't think the "bendy" coax will really do it, unless you don't mind having some loss at 2m and not having a very deep notch at 135 MHz. Problem with smaller cables is they don't have low enough loss to maintain a very high "Q."

    But give it a shot, it might work "a bit," and demonstrate it can work a bit, which would lead to making a fatter stub that will work a lot better!
  9. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    This is really where a band reject cavity would do the job, this could be made with only a moderate level of skill.
  10. KA0GKT

    KA0GKT Ham Member QRZ Page

    Years ago when the cable companies used a pulsed carrier inserted between the NTSC Chroma and audio carriers to scramble the picture and sound, I was able to make a 1/4-wave open-stub notch filter which nicely notched out the offending carrier and left the rest of the TV channel quite viewable.

    The channel in question was US TV channel 4 (66-72 MHz), and the top of Chroma would have been 70.82955 MHz and the Aural carrier would have been 71.75 MHz.

    I made the stub out of RG-6 coaxial cable and carefully trimmed it to frequency using a Dremel tool with a carborundum cut-off disc while watching the signal strength of the interfering carrier on a TV field strength meter.

    I would attempt a 1/4-wave open stub cut to the offending frequency. A short piece of coaxial cable connectors and a "T" connector won't break the bank, and just might correct your problem.
  11. WA7PRC

    WA7PRC Ham Member QRZ Page

    I've done that too, in 50-75 ohm unbalanced and 300 ohm balanced systems. It's worth a try, and not much to lose.

    Ian GM3SEK has some information here:
  12. W9XMT

    W9XMT Ham Member QRZ Page

    Probably better for a 1/4-wave stub used for this purpose to be shorted at the far end, because that will be transformed by the electrical length of that stub to a high impedance at the reject frequency where the stub is connected to the main transmission line.

    To get those same conditions for a 1/2-wave stub the far end should be open.

    A 1/2-wave stub will have a little less bandwidth at the reject frequency than the 1/4-wave stub (which is good, in this case). But for this small frequency separation, probably there will be some loss of ~144 MHz transmitter power when using either stub.

    The stub should be attached very close to the output port of the transmitter. That physical/electrical distance may need to be adjusted for best system performance, including the control of any spurs from the final r-f amp in the transmitter.

    R. Fry
  13. W0BTU

    W0BTU Ham Member QRZ Page

    When I lived in Ohio, I used that method to successfully notch out interference on the low end of 2 meters from a nearby FM station. I just used RG-59 for the quarter-wave stub.

    The location of the stub along the feedline matters, since the SWR of the interfering FM signal on the main feedline is high.
  14. KM1H

    KM1H Ham Member QRZ Page

    Does it really? Not that Ive noticed when using stubs in feedlines from 160 to 2M and even the stub coax impedance doesnt matter.

    The 160-10M stubs here were to use 2 rigs in a contest enviroment to reduce harmonics and kill broadband phase noise at the 1200W level. The ones for 6 and 2M were to clean up various 2 way and broadcast FM crud from a nearby site. I also installed one on a 152 MHz 250W paging transmitter on a tower here about 150' from the 2M yagis and dropped the phase noise into the noise.
    The coax used in all cases was CATV RG-11 type foam which has a reasonably high Q for about 25dB notches. Additional cables can be placed 1/4 wave away to get an additive attenuation.

  15. WX7G

    WX7G Ham Member QRZ Page

  16. KA9JLM

    KA9JLM Platinum Subscriber QRZ Page

    RG6 or even 59 may be a good choice, for this application. RG11 is good also, if you have some.

    Notch filters like the one above are not cheap. MA makes nice ones also.

    You can build your own, with a few Hi-Q coils and caps.

    You can also use multiple stubs to get the correct notch.

    Have Fun.
  17. W0BTU

    W0BTU Ham Member QRZ Page

    I believe you, Carl. But it certainly mattered for me, and others I met --and read about-- after that. I moved the stub while changing nothing else, and I couldn't figure out why the broadband FM broadcast crud was back. I forget all the details, this was over 30 years ago.

    Think about it. The Z of the FM broadcast band signal at any given point along the feedline changes due to antenna/preamp output and receiver input mismatches. The in-band signals do not because the line is flat. In my case, the location of the open 1/4 wave stub mattered.

    Last edited: Oct 13, 2013
  18. GJ6WRI

    GJ6WRI XML Subscriber QRZ Page

    Look what I found in my box of junk !!! :rolleyes:


    Found some other bits too!!!
    Well, that's this weeks twiddling sorted!
    Thanks for all of your posts.
  19. WX7G

    WX7G Ham Member QRZ Page

    A VHF aircraft band 90-degree stub is not a viable solution if 2 meter transmit operation is desired.
  20. GJ6WRI

    GJ6WRI XML Subscriber QRZ Page

    "A VHF aircraft band 90-degree stub is not a viable solution if 2 meter transmit operation is desired"

    L & C it is then !!! (now that I've found some!!!)
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