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Thread: VHF Notch filter with Coax???

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  1. #1
    Join Date
    May 2001
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    Jersey, British Channel Islands
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    Default VHF Notch filter with Coax???

    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!
    73
    Steve, GJ6WRI

  2. #2
    Join Date
    Jan 2011
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    Houston Texas
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    Default

    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.
    "Theory only works perfect in a vacuum." KA9JLM Don

  3. #3
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    Is it literally just a 1/4 wavelength stub (open circuit)?
    Last edited by GJ6WRI; 10-12-2013 at 06:09 PM.

  4. #4

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    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.
    What if soy milk is just regular milk introducing itself in Spanish?

  5. #5
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    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!!!
    Thanks again.
    Steve

  6. #6
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    Sep 2011
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    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. #7
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    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. #8

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    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!
    What if soy milk is just regular milk introducing itself in Spanish?

  9. #9
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    This is really where a band reject cavity would do the job, this could be made with only a moderate level of skill.
    When it's time, and it may be sooner than you think.

  10. #10
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    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.
    73 DE KAGKT/7

    --Steve

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