cyamamot@nunki.usc.edu (Cliff Yamamoto) (07/20/89)
Greetings, I am trying to make a sharp notch filter to cut out an overbearing paging transmitter that swamps my PRO-2004 scanner on the VHF bands. I *think* I've read somewhere before about how one can cut a piece of cable to a certain length and short it at the end to kill the frequency of said wavelength. Something like this... |<------------feed line cable length important??--------->| ----------------------------------------------------------- Antenna ------------------------------+---------------------------- Receiver ----------------------------+ | +-------------------------- ^ | | | \ | | | | \ The length of this | | | | \___location of BNC "T" connector "stub" is cut to the | | | | important?? wavelength of the | | | | offending frequency | | | | v |_|_| <---Short center conductor to shield The pager frequency is 152.4 Mhz. So using the ol' ARRL handbook I get a wavelength of 1.96 meters (length of the "stub"). Has anyone heard of this before? I know it's cheap and dirty, but I'm not sure if the location of the "T" along the feedline is important. There's no mention of this kind of filter in the handbook, but I'm sure I've seen it before. Thanks in advance for your input. Cliff Yamamoto
dvh@cci632.UUCP (David Hallidy) (07/21/89)
In article <4436@merlin.usc.edu>, cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: > Greetings, > > I am trying to make a sharp notch filter to cut out an overbearing > paging transmitter that swamps my PRO-2004 scanner on the VHF bands. I > *think* I've read somewhere before about how one can cut a piece of cable > to a certain length and short it at the end to kill the frequency of said > wavelength. Something like this... > > |<------------feed line cable length important??--------->| > > ----------------------------------------------------------- > Antenna ------------------------------+---------------------------- Receiver > ----------------------------+ | +-------------------------- > ^ | | | \ > | | | | \ > The length of this | | | | \___location of BNC "T" connector > "stub" is cut to the | | | | important?? > wavelength of the | | | | > offending frequency | | | | > v |_|_| <---Short center conductor to shield > > The pager frequency is 152.4 Mhz. So using the ol' ARRL handbook I get a > wavelength of 1.96 meters (length of the "stub"). > > Has anyone heard of this before? I know it's cheap and dirty, but I'm not sure > if the location of the "T" along the feedline is important. There's no mention > of this kind of filter in the handbook, but I'm sure I've seen it before. > > Thanks in advance for your input. > Cliff Yamamoto Well, actually, you don't want the stub to be a full wavelength long- actually a quarter wavelength. Also, it should be open ended, not shorted. An quarter wave stub with one end open looks like a short at the other end at its resonant frequency. To design this filter, you need to know the free space wavelength (1.96 meters) as you determined. Divide by four to get a free space quarter wave. Now you need to know the velocity of propagation (velocity factor) of the coax you're using. RG-58 (not foam but solid polyethylene) has a velocity factor of about 66% (.66). Foam RG-58 is about 79%. Anyway, multiply the free-space quarter wavelength you just came up with by the velocity factor of the cable you're using and this is the correct length. A convenient formula for VHF lengths is: L (inches)= 2952/F(MHz) X V where L is the length of the stub, F is the Frequency in MHz and V is the Velocity Factor of the cable. For your example, L=12.8 inches, using RG-58 cable. Build it that way, leaving the free end open, and it should work. What most guys do is build it long (also remember the connector adds length) and trim it about 1/8 or 1/4 inch at a time while listening to the offending signal. The notch is very deep and will be very apparent. Just remember to make small cuts, because after you go too far, you get to start over! The length of the antenna feedline is not important, also, theoretically it shouldn't matter where you place the filter- in reality, you want to put it near the receiver so that stray pickup by the cable after the filter won't be significant. As with everything, there is a down side to this type of filter. Though the notch depth is good, the filter is somewhat broad and may (will) also attenuate nearby frequencies to which you may want to listen. The better the cable quality you use, the higher the Q of the tuned circuit, and the narrower the notch. BTW, you can add more sections to the filter by adding more Tees and more open stubs- probably the only thing that will happen though is the notch will be broader still. Hope this helps. 73 Dave KD5RO
tomb@hplsla.HP.COM (Tom Bruhns) (07/21/89)
cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: > > (stuff deleted about trying to notch out a freq. with a stub) > ----------------------------------------------------------- >Antenna ------------------------------+---------------------------- Receiver > ----------------------------+ | +-------------------------- > ^ | | | \ > | | | | \ > The length of this | | | | \___location of BNC "T" connector > "stub" is cut to the | | | | important?? > wavelength of the | | | | > offending frequency | | | | > v |_|_| <---Short center conductor to shield > >The pager frequency is 152.4 Mhz. So using the ol' ARRL handbook I get a >wavelength of 1.96 meters (length of the "stub"). > Yes, this will work, but just use 1/2 wavelength stub shorted as shown, or 1/4 wave open on the free end. The 1/2 or 1/4 wave is IN THE TRANSMISSION LINE, so if you are using std. polyethelene insulated coax (not foam), multiply the lenght you get for free-space by about .66; see coax refs for other types of insulation. Note that this will have attenuation quite a ways away from the design frequency (not a super-sharp null, though pretty deep with good coax). Should not be very dependent on location along the line, if the line is operated at low SWR.
dvh@cci632.UUCP (David Hallidy) (07/21/89)
In article <4436@merlin.usc.edu>, cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: > Greetings, > > I am trying to make a sharp notch filter to cut out an overbearing > paging transmitter that swamps my PRO-2004 scanner on the VHF bands. I > *think* I've read somewhere before about how one can cut a piece of cable > to a certain length and short it at the end to kill the frequency of said > wavelength. Something like this... > > |<------------feed line cable length important??--------->| > > ----------------------------------------------------------- > Antenna ------------------------------+---------------------------- Receiver > ----------------------------+ | +-------------------------- > ^ | | | \ > | | | | \ > The length of this | | | | \___location of BNC "T" connector > "stub" is cut to the | | | | important?? > wavelength of the | | | | > offending frequency | | | | > v |_|_| <---Short center conductor to shield > > The pager frequency is 152.4 Mhz. So using the ol' ARRL handbook I get a > wavelength of 1.96 meters (length of the "stub"). > > Has anyone heard of this before? I know it's cheap and dirty, but I'm not sure > if the location of the "T" along the feedline is important. There's no mention > of this kind of filter in the handbook, but I'm sure I've seen it before. > > Thanks in advance for your input. > Cliff Yamamoto These filters work, though they tend to be a bit broad. Use of very good coax helps keep the Q high and thus narrow the notch. However, you don't want to use a free space wavelength, as you calculated. You need to add in the velocity of propagation (velocity factor) which will make the actual length somewhat shorter. Also, you needn't use a full wave of cable - a half-wave or any multiple thereof will work. Also, you can use an open quarter-wave stub just as effectively. If you're using RG-58 (solid dielectric, not foam), the velocity factor is 66% (.66). For a half-wave shorted stub, the formula is: L(inches)= 5904 / Freq in MHz X VF where L is the length of the stub in inches (convenient at VHF) and VF is the Velocity Factor of your cable. For a quarter-wave open stub, it's 2952 / F (MHz) X VF. In your case then, using RG-58, you want a cable length of about 25.6 inches. Or, 12.8 in the case of the quarter- wave stub. The connector adds a little length. Best results are to start an inch or so too long, listen to the offending signal, and trim the end (the reason why I like the open stub better- you don't have to keep remaking the connection on the end) in small (1/8 or 1/4 inch) steps until you observe the drop in the signal. It will be quite apparent. Remember, if you go too far and cut too much off, you get to start over with a new piece of cable. In theory, it doesn't matter where in the feedline you place the filter, but in reality, you want it as near to the receiver as possible to minimize stray pickup of the offending signal after the filter. The only real down side to this circuit is that signals a couple of MHz away will also be significantly attenuated. This may or may not matter. Hope this helps. 73 Dave KD5RO
ferguson@gorby.SRC.Honeywell.COM (Dennis Ferguson) (07/22/89)
In article <4436@merlin.usc.edu> cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: >Greetings, > > I am trying to make a sharp notch filter to cut out an overbearing >paging transmitter that swamps my PRO-2004 scanner on the VHF bands. I >*think* I've read somewhere before about how one can cut a piece of cable >to a certain length and short it at the end to kill the frequency of said >wavelength. Something like this... > > |<------------feed line cable length important??--------->| > > ----------------------------------------------------------- >Antenna ------------------------------+---------------------------- Receiver > ----------------------------+ | +-------------------------- > ^ | | | \ > | | | | \ > The length of this | | | | \___location of BNC "T" connector > "stub" is cut to the | | | | important?? > wavelength of the | | | | > offending frequency | | | | > v |_|_| <---Short center conductor to shield > >The pager frequency is 152.4 Mhz. So using the ol' ARRL handbook I get a >wavelength of 1.96 meters (length of the "stub"). > The effective wavelength of your stub is the free space wavelength divided by the square root of the dielectric constant of the coax. RG58 or RG59 usually has a polyethelene dielectric with Er=2.2-2.6. Therefore the real wavelength is approximately 1.25 meters. A full wavelength is not necessary, for the shorted stub one half wavelength will do and for an open stub, one quarter wavelength will work. The placement of the stub with respect to the antenna and receiver depends upon if the antenna and receiver impedances look like the standard RF transmission line impedance of 50 ohms. Probably the receiver does but the antenna does not. A different notch filter that works just as well does not require placement at a specific distance from the antenna is a transversal filter. Take a piece of coax one half wavelength of the frequency you wish to notch (152MHz) and provide two paths for the signal, one normal path and one which is parallel but 1/2 wavelength longer. Since the signals sum 180 degrees out of phase, the unwanted signal is cancelled out (as well as all harmonics of the signal). Probably the best approach is to build a twin tee notch filter. Opps.... my notes aren't laying around... I'll email you the details. Dennis
john@frog.UUCP (John Woods) (07/25/89)
In article <29800@cci632.UUCP>, dvh@cci632.UUCP (David Hallidy) writes: > In article <4436@merlin.usc.edu>, cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: > > Greetings, > > I am trying to make a sharp notch filter to cut out an overbearing > > paging transmitter that swamps my PRO-2004 scanner on the VHF bands. I > > *think* I've read somewhere before about how one can cut a piece of cable > > to a certain length and short it at the end to kill the frequency of said > > wavelength. Something like this... > > The pager frequency is 152.4 Mhz. So using the ol' ARRL handbook I get a > > wavelength of 1.96 meters (length of the "stub"). This may not be a practical solution to your problem, but it's an interesting one: some years back in QST magazine (probably between '76 and '78) someone had an article about a 2-meter (~146Mhz) resonator used for allowing two stations to share the same antenna (during contests, I believe). The author had discovered that a standard 55-gallon garbage can was very close to being a resonant cavity for that band, requiring only a modest (adjustable) 2-plate capacitor in the middle (which was large enough that hot dogs could be rested on it and heated while transmitting (with a little retuning, of course)). If you can dig up the article, and if it appeals to you, you might be able to get away with it for this application. Just think -- someone comes in to your den, sees a garbage can, asks you what it is, and you can reply, "Oh, that's where I throw away unwanted signals from pagers." John Woods, WB7EEL/1 -- John Woods, Charles River Data Systems, Framingham MA, (508) 626-1101 ...!decvax!frog!john, john@frog.UUCP, ...!mit-eddie!jfw, jfw@eddie.mit.edu People...How you gonna FIGURE 'em? Don't bother, S.L.--Just stand back and enjoy the EVOLUTIONARY PROCESS...
DAVIS@ohstpy.mps.ohio-state.edu (JOHN E. DAVIS) (07/26/89)
In article <29800@cci632.UUCP>, dvh@cci632.UUCP (David Hallidy) writes: > In article <4436@merlin.usc.edu>, cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: >> Greetings, >> >> I am trying to make a sharp notch filter to cut out an overbearing >> paging transmitter that swamps my PRO-2004 scanner on the VHF bands. I >> *think* I've read somewhere before about how one can cut a piece of cable >> to a certain length and short it at the end to kill the frequency of said >> wavelength. Something like this... >> >> |<------------feed line cable length important??--------->| >> >> ----------------------------------------------------------- >> Antenna ------------------------------+---------------------------- Receiver >> ----------------------------+ | +-------------------------- >> ^ | | | \ >> | | | | \ >> The length of this | | | | \___location of BNC "T" connector >> "stub" is cut to the | | | | important?? >> wavelength of the | | | | >> offending frequency | | | | >> v |_|_| <---Short center conductor to shield >> >> The pager frequency is 152.4 Mhz. So using the ol' ARRL handbook I get a >> wavelength of 1.96 meters (length of the "stub"). >> >> Has anyone heard of this before? I know it's cheap and dirty, but I'm not sure >> if the location of the "T" along the feedline is important. There's no mention >> of this kind of filter in the handbook, but I'm sure I've seen it before. This is interesting. Can this type of filter be used to remove the the "buzzing" sound and picture interference my cable company uses to scramble some of it's channels. Someone told me to use 300 ohm twin lead and tin foil to do the job. Unfortunately I do not know the "scrambling frequency" but I imagine it is somewhat standard. Thanks John E. Davis OSU Physics