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TECHNOTE No. 6
Joe Carr's Radio Tech-Notes Tech-Notes
Dealing With AM Broadcast Band Interference to Your Your Receiver Receiver Joseh J! Carr Universal Radio Research 6830 Americana Parkwa Renolds!"r#$ Ohio %3068 2
Dealing With AM Broadcast Band Interference to Your Receiver Joseph J. Carr If you live anywhere near an AM broadcast band (BCB) station, then you might have serious reception problems, even with a high quality receiver Although one of the things you get when you pay the premium price for a high quality receiver is superior overload protection, the signal from a local AM BCB station might overwhelm its defenses In the !"A we use the #$%&'%% h* medium wav e AM BCB Most stations operate with ',%%% to '%,%%% watts of + output power (although a few -#% to #%% watt local fi**lers also e.ist) A few stations are designated /clear channe l0 stations, and operate with #%,%%% watts, -1&hours a day 2hese stations (eg 3"M 4ashville, 5#% h*) are on frequencies that are not assigned to other stations for a distance of, I believe, '$%% miles radius If you live within a few hundred yards of an AM BCB station, then it6s possible to see more than one volt of + appearing at your receiver antenna an tenna terminals (a laboratory measured 1 volts in one case7) 8iven that your receiver li9es to see signals in the do*ens of microvolts level, then you can understand un derstand the problem The Prole! "o what is the problem: ;our ;our receiver, no matter what frequency it receives, is designed to accept only a certain ma.imum amount of radio frequency energy in the frontend If more energy is present, then one or more of several overload conditions results 2he overload could result from a desired station is too strong In other cases, there are simply too many signals within the passband for the receiver front&end to accommodate In still other cases, a strong out&of&band signal is present igure ' shows several conditions that your receiver might have to survive "everal different receiver problems result from the various types of overload, all of which are species intermodulation and) and &' dB compression point specifications 2he strong out&of&band signal ta9es up so much of the receiver?s dynamic range /head room0 that only a small amount of capacity ca pacity remains for the desired signal 2he signal level of the desired signal is thereby reduced to a smaller level In some cases, the overload is so
severe that the desired signal becomes inaudible If you can filter out or otherwise attenuate the strong out&of&band signal , then the head room is restored, and the receiver has plenty of capacity to accommodate both signals 3
=ne thing that?s important today is what happens when signals are received that are much stronger than the input signal that produce s the flattening of the response in the output&vs&input curve =ne unfortunate factor is the generation of harmonics that were not present in the original signal 2he harmonics are integer multiples of the input signal frequency, so will appear at higher points on the frequency dial 2he harmonics ma y fall within the passband of your receiver, and are seen as valid signals even though they were generated in the receiver itself7 2he strong intermodulation products are created when two of these signals heterodyne together 2he heterodyne (/mi.ing0) action occurs because the receiver frontend is non&linear at this point 2he frequencies produced by @ust two input frequencies (' and -) are described by m' n-, where m and n are integers As you can see, depending on how many frequencies are present and how strong they are, a huge number of spurious signals can be generated by the receiver front&end "o, what about I selectivity: ;ou have an I filter of -% * to h* (depending on model and mode), so why doesn6t it re@ect the dirty smelly bad&guy signals: 2he problem is that the damage occurs in the front&end section of the receiver, before the signals encounter the I selectivity filters 2he problem that causes all of these problems is an overdriven + amplifier, mi.er or both 2he only really effective way to deal with these problems is to reduce the level of the offending signals In this paper we will investigate the use of front&end attenuators, highpass filtering and single&frequency wavetraps The Attenuator "olution "ome modern receivers are equipped with one or more switchable attenuators in the front&end "ome receivers also include an + gain control that sometimes operates in the same manner "ome receiver operators use e.ternal in&line single&range or switchable attenuators for e.actly the same purpose 2he idea b ehind the attenuator is to reduce all of the signals to the front&end enough to drop the overall energy in the circuit to below the ma.imum level that can be accommodated without either overload or intermodulation occurring at significant levels 2he attenuator reduces both desired and undesired signals, but the perceived ratio is altered when the receiver front&end is de&loaded to a point where desensiti*ation occurs, or intermods and harmonics pop up The Antenna "olution 2he antenna that you select can ma9e some difference in AM BCB problems 8enerally, a resonant antenna with its end nulls pointed toward the offending station will provide marginally better performance than a random length wire antenna (which are popular amongst "3Ds) 4
Also, it is well 9nown that vertical antennas are more susceptible to AM BCB because they respond better to the ground wave electrical field generated by the BCB station (EeMaw) The #ilter "olution =ne of the best solutions is to filter out the offending signals before they hit the
receiver front&end, while affecting the desired signals minimally 2his tas9 is not possible with the attenuator solution, which is an /eq ual opportunity0 situation because it affects all signals equally A signal that is outside the passband of a frequency selective filterF it is severely attenuated It does not drop to *ero, but the reduction can be quite profound in some designs "ignals within the receiver6s passband are not unaffected by the filter 2he loss for in&band signals is, however, considerably less than for out&of&band signals 2his loss is called insertion loss, and is usually quite small (' or - dB) compared to the loss for out&ofband signals (lots of dB) "everal different types of filter are used in reducing interference Ahigh-pass filter passes all signals above a specified cut&off frequency (c) 2he low-pass filter passes all signals below the cut&off frequency 2his filter is similar to the type of filter that hams using transmitters place between the transmitter and antenna to prevent harmonic radiation from interfering with television operation A bandpass filter passes signals between a lower cut&off frequency (D) and an upper cut&off frequency () A stop-band filter is @ust the opposite of a bandpass filterF it stops signals on frequencies between D and , while passing all others A notch filter, also called a wave trap, will stop a particular frequenc y (o), but not a wide band of frequencies as does the stopband filter In all cases, these filters stop the frequencies in the designated band, while passing all others More or less 2he positioning of the filter in your antenna system is shown in ig ' below 2he ideal location is as close as possible to the an tenna input connector 2he best practice, if you have the space at your operating position, is to use a double&male coa.ial connector to connect filter output connector to the antenna connector on the receiver A short piece of coa.ial cable can connect the two terminals if this approach is not suitable in your case Be sure to ground both the ground terminal on the receiver and the ground terminal of the filter (if one is provided) =therwise, depend on the coa.ial connectors6 outer shell ma9ing the ground conn ection 5
#I$. % Different #or!s of #ilter Wave traps A wave trap is a tuned circuit that causes a specific frequency to be re@ected 2wo forms are usedF series tuned (ig -A) and parallel tuned (ig -B) 2he series tuned version of the wavetrap is p laced across the signal line (as in ig -A), and wor9s because it produces a very low impedance at its resonant frequency and a high impedance at frequencies removed from resonance As a result, the interfering signal will see a resonant series&tuned wave trap as a short circuit, while other frequencies do not see it at all 2he parallel resonant form is placed in series with the antenna line (as in ig -B) It provides a high impedance to its resonant frequency, so will bloc9 the offending signal before it reaches the receiver It provides a low impedance to frequencies removed from resonance 2he wave traps are useful in situations where a single station is causing a problem, and you don6t want to eliminate nearby stations or e.ample, if you live close to an M3 AM BCB signal and don6t want to interrupt reception of other M3 AM BCB signals or D3 6
AM BCB signals Gven AM BCB EHers often have wavetraps tuned to the frequen cy of a nearby station in order to either increase the available dynamic range or eliminate other problems 2he values of components shown in igs -A and -B are suitable for the M3 AM BCB, but can be scaled to the D3 BCB if desired A good starting point is $5# p for the variable capacitor, and -%% µ or --% µ for the inductor Both components should be variable in order to ma9e tuning easier owever, if one component is fi.ed, the filter will do its @ob @ust as well If there are two stations causing significant interference, then two wave traps will have to be provided, separated by a short piece of coa.ial cable In that case, use a parallel tuned wave trap for one frequency, and a series tuned wave trap for the other =therwise, interaction between the wave traps will cause problems &igh'Pass #ilters =ne very significant solution is to use a high&pass filter with a cut&off frequency between '%% and $%%% h* It will pass the shortwave frequencies, and severely attenuate AM BCB signals in both M3 and D3 bands, causing the desired improvement in performance igure $ shows a design used for many decades It is easily built because the capacitor values are %%%' and %%%- (which some people ma9e by parallelling two %%%' capacitors) #I$. ( 2he inductors are both $$ , so can be made with toroidal cores If the 2&#%&+GE cores are used (AD J 1K), then -5 turns of small diameter enameled wire will suffice =r if the 2&#%&'# +GE<3I2G cores are used (AD J '$#), then '# turns are used 2he circuit of ig $ produces pretty decent results for low effort A number of readers contacted me with success stories when this circuit was published once before, a result that gives me pleasure But there is a better way Asorptive #ilters 2he absorptive filter (=rr 'KK5 and 3einreich
C5
daunting due to interactions between the sections A better approach is to use toroid core 8
homebrew inductors 2he toroidal cores reduce interaction between the coil6s magnetic fields, so simplifies construction >ossible alternatives are shown below in 2able -F Tale ) Coil *alue Core A + *alue Turns D' 1' 2&#%&'# +GE<3I2G '$# ' D- 1' 2&#%&'# +GE<3I2G '$# ' D$ - 2&#%&'# +GE<3I2G '$# 'D1 '# 2&#%&- +GE 1K ' 2&#%&5 ;GD 1% -% D# -- 2&#%&- +GE 1K -' 2&#%&5 ;GD 1% -1 D5 '%- 2&#%&- +GE 1K 15 2&#%&5 ;GD 1% #' or all coils use wire si*e to -1 to $% A38 enamel insulation 2he dummy load used at the output of the low&pass filter (+' in ig 1) can be made using a #' ohm carbon or metal film resistor, or two '%% ohm resistors in parallel In a pinch a 1 ohm resistor could also be used, but is not preferred In any event, use only noninductive resistors such as carbon composition or metal film '<1 to -&watt resistors If you would li9e to e.periment with absorptive filters at other cut&off frequencies than $ Mh*, then use the reactance values in 2able $ to calculate component valuesF Tale ( Co!ponent , -, + or ,C D' - Ω D- 1 Ω D$ $ Ω D1 - Ω D# 1- Ω D5 'K$ Ω C' - Ω C- 1- Ω C$ 'K$ Ω C1 1 Ω C# 1 Ω C5 $ Ω 2he e.act component values can be found from variations on the standard inductive and capacitive reactance equationsF 9 HL µ -=π Fc%'×
Microhenrys and, picofarads 2hese component values are bound to be non&standard but can be made either using coil forms (for inductors) or series¶llel combinations of standard value capacitors "hielding FC - pFXcπ
"hielding is a non&negotiable requirement of filters used for the N+M reduction tas9 =therwise, signal will enter the filter at its output and will not be attenuated !se an aluminum shield bo. of the sort that has at least #&5 mm of overlap of the flange between upper and lower portions I used a tinned steel + bo. for this purpose when building the prototype for this filter /0pected Results If the correct components are selected, and good layout practice is followed (which means separating input and output ends), then the absorptive filter offers stopband attenuation of &-% dB at one octave above c, &1%O dB at two octaves and &5% dB at three octaves or a $ Mh* signal, one octave is 5 Mh*, two octaves are '- Mh* and three octaves are -1 Mh* My results were slightly less than these figures because some of my components were ill&matched (eg slug&tuned commercial inductors were used rather than toroid core coil) A design suitable for the !" television bands is provided by 3einreich
References EeMaw, Eoug, 3'CG+ ('K5) /+e@ecting Interference rom Broadcast "tations0 N"2, Eecember 'K5, pp $#&$ =rr, Bill 35"AI ('KK5) Antenna andboo9 ic9sville, 4;F CN Communications, Inc p5&'K 3einreich, +ichard 5!P! and +3 Carroll ('K5) /Absorptive ilters or 2P armonics0 N"2, 4ovember 'K5, pp -%&-#
