1.        Most receivers require antenna terminal (source) impedances in the range of 36 ohms to 2K ohms. Depending on the specific
input circuitry this is usually a suitable range for an acceptable impedance match, and fairly efficient energy transfer of the received
signals.
2.        Most communications receivers have provisions for “low’ and “high” impedance antenna inputs. Based on a broad sampling of
“rated” input impedance and “actual” impedance, the following has been observed:
2a. The low impedance is generally on the order of 36 to 100 ohms across the receiver’s operational bandwidth.
2b. The high inmedance is from 600 to 5k ohms.
3.        Since series-tuned and parallel-tuned resonant filters behave in a (sort of ) reciprocal manner, it becomes obvious that for the
impedances of concern that, the series-tuned family is the logical choice for designing efficient filters for the (less-than) 5-digit
impedance ranges. Conversely, for active filters using interstage amplification and/or high-ratio transformer coupling, parallel-tuned
(tank) circuits may better serve the designer.
4.        The designer must recall the following filter characteristics;
4a. Series tuned L/C filters are at MINIMUM impedance at resonance.
4b. Parallel tuned filters are at MAXIMUM impedance at resonance.
5.        When designing a filter, it’s important to note that the efficiency (loop Q) is directly proportional to the reactance of its individual
components within the design frequency.
EG: Assume a series-tuned filter designed for 1MHz. At resonance the impedance through the filter will be almost zero (minus the
effects of losses within the L & C components). This condition allows the filter to pass, with minimal loss, the signal at 1 MHz. However,
since the center frequency is a given, the designer must be concerned about the sharpness of the filter; IE: its bandwidth, or selectivity,
or “Q”.
6.        See the chart for reference for the following examples of how the L/C ratio affects filter “loop-Q”.
6a. Our first filter (lower blue “X”) uses an L/C combination that resonates at 1 MHz. The components are 500pF and 50uH. If you read
across the chart to the reactance scale, you can see that the off-frequency signals (the ones we wish to attenuate) are presented with a
reactance of about 300 ohms. In a 50 ohm antenna circuit, that is only an attenuation of 6:1 (about 15dB or approximately 2 ½ S-units).
6b. The second filter (upper blue “X”) has an L/C combination that also resonates at 1 MHz. But this time we have shifted the inductor
value up by a factor of 10:1 and the capacitor down by 10:1. Now read across and note that the off-center frequency reactance is about 10
times greater than the original circuit – 3,000 ohms. Now the attenuation ratio is 60:1 (about  36 dB or approximately 6 S-units).
7.        The above illustration is purely theoretical and subject to component losses, but it does help to clarify why filters are designed with
such great L-to-C ratios.
8.        Based on the above examples, suitable bandwidth/Q parameters may be established. Often, it is best to calculate the filter’s
performance or develop a design using a software program (such as Elsie), and then actually measuring the filter performance on the
test bench. For lack of test equipment, you may check the efficiency of your design in the following manner:
8a. Tune a digital radio, with it’s filter setting set to wide, to a strong station and note the frequency and s-meter reading.
Then detune the radio until artifacts of the signal are barely audible. Note the frequency and s-meter readings again.
Repeat the above two steps, but this time with the filter tuned to the dead center of the station. Note the on-frequency and skirt s-meter
readings.
You now have a feeling for the approximate effectiveness of the filter:
The on-frequency difference in signal level is the loss through the filter. High-Q coils will have less loss (I^sqR) than low Q coils.
The skirt signal differences will define the filter’s loop Q (selectivity).
9. For general and practical use throughout the AM broadcast band and the commercial shortwave bands, a desirable skirt attenuation
would be anything that is greater than 12-18dB (2-3 s-units) when tuned to 5 kHz off center frequency. Examples of actual plots can be
seen here. Of course, there is no better test than actually using the filter over a long enough period to determine your subjective
evaluation of its utility.
Basic Series-Resonant Preselector Filters
THEORY OF OPERATION
(T-OPS)
Noise and Bleed Protection Filters

Design Notes

Note 1: All reactive filter circuits are source and load impedance selective and impedance dependent. The two reactive circuits
described here were designed for a nominal source (antenna) impedance of 52 ohms and a similar load (receiver) impedance.
In practical application, a mismatch of 2:1 is functionally tolerable, and even at 3:1 may still be of applicable utility.

Note 2: The trigger (voltage) level circuits described here are designed for use in antenna circuits, which have a nominal
impedance of 52 ohms, and therefore a calculated range of signal voltage levels. Since virtually any form of antenna input
impedance transformation will produce a corresponding change in signal voltage level, appropriate circuit modifications must be
made to accommodate that condition. These circuits are designed with a latitude of impedance mismatch of up to 3:1, with 2:1
being more acceptable for general practical use.

Note 3: As discussed above, provisions for proper source and load impedance matching must be made for all of these circuits to
function properly and efficiently. There is information given elsewhere in this program on the construction and application of
tapped broadband matching transformers and tuners (transmatches, couplers, etc).

Note 4: These filters are discussed here purely for technical purposes. None are absolutely required, and none are totally and
completely effective. Additionally, the functionality of some of the circuits overlaps that of other circuits and therefore may be
somewhat redundant. Depending on your circumstances, you may elect to take advantage of some, all, or none of these circuits.

RF Hash (High Pass Shunt)

The value for C should be selected for a reactance level that is equal to the nominal input impedance times 4 at the highest
operating frequency (Xc = 4 Zin).
The value for R should be equal to the nominal source impedance.
The value for Rv should be 10X, the value of R.

EG: For a general coverage receiver capable of receiving up to 30MHz, and connected to a (nominal) 52 ohm antenna source, the
Xc should be about 200 ohms at 30 MHz … a capacitor of about 30pF. R would be about 47 ohms, and Rv about 500 ohms.

DC Bleed (Low Pass Shunt)

The value for L should be selected for a reactance level that is equal to the nominal input impedance times 4 at the lowest
operating frequency (Xl = 4 Zin).
The value for R should be equal to the nominal source impedance.
The value for Rv should be 10X, the value of R.

EG: For a general coverage receiver capable of receiving down to 150 kHz, and connected to a (nominal) 52 ohm antenna
source, the Xl should be about 200 ohms at 150 kHz … an inductor of about 300uH. R would be about 47 ohms, and Rv about
500 ohms.

Impulse Shunt (Transient Threshold Limiter)

This back-to-back double diode is more or less a traditional method of clipping impulse noise peaks and bursts. There are many
variations of this basic circuit, the simplest of which would be where the diodes are connected directly across the antenna input.
In our circuit, we have added a fixed resistor to provide some basic current limiting. Although not absolutely necessary, it will
provide some diode protection for severe cases of high level, near field energy. The potentiometer allows for some desensitizing
of the circuit in the event of  temporary strong local transmissions or for establishing a trigger threshold where strong signals are
present. These adjustments might be appreciated in several possible situations: a local, powerful AM broadcaster who goes
from high daytime power to low nighttime power, a passing mobile ham or CBer, an approaching or passing electrical storm, or
the intermittent use of local, noisy electrical motors, bug zappers, or other impulse QRM.

No recommendation for the diode type is made, since it is widely considered a personal opinion on how the clipping circuit
should work. I personally (and generally) use 1N914 diodes as a good middle-road compromise. Some experimenters like
Shockley diodes for their speed and low threshold, while still others have used the old, fragile germanium diodes with their lower
junction breakdown. This is something that really must be determined for the best performance in the listener’s area, the type of
antenna, and the surrounding noise environment.
The resistor should be on the order of about one-tenth to one quarter of the antenna feed impedance, maybe 6 to 15 ohms. Rv
should be several times the input impedance, say 300 to 1k ohms, with 500 ohms being a good starting value.

EMP Shunt (High Voltage Suppressor)
This also, is an old school practice. An NE2 neon tube will fire (conduct) at about 87 volts. You are not likely to see that kind of
voltage level on your antenna feed line except for two conditions:
1.        Dry winds, dry snowfall, and dust storms are notorious for generating a static-electric charge on wire antennas. This is
particularly true for unbalanced antennas that are end fed to the radio and have otherwise been unterminated for some period of
time. A 90 foot long, end fed wire can generate several thousand volts of static electricity in minutes, during a dry, gentle snowfall!
If you were to contact the connector of this antenna’s feed line while it lay unterminated, during this kind of situation, you could be
subject to a very painful electrical shock. To plug this antenna into a solid state receiver under these conditions, could be the
recipe for serious damage to the radio.
2.        Nearby lightning strikes. This is a totally unpredictable circumstance. Suffice it to say that any nearby lighting event will
produce a potentially damaging electromotive pulse. The pulse will contain energy from DC all the way up into the high MHz
frequencies – very broadband and very high energy.
3.        My NE2 uses a 5K ohm resistor and a 20k Rv. Again, you may want to experiment on the effectiveness of these values for
your location and conditions.

If you are using the “Impulse” or “DC Bleed” circuits, you may decide to not use the NE2 flashover. I generally do, just in the event
of nearby lightning strikes and the loss of the diodes.
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