alex9ufo 聰明人求知心切: 1月 2014

2014年1月29日星期三

The Ultimate AM Radio Antenna

The Ultimate AM Radio Antenna
as realized by Ted Tenny

When I searched on “AM radio antenna design”, the best result I found was How to Make the Ultimate AM Antenna by R. Wagoner. The design looked right, but only by making one could I find out how good it was. The result is audibly better AM reception at my home in Mesa, Arizona.

The original wooden frame was problematic to build (and alas, not very substantial) so I tried substituting plastic pipe for the wood, with the antenna wires on the inside. This proved to be durable and easy to construct.

Here are the parts ...

6' of telephone cable. Round telephone cable with four solid #22 wires works best. Cable with twisted pairs of wires interferes with AM reception, and therefore must be avoided.
Thanks to Gary Camp for advising me about this.
three 11" sections of 1/2" plastic pipe.
three 5" sections of 1/2" plastic pipe.
four slip elbows for 1/2" plastic pipe.
one slip “T” for 1/2" plastic pipe.
wire cutting and stripping tool
radio and TV solder
electrical tape

Here’s how to assemble the antenna:

Thread the telephone cable through the plastic pipes and fittings, like stringing beads. The ends of the cable coming out the bottom should be of approximately equal length. It’s easier to get them even if you start by threading the 11" plastic pipe on top.

Press the plastic pipes and fittings together, with the telephone cable inside. You can glue the plastic parts with PVC cement, but you don’t have to. They fit snugly.

Strip the outer insulation from both ends of the telephone cable.

If you don’t already have a wire cutting and stripping tool it’s well worth buying one. Mine is from JerryCo, part no. 40587.

Cut all but the white wire on the left, and all but the black wire on the right, leaving just enough to connect the wires that remain. Save the pieces of wire you cut off.

Strip and connect each short wire on the left to a short wire on the right. Make sure each connection is from a left wire to a right wire. Remember that left and right wires of the same color are opposite ends of the same wire. An effective connection pattern is

left —	right
white —	red
red —	green
green —	black

You are transforming the 4-conductor telephone cable into a loop with 4 turns of wire.

Test the loop for circuit continuity at the ends of the two long wires you didn’t cut.

My home-made circuit continuity tester has both a beeper and a light that come on when the circuit is closed.

Twist the two long wires together so that they have approximately 2 turns per inch.

If the wires aren’t too long you can hold the loose ends up with one hand while turning the antenna assembly below them. Watch out for kinking.

If the resulting antenna leads aren’t long enough, extend them using the pieces of wire you cut off in step 4, all twisted so they have approximately 2 turns per inch.

Build a base for your antenna out of wood, plastic pipe, or other insulating material. Or, depending on the location, you could hang the antenna from a ceiling.

Here’s where I used that third 5" plastic pipe. Drill a hole in one side to let the wires through.

You can turn the antenna in any direction to get the best AM radio reception.

Ted,

This was fascinating and I have a few observations to share.

The verbal description was excellent, yet when I referred to the photographs there seemed to be disconnect when you said “Cut all but one of the wires on the left, and all but one of the wires on the right,” since when I referred to the photo I saw only one long wire. Turns out that the background color, the wire color, and my “first look” observation skills did not allow me to see the black wire. Fortunately I continued to follow the instructions to the end, then went back and read it again with the knowledge that there had to be two long wires, and walla there they were. Once the black wire is seen it is obvious in subsequent looks. Working in medical imaging, I am always fascinated by the way the human visual system filters image information, causing some things to be invisible until they are pointed out.

I am curious whether the shape and size of the loop is critical to the efficiency of the AM reception and how you determined the optimum design.

For those that don’t have any engineering or science background, it would help to show how the antenna is connected to the radio, perhaps even several different radios to demonstrate different means/locations of the connection.

You might also want to check out the broadcast Digital TV Antenna described in the attached document. You might also want to check out the broadcast Digital TV Antenna described in Makezine.

Loved your post. Thank you for sharing.

Phil Rauch

Phil,

Thanks! It’s great to hear from you.

The green carpet in my living room might not have been the best background. I’d thought about using a table, or the concrete floor on the patio, but wound up on my living room carpet.

The shape and size of the AM antenna are optimal, according to Wagoner. I don’t know enough about antenna design to say much more, so I adopted Carver’s design as given.

On the back of my stereo receiver (and apparently most) there are antenna jacks marked for AM and FM. For best reception you should use both an AM antenna and an FM antenna, just by inserting the wires in their corresponding jacks.

The Digital TV Coat Hanger Antenna looks interesting. We didn’t throw away all of our metal coat hangers when my family switched to plastic.

Ted


updated October 14, 2013	Mile 204	Ted Tenny

FM, AM/MW and SW Antenna Amplifier

FM, AM/MW and SW Antenna Amplifier
Here is a schematic of FM, AM/MW and SW antenna amplifier circuit or we can also say it antenna preamplifier circuit which can be used to increase the faint signals of FM, AM/MW & SW bands. The circuit is very simple and easy to build using only one transistor MPF 102 and few other components

But if you didn't found MPF 102 transistor then you can use NTE 451 or 2N4406 as substitutes. For L1 use 470uH coil for AM and for shortwave use 20uH coil. This circuit can be powered with a 9V alkaline battery or if you are using power supply then bypass power supply with a 0.04 uF capacitor to decrease noise. Antenna can be a 18 inch telescope or 18 inch thick piece of copper wire.

2014年1月23日星期四

Active Band Pass Filter

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Tutorial: 7 of 8

Active Band Pass Filter

As we saw previously in the Passive Band Pass Filter tutorial, the principal characteristic of a Band Pass Filter or any filter for that matter, is its ability to pass frequencies relatively unattenuated over a specified band or spread of frequencies called the "Pass Band". For a low pass filter this pass band starts from 0Hz or DC and continues up to the specified cut-off frequency point at -3dB down from the maximum pass band gain. Equally, for a high pass filter the pass band starts from this -3dB cut-off frequency and continues up to infinity or the maximum open loop gain for an active filter.

However, the Active Band Pass Filter is slightly different in that it is a frequency selective filter circuit used in electronic systems to separate a signal at one particular frequency, or a range of signals that lie within a certain "band" of frequencies from signals at all other frequencies. This band or range of frequencies is set between two cut-off or corner frequency points labelled the "lower frequency" ( ƒL ) and the "higher frequency" ( ƒH ) while attenuating any signals outside of these two points.

Simple Active Band Pass Filter can be easily made by cascading together a single Low Pass Filterwith a single High Pass Filter as shown.

The cut-off or corner frequency of the low pass filter (LPF) is higher than the cut-off frequency of the high pass filter (HPF) and the difference between the frequencies at the -3dB point will determine the "bandwidth" of the band pass filter while attenuating any signals outside of these points. One way of making a very simple Active Band Pass Filter is to connect the basic passive high and low pass filters we look at previously to an amplifying op-amp circuit as shown.

Active Band Pass Filter

This cascading together of the individual low and high pass passive filters produces a low "Q-factor" type filter circuit which has a wide pass band. The first stage of the filter will be the high pass stage that uses the capacitor to block any DC biasing from the source. This design has the advantage of producing a relatively flat asymmetrical pass band frequency response with one half representing the low pass response and the other half representing high pass response as shown.

The higher corner point ( ƒH ) as well as the lower corner frequency cut-off point ( ƒL ) are calculated the same as before in the standard first-order low and high pass filter circuits. Obviously, a reasonable separation is required between the two cut-off points to prevent any interaction between the low pass and high pass stages. The amplifier also provides isolation between the two stages and defines the overall voltage gain of the circuit.

The bandwidth of the filter is therefore the difference between these upper and lower -3dB points. For example, if the -3dB cut-off points are at 200Hz and 600Hz then the bandwidth of the filter would be given as: Bandwidth (BW) = 600 - 200 = 400Hz. The normalised frequency response and phase shift for an active band pass filter will be as follows.

Active Band Pass Frequency Response

While the above passive tuned filter circuit will work as a band pass filter, the pass band (bandwidth) can be quite wide and this may be a problem if we want to isolate a small band of frequencies. Active band pass filter can also be made using inverting operational amplifier. So by rearranging the positions of the resistors and capacitors within the filter we can produce a much better filter circuit as shown below. For an active band pass filter, the lower cut-off -3dB point is given by ƒC2 while the upper cut-off -3dB point is given by ƒC1.

Inverting Band Pass Filter Circuit

This type of band pass filter is designed to have a much narrower pass band. The centre frequency and bandwidth of the filter is related to the values of R1, R2, C1 and C2. The output of the filter is again taken from the output of the op-amp.

Multiple Feedback Band Pass Active Filter

We can improve the band pass response of the above circuit by rearranging the components again to produce an infinite-gain multiple-feedback (IGMF) band pass filter. This type of active band pass design produces a "tuned" circuit based around a negative feedback active filter giving it a high "Q-factor" (up to 25) amplitude response and steep roll-off on either side of its centre frequency. Because the frequency response of the circuit is similar to a resonance circuit, this centre frequency is referred to as the resonant frequency, ( ƒr ). Consider the circuit below.

Infinite Gain Multiple Feedback Active Filter

This active band pass filter circuit uses the full gain of the operational amplifier, with multiple negative feedback applied via resistor, R2 and capacitor C2. Then we can define the characteristics of the IGMF filter as follows:

We can see then that the relationship between resistors, R1 and R2 determines the band pass "Q-factor" and the frequency at which the maximum amplitude occurs, the gain of the circuit will be equal to-2Q2. Then as the gain increases so to does the selectivity. In other words, high gain - high selectivity.

Example No1

An active band pass filter that has a gain Av of one and a resonant frequency, ƒr of 1kHz is constructed using an infinite gain multiple feedback filter circuit. Calculate the values of the components required to implement the circuit.

Firstly, we can determine the values of the two resistors, R1 and R2 required for the active filter using the gain of the circuit to find Q as follows.

Then we can see that a value of Q = 0.7071 gives a relationship of resistor, R2 being twice the value of resistor R1. Then we can choose any suitable value of resistances to give the required ratio of two. Then resistor R1 = 10kΩ and R2 = 20kΩ.

The centre or resonant frequency is given as 1kHz. Using the new resistor values obtained, we can determine the value of the capacitors required assuming that C = C1 = C2.

The closest standard value is 10nF.

Resonant Frequency

The actual shape of the frequency response curve for any passive or active band pass filter will depend upon the characteristics of the filter circuit with the curve above being defined as an "ideal" band pass response. An active band pass filter is a 2nd Order type filter because it has "two" reactive components (two capacitors) within its circuit design and will have a peak response or Resonant Frequency ( ƒr ) at its "centre frequency", ƒc. The centre frequency is generally calculated as being the geometric mean of the two -3dB frequencies between the upper and the lower cut-off points with the resonant frequency (point of oscillation) being given as:

Where:
ƒr is the resonant or Centre Frequency
ƒL is the lower -3dB cut-off frequency point
ƒH is the upper -3db cut-off frequency point

and in our simple example above the resonant centre frequency of the active band pass filter is given as:

The "Q" or Quality Factor

In a Band Pass Filter circuit, the overall width of the actual pass band between the upper and lower -3dB corner points of the filter determines the Quality Factor or Q-point of the circuit. This Q Factor is a measure of how "Selective" or "Un-selective" the band pass filter is towards a given spread of frequencies. The lower the value of the Q factor the wider is the bandwidth of the filter and consequently the higher the Q factor the narrower and more "selective" is the filter.

The Quality Factor, Q of the filter is sometimes given the Greek symbol of Alpha, ( α ) and is known as the alpha-peak frequency where:

As the quality factor of an active band pass filter (Second-order System) relates to the "sharpness" of the filters response around its centre resonant frequency ( ƒr ) it can also be thought of as the "Damping Factor" or "Damping Coefficient" because the more damping the filter has the flatter is its response and likewise, the less damping the filter has the sharper is its response. The damping ratio is given the Greek symbol of Xi, ( ξ ) where:

The "Q" of a band pass filter is the ratio of the Resonant Frequency, ( ƒr ) to the Bandwidth, ( BW ) between the upper and lower -3dB frequencies and is given as:

Then for our simple example above the quality factor "Q" of the band pass filter is given as:

346Hz / 400Hz = 0.865. Note that Q is a ratio and has no units.

When analysing active filters, generally a normalised circuit is considered which produces an "ideal" frequency response having a rectangular shape, and a transition between the pass band and the stop band that has an abrupt or very steep roll-off slope. However, these ideal responses are not possible in the real world so we use approximations to give us the best frequency response possible for the type of filter we are trying to design.

Probably the best known filter approximation for doing this is the Butterworth or maximally-flat response filter. In the next tutorial we will look at higher order filters and use Butterworth approximations to produce filters that have a frequency response which is as flat as mathematically possible in the pass band and a smooth transition or roll-off rate.