223 lines
8.3 KiB
Plaintext
223 lines
8.3 KiB
Plaintext
Subject: Phone line as SW antenna
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Date: 1 Aug 1995 07:01:59 GMT
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[Last modified 07-Mar-95] Changes preceeded by "|".
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This article describes how to use a phone line as a shortwave antenna.
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WARNING: Connecting unapproved devices to phone lines may be illegal in
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your area. There may also be a potential shock hazard. Use at your own
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risk.
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Performance will vary depending on the kind of line you have. Overhead
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lines usually make okay antennas, while underground lines generally don't.
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A highpass filter is used to remove signals below the shortwave bands.
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It rejects interference from local AM stations and decouples the receiver
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from the phone line. In addition, all phone line voltages, including
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ringing, are eliminated and thus will not harm the receiver.
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In addition, a lowpass filter can be used to reject interference from FM
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stations. If you use both filters, connect the output of the first filter
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to the input of the second (don't forget to connect their ground terminals
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together). It doesn't matter which filter is connected first.
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By the way, these filters also work well with random wire antennas. Just
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connect the antenna in place of the phone line.
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The filter(s) should be connected to the phone line in this manner:
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Phone line RF connector
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red -------- center cond. / \
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or o----| |--------------------------|-o | To receiver
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green | | \ /
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| FILTER | 50 ohm coax |
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ground | | |
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or o----| |----------------------------+
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N.C. -------- shield
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N.C. = no connect. I have tried connecting the phone line ground (yellow
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wire) to this teminal, but reception has been better without it (if you do
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this, play it safe and put a .01 uF capacitor in series). If another
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ground is available, you can connect it here.
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Someone wrote asking about lightning protection. I haven't thought much
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about it because thunderstorms are rare in my area. Most phone lines have
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lightning arrestors on them where they enter the house, but my suggestion
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is to unplug the antenna when not in use if you experience frequent
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thunderstorms.
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The original credit for the highpass and lowpass filters goes to Paul
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Blumstein and John Shalamskas, respectively. Edited versions of their
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articles are included here:
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Date: 09 Jan 91 00:54:08 GMT
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From: paulb@harley.TTI.COM (Paul Blumstein)
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Subject: BC Band Hi-Pass Filter
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The following ascii-schematic diagram is a high pass filter that will filter
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out Broadcast Band (MW) stations. I found it a great boon to my shortwave
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listening since local MW stations overload my ATS-803A front end & appear in
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SW, especially with a long antenna.
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If you remember my antenna saga, I went from 50 feet to 150 feet & had
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overload problems causing me to cut back to 50 feet. (Even at 50 feet, I
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still have some MW interference). I took the advice of Gary Coffman and
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looked up filters in the ARRL Handbook. With the filter in place, I intend
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to try to increase my antenna length again.
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Anywho, here is the filter, for interested parties.
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--------||---+----||-----+----||-----------
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} }
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{ {
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} }
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-------------+-----------+-----------------
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The outer capacitors are 1500 pf ceramic disks.
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The inner capacitor is 820 pf ceramic disk.
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The squiggly things are coils (two total). Each one is 2.7 uh. (a close
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value will do).
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Date: 12 Jan 91 00:44:25 GMT
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From: bill@videovax.tv.tek.com (William K. McFadden)
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Subject: Re: BC Band Hi-Pass Filter
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I built the filter that Paul Blumstein posted recently and measured it on a
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gain-phase analyzer. Here are its characteristics:
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100 KHz -120dB
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500 KHz -68dB
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1000 KHz -38dB
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1600 KHz -15dB
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2100 KHz -3dB
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The source and load impedances were 50 ohms. Because the filter has five
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elements, the attenuation is 30dB per octave. The measurements confirmed
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this.
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This filter seems to be a pretty good compromise between interference
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attenuation and passband response. There is very little attenuation in the
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120m band and above. It could use a little more attenuation at the upper end
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of MW, which could be done with more stages or a higher cutoff frequency.
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Alternatively, you could build two of these filters and put them in series.
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(Since two 1500pF capacitors in series are really 750pF, you could eliminate
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one cap.)
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Just for fun, I decided to put 470 ohms in series with the input to see how
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the filter performs with an antenna mismatch. The characteristics were:
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100 KHz -105dB
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500 KHz -60dB
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1000 KHz -35dB
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1600 KHz -15dB
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2300 KHz -3dB
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These figures are normalized to the passband response of -15dB, which is due
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to the impedance mismatch between the source and load and would have been
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there without the filter. Hence, the filter works almost as well in spite of
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the mismatch, which is good news to those who use longwire antennas.
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Date: 22 Apr 92 08:59:33 GMT
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From: johns@uhunix.uhcc.Hawaii.Edu (John Shalamskas)
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Subject: Construction of filters for SW reception
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Several people have asked for construction details of the filters I
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built for my DX-440.
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The high-pass filter helped some, but in my location the VHF/UHF
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broadcasters are also causing problems. So, I dug out the ARRL
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handbook and chose a 7-element Chebyshev low-pass design that is -3
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dB at 35 MHz, -20 dB at 43 MHz, and -50 dB at 64 MHz (all calculated;
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it works well in practice!)
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LOW-PASS FILTER (Rejects FM, TV, etc.)
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0.36 uH 0.42 uH 0.36 uH
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signal -------+--UUU--+--UUUUU--+--UUU--+------- signal
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82 ___ 180___ 180___ ___ 82
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pF --- pF--- pF--- --- pF
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shield | | | | shield
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braid -------+-------+----+----+-------+------- braid
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chassis ground
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I had to do a little more improvising at this point. I used .33 uH
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instead of .36, and .66 uH instead of .42, but it works fine.
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The 5-lug terminal strips were perfect for these circuits, since
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there are 4 lugs plus a grounded lug. All "ground" connections go to
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the lug that is mounted to the chassis, and the other 4 lugs are used
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for each of the connections on the signal line. One terminal strip
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is used per filter. Since both filters were necessary to clean up
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the hash, I am going to put them both into one box when I get the
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time.
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The proper way to connect them is in series, i.e.
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signal in ------- filter 1 -------- filter 2 -------- signal out
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There is no difference between ends. They are "bilateral" which
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means you can't possibly hook them up backwards. (In the above
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schematics, left and right ends are interchangeable.)
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COIL WINDING
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You can make the coils yourself using this formula:
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L = 0.2 * B^2 * N^2 / (3B + 9A + 10C)
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L is inductance, in uH
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A is length of coil, in inches
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B is mean diameter of coil, in inches
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C is the diameter of the wire, in inches
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N is the number of turns
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For small wire, you can assume C = 0.
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PARTS SOURCE
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All of the parts for these filters can be obtained from the following source:
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Digi-Key
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701 Brooks Av S
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P.O. Box 677
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Thief River Falls, MN 56701-0677 USA
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Voice: 800-344-4539
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FAX: 218-681-3380
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PART DIGI-KEY PART NO.
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82 pF capacitor P4023
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180 pF capacitor P4027
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820 pF capacitor P4184
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1500 pF capacitor P4187
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0.33 uH inductor M8007
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0.39 uH inductor M8008
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2.7 uH inductor M8018
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The cost of these parts is less than $1 each.
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--
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Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077
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bill@tv.tv.tek.com, ...!tektronix!tv.tv.tek.com!bill Phone: (503) 627-6920
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CAUTION: Don't look into laser beam with remaining eye.
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