162 lines
6.3 KiB
Plaintext
162 lines
6.3 KiB
Plaintext
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Short Wave Antenna basics
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This article describes how to use a phone line as a shortwave antenna.
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Performance will vary depending on the kind of line you have. Overhead
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lines make fairly good antennas, while underground lines generally
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don't.
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A highpass filter is used to remove signals below the shortwave bands.
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This improves rejection from local AM stations. In addition, a lowpass
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filter can be used to reject interference from FM stations.
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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. I have included edited
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versions of their articles below.
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The filter(s) should be connected to the phone line in this manner:
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Phone line RF plug
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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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N.C. o----| |----------------------------+
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-------- shield
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N.C. = no connect. Alternatively, you could connect this to a ground.
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I tried the phone line ground (yellow wire) and it worked more poorly
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than no ground at all. I haven't tried any other ground, because that
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would defeat the purpose of a portable antenna.
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If the highpass filter is used, all phone line voltages, including
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ringing, are eliminated and thus will not harm the receiver. I should
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also mention that this filter works very well with random wire
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antennas.
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Someone who wrote asked about lightning protection. I haven't thought
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much about this because thunderstorms are rare in my area. However, I
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believe most phone lines have lightning arrestors on them where they
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enter the house. Anyway, my suggestion would be to unplug the antenna
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when not in use if you experience frequent thunderstorms.
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The following ascii-schematic diagram is a high pass filter that will
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filter out Broadcast Band (MW) stations. I found it a great boon to my
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shortwave listening since local MW stations overload my ATS-803A front
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end & appear in 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,
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I still have some MW interference). I took the advice of Gary Coffman
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and looked up filters in the ARRL Handbook. With the filter in place,
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I intend 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.
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(a close value will do).
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I built the filter that Paul Blumstein posted recently and measured it
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on a 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
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has five elements, the attenuation is 30dB per octave. The
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measurements confirmed 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
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in the 120m band and above. It could use a little more attenuation
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at the upper end of MW, which could be done with more stages or a
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higher cutoff frequency. Alternatively, you could build two of
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these filters and put them in series. (Since two 1500pF capacitors
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in series are really 750pF, you could eliminate one cap.)
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Just for fun, I decided to put 470 ohms in series with the input
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to see how the filter performs with an antenna mismatch. The
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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,
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which is due to the impedance mismatch between the source and load
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and would have been there without the filter. Hence, the filter
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works almost as well in spite of the mismatch, which is good news
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to those who use longwire antennas.
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Several people have asked for construction details of the filters
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I 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
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ARRL handbook and chose a 7-element Chebyshev low-pass design
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that is -3 dB at 35 MHz, -20 dB at 43 MHz, and -50 dB at 64 MHz
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(all calculated; 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.
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I used .33 uH instead of .36, and .66 uH instead of .42,
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but it works fine.
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The 5-lug terminal strips were perfect for these circuits,
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since there are 4 lugs plus a grounded lug. All "ground"
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connections go to the lug that is mounted to the chassis,
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and the other 4 lugs are used for each of the connections
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on the signal line. One terminal strip is used per filter.
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Since both filters were necessary to clean up the hash,
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I am going to put them both into one box when I get the 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"
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which means you can't possibly hook them up backwards.
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(In the above schematics, left and right ends are interchangeable.)
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