Phone line as SW antenna

[Fansome]

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Posted to rec.radio.shortwave:

[Last modified 12-Jul-06]

The latest version of this file can be accessed via my web page at:
http://www.rdrop.com/users/billmc

Phone line as SW antenna
Bill McFadden
billmc@agora.rdrop.com

Changes preceded by "|".

  WARNINGS AND DISCLAIMERS

  Connecting unapproved devices to phone lines may be illegal in your area.
  Telephone lines present a potential shock hazard.  Do not use this antenna
  for transmitting.  Use at your own risk.


  INTRODUCTION

  This article describes how to use a phone line as a shortwave receiving
  antenna.  Performance will vary depending on the kind of phone line you
  have.  Overhead lines usually make okay antennas, while underground lines
| usually don't.  Phone lines carrying DSL service may be unsuitable because
| DSL operates over a wide spectrum.  A phone line antenna will generally
  pick up more power line noise than a dedicated antenna, so don't expect to
| use it for DX work.  Nevertheless, I found that it outperformed the
| telescopic whip antennas built into many portables.

  A reader sent in this suggestion:  If you're concerned about connecting
  the phone line directly to the receiver, try wrapping the phone cord
  several times around the receiver's telescopic whip antenna instead.
  Performance may not be as good as a direct connection, but it will
  probably be better than the whip antenna by itself.

  Someone else wrote asking about lightning protection.  I haven't thought
  much about it because thunderstorms are rare in my area.  Most phone lines
  have lightning arrestors on them where they enter the house, but the
  lightning arrestors may pass enough energy during a lightning strike
  damage a receiver.  My suggestion is to disconnect the antenna when not in
  use if you experience frequent thunderstorms.


  HOW TO DO IT

  The simplest connection is a single capacitor between the phone line and
  receiver.  The capacitor eliminates all phone line voltages, including
| ringing, so that they will not harm the receiver.  The capacitor should
| be rated 250 volts or higher.


   Phone line                                    RF connector
      red          .01 uF     center cond.            / \
       or   o--------||------------------------------|-o |  To receiver
     green                                            \ /
                             50 ohm coax               |
     ground                                            |
       or   o------------------------------------------+
      N.C.                      shield


  N.C. = no connect.  I originally connected the phone line ground (yellow
  wire) to this terminal, but reception was better without it (if you do
  this, play it safe and put a .01 uF capacitor in series).  If another
  ground is available, you can connect it to this terminal.

  If you are picking up local AM or FM radio stations on the shortwave
  bands, it is likely that your receiver is overloaded.  Inexpensive and
  portable receivers usually lack the filtering needed to eliminate this
  interference, but you can make your own filters.  Filters can also be
| purchased from shortwave dealers (see below for recommendations).

  A high-pass filter can be used to remove signals below the shortwave
  bands.  It rejects interference from local AM stations and presents a high
  impedance to the phone line at audio frequencies so that the phone line
  remains balanced.

  A low-pass filter can be used to remove signals above the shortwave bands.
  It rejects interference from FM and TV stations, as well as VHF 2-way
  radio transmitters.  If you use both filters, connect the output of the
  first filter to the input of the second (don't forget to connect their
  ground terminals together).  It doesn't matter which filter is connected
  first.

  These filters also work well with random wire antennas.  Just connect the
  antenna in place of the phone line.

  The filter(s) should be connected to the phone line in this manner:


   Phone line                                    RF connector
      red         --------    center cond.            / \
       or   o----|        |--------------------------|-o |  To receiver
     green       |        |                           \ /
                 | FILTER |  50 ohm coax               |
     ground      |        |                            |
       or   o----|        |----------------------------+
      N.C.        --------      shield


| RECOMMENDED FILTERS
|
| If you don't want to make your own filters, the following have good
| performance, are well-built and affordable:
|
|   Model 400 BCB interference filter (3.5 MHz highpass)
|   http://www.iceradioproducts.com/filtersrf.html#bcb
|
|   Model 420 HF lowpass filter (30 MHz lowpass)
|   http://www.iceradioproducts.com/filtersrf.html#3
|   (scroll down a bit to see the model 420)
|
| If you have a random-wire antenna, the following unit will provide
| impedance matching and lightning protection (connect antenna to 450-ohm
| terminal):
|
|   Model 182A beverage matching unit
|   http://www.iceradioproducts.com/reconly.html#Beverage Matching


  FILTER PLANS

  The original credit for the high-pass and low-pass filters goes to Paul
  Blumstein and John Shalamskas, respectively.  Edited versions of their
  articles are included here:


    Date:    09 Jan 91 00:54:08 GMT
    From:    paulb@harley.TTI.COM (Paul Blumstein)
    Subject: BC Band Hi-Pass Filter

    The following ascii-schematic diagram is a high pass filter that will
    filter out Broadcast Band (MW) stations.  I found it a great boon to my
    shortwave listening since local MW stations overload my ATS-803A front
    end & appear in SW, especially with a long antenna.

    If you remember my antenna saga, I went from 50 feet to 150 feet & had
    overload problems causing me to cut back to 50 feet.  (Even at 50 feet,
    I still have some MW interference).  I took the advice of Gary Coffman
    and looked up filters in the ARRL Handbook.  With the filter in place, I
    intend to try to increase my antenna length again.

    Anywho, here is the filter, for interested parties.


           --------||---+----||-----+----||-----------
                        }           }
                        {           {
                        }           }
           -------------+-----------+-----------------

    The outer capacitors are 1500 pf ceramic disks.
    The inner capacitor is 820 pf ceramic disk.
    The squiggly things are coils (two total).  Each one is 2.7 uh.
    (a close value will do).


    Date:    12 Jan 91 00:44:25 GMT
    From:    bill@videovax.tv.tek.com (William K. McFadden)
    Subject: Re: BC Band Hi-Pass Filter

    I built the filter that Paul Blumstein posted recently and measured it
    on a gain-phase analyzer.  Here are its characteristics:

     100 kHz -120dB
     500 kHz  -68dB
    1000 kHz  -38dB
    1600 kHz  -15dB
    2100 kHz   -3dB

    The source and load impedances were 50 ohms.  Because the filter has
    five elements, the attenuation is 30dB per octave.  The measurements
    confirmed this.

    This filter seems to be a good compromise between interference
    attenuation and passband response.  There is very little attenuation in
    the 120m band and above.  It could use a little more attenuation at the
    upper end of MW, which could be done with more stages or a higher cutoff
    frequency.  Alternatively, you could build two of these filters and put
    them in series.  (Since two 1500pF capacitors in series are really
    750pF, you could eliminate one cap.)

    Just for fun, I put 470 ohms in series with the input to see how the
    filter performed with an antenna mismatch.  The response was:

     100 kHz -105dB
     500 kHz  -60dB
    1000 kHz  -35dB
    1600 kHz  -15dB
    2300 kHz   -3dB

    These figures are normalized to the passband response of -15dB, which is
    due to the impedance mismatch between the source and load and would have
    been there without the filter.  Hence, the filter works almost as well
    in spite of the mismatch, which is good news to those who use long-wire
    antennas.


    Date:    22 Apr 92 08:59:33 GMT
    From:    johns@uhunix.uhcc.Hawaii.Edu (John Shalamskas)
    Subject: Construction of filters for SW reception

    Several people have asked for construction details of the filters I
    built for my DX-440.

    The high-pass filter helped some, but in my location the VHF/UHF
    broadcasters are also causing problems.  So, I dug out the ARRL handbook
    and chose a 7-element Chebyshev low-pass design that is -3 dB at 35 MHz,
    -20 dB at 43 MHz, and -50 dB at 64 MHz (all calculated; it works well in
    practice!)


                LOW-PASS FILTER  (Rejects FM, TV, etc.)

                        0.36 uH  0.42 uH   0.36 uH         
       signal  -------+--UUU--+--UUUUU--+--UUU--+-------  signal
                      |       |         |       |
                 82  ___  180___    180___     ___  82
                 pF  ---   pF---     pF---     ---  pF
       shield         |       |         |       |         shield
        braid  -------+-------+----+----+-------+-------  braid
                                   |
                             chassis ground


    I had to do a little more improvising at this point.  I used .33 uH
    instead of .36, and .66 uH instead of .42, but it works fine.

    The 5-lug terminal strips were perfect for these circuits, since there
    are 4 lugs plus a grounded lug.  All "ground" connections go to the lug
    that is mounted to the chassis, and the other 4 lugs are used for each
    of the connections on the signal line.  One terminal strip is used per
    filter.  Since both filters were necessary to clean up the hash, I am
    going to put them both into one box when I get the time.

    The proper way to connect them is in series, i.e.

    signal in ------- filter 1 -------- filter 2 -------- signal out

    There is no difference between ends.  They are "bilateral" which means
    you can't possibly hook them up backwards.  (In the above schematics,
    left and right ends are interchangeable.)


  COIL WINDING

  You can make the coils yourself using this formula:

  L = 0.2 * B2 * N2 / (3B + 9A + 10C)

  L is the inductance, in uH
  A is the length of the coil, in inches
  B is the mean diameter of the coil, in inches
  C is the diameter of the wire, in inches
  N is the number of turns

  For small wire, you can assume C = 0.


  PARTS SOURCE

  Parts for the filters can be obtained from the following source:
  http://www.digi-key.com/

--- end ---

-- Bill McFadden billmc@agora.rdrop.com http://www.rdrop.com/users/billmc CAUTION: Don't look into laser beam with remaining eye.

[cpuerror]

When ringing, the phone line voltage will increase to 90VAC. A .01uf capacitor might attenuate it but why even risk having that voltage anywhere near your receiver's front end? If it isn't possible to setup a dipole, a random wire antenna would still work better then a phone line picking up every AC noise in the neighberhood and there is much less possibility of frying your receiver.

[Pigmeat]

I've wondered how to do this with modern phones.

Back in the days of heavy duty dial phones you could hook your radio to the metal stop on the dial,it went to ground. I'd use a short piece of wire and an alligator clip to do the job. Damage from overloading wasn't much of a problem with those old tube radios. If you didn't live near an AM transmitter it worked better than most longwires.

[Zoidberg]

I tried a variation of this trick that's reasonably safe without special precautions.  Use the inductive coupling trick, wrap several turns of insulted phone line around the portable's whip.  Plug other end into phone jack.  Unless the lines are very noisy this will boost signals just a bit.  Virtually no risk of damage.  Worked fairly well in an office building several years ago that blocked out most radio signals.  But in my apartment, just locating a small antenna near the window does the trick.

[cmradio]

Personally, I'd stick a MOV in there... the voltage buildup on the lines can get pretty hairy at times.

Peace!