Phone line as SW antenna [04-Apr-00]
 

[Last modified 04-Apr-00]

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Phone line as SW antenna
Bill McFadden


Changes preceeded 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. 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 find that it outperforms the telescopic whip
antennas built into most 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
| lightening arrestors may pass enough energy during a lightening 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.


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 teminal, 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 some shortwave dealers.

A highpass 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 lowpass 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.

By the way, 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



FILTER PLANS

The original credit for the highpass and lowpass filters goes to Paul
Blumstein and John Shalamskas, respectively. Edited versions of their
articles are included he


Date: 09 Jan 91 00:54:08 GMT
From: (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: (William K. McFadden)
Subject: 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
longwire antennas.


Date: 22 Apr 92 08:59:33 GMT
From: (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 * B^2 * N^2 / (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

All of the parts for the filters can be obtained from the following
source:

Digi-Key
http://www.digi-key.com/
701 Brooks Av S
P.O. Box 677
Thief River Falls, MN 56701-0677 USA
Voice: 800-344-4539
FAX: 218-681-3380

PART DIGI-KEY PART NO.
82 pF capacitor P4023
180 pF capacitor P4027
820 pF capacitor P4184
1500 pF capacitor P4187
0.33 uH inductor M8007
0.39 uH inductor M8008
2.7 uH inductor M8018

The cost of these parts is less than $1 each.

--- end ---

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