Appropriate Type of Antenna
To achieve the best
quality recordings, it is important to use a sensitive antenna and receiver system. A brief overview of antenna and receiver techniques will be
discussed as follows.
Newcomers to the TV DX
interest can start with relatively simple antennas, since signals propagated via Sporadic E can be strong. However, always
attempt to make the antenna system as efficient as
possible.
One word of caution -
don't be tempted to use an FM radio antenna for DXing channels in VHF bands 1
and 3. The results will be disappointing. Always use an antenna designed to cover the appropriate channels.
Since signals received via Sporadic E
often reach high signal levels, a simple folded dipole cut to approximately 100" can be used for 45-70 MHz band 1 DX. The dipole can be mounted vertically for an omnidirectional
response, will not need to be rotated. If the dipole is mounted
horizontally, it will need to be rotated 180 degrees, so it is facing broadside to the
direction of the desired DX station. A horizontal dipole will also have slightly
more gain with respect to a vertical dipole.
It is also worth noting that most DXers have found their domestic "garden variety" TV antennas are poor performers for
DX reception.
To capture the weaker
signals, a more elaborate antenna system is recommended. A 2 element wideband
yagi array for band 1 can be quite adequate and takes up little space, while a 4
or 5 element yagi will offer greatly improved results in terms of signal level
(gain) and directivity. It is important to have
the reflector element 125" in length, so it will receive down to NZ Ch 1 video
(45.25 MHz). The dipole is usually 100" and the reflectors are cut to
80" for coverage up to Ch 2 audio (69.75 MHz). Some DX enthusiasts use
single yagi arrays for each band, while others use combined arrays covering
bands 1 & 3 or bands 1, 2 & 3 (45-220MHz). An example of an excellent
all channel antenna is the Channel Master CX 28 (28 element) log periodic array.
This type of antenna takes up less space on the mast and requires only one coax
line. The CLP5130 log periodic 50-1300 MHz antenna is currently popular for DX TV and FM reception in Japan. The official web site for the CLP5130 can be found at: http://www.cd-corp.com/eng/ama/5130.htm
It is not recommended that
an all channel antenna, covering the entire VHF/UHF (45-860MHz) bands be
used, due to its relatively poor signal gain. When possible, use separate TV antennas for VHF and UHF. Yagi and Log Periodic
arrays for band 1 are available from antenna installations firms in both
Australia and New Zealand.
The video cassette recorder (VCR) is gradually becoming very popular with TV DXers for keeping a
permanent record of reception. One advantage is that distant signals can be
recorded and played back to review specific items of interest, such as exotic
rare signals or unidentified test cards or logos. The "freeze frame" or pause
facility that is available on VCR machines will enable an unknown pattern to be
examined frame by frame to enable identification. If a DXer has access to a
second compatible VCR machine, it is possible to dub items of interest on to a
the second VCR to build up an edited library of test card and other DX
material.
86-108 MHz antennas
A wide variety of
antennas are now available for the FM band and Australian TV channels 3, 4 and
5. They range from a simple folded dipole to a 10 element yagi. Some examples of suitable band 2 antennas are the Matchmaster FM G8 (8 element)
yagi, Triax FM-8, Hills 3-4-5, or Antenna Performance APS 13.
175-230 MHz TV antennas
Single band yagi arrays
are available with up to 14 elements. As mentioned earlier, combination antennas are also available. If a combination VHF antenna is desired, it is wise to purchase
a "deep fringe" antenna, designed for weak signal
reception.
Rotating the antennas
Since sporadic E and
tropospheric signals can come from almost any direction, it is desirable that
some method of antenna rotation is provided especially when an antenna has
several elements. Beginners can try the "armstrong method", which is a hand
rotatable mast with a cross bar. Mechanical antenna rotators are also available
from most amateur and TV installation firms.
TV DX receiving systems
Small screen portable TV
sets are very popular with DXers and are also ideal for mobile/holiday use, as
well as in the home.The set must have its tuners marked with the correct
channels (Australia: VHF 0-11, UHF 28-69). Some sets have a continuous (varicap)
tuning dial, covering VHF and UHF. Some examples of high sensitive portable TV
sets are: Toshiba C531 VHF/UHF 5" Colour TV: This particular TV has a sharp SAWF, hence is very selective. The D100 DX TV and Toshiba C-531 offer excellent results when used together.
Phillips P45 VHF/UHF 4" B/W TV: Can receive DX on its
own inbuilt rod antenna and also is lightweight for
travelling. Features a varicap tuning dial for VJF and UHF. National Ranger TR505DU VHF (only) B/W TV: This
particular TV is extremely sensitive on band 1 and can receive Sporadic E DX signals
quite well even just on its own inbuilt rod antenna. Go to: TV and FM receivers, currently used by DXers for pictures and details of these sets. All these models have long
since ceased production, but can still be purchased second
hand.
TV vision IF bandwidth
All modern TV receivers
are designed with wide IF bandwidth stages, which are necessary in order to
provide high definition pictures for domestic viewing under normal conditions.
However, a narrow If bandwidth is essential when attempting to resolve
extremely weak DX signals under difficult conditions. Narrow IF bandwidth helps
to display extremely weak signals, which are normally "lost in the noise", when
wide bandwidth is used. By using narrow IF bandwidth, some DXers can detect low
level signals from transmitters located 150-300 miles away, on a daily basis
under normal reception conditions.
Selectivity
Single or multi-system TV
receivers, employing wide IF bandwidth will often suffer from adjacent channel
problems, such as pictures floating over one another, (Australian Ch 2 over NZ Ch 3 or
Australian Ch 0 over NZ Ch 1, etc). TV signals only 1-2MHz apart can easily be
separated, when using a narrow IF bandwidth receiver
system. For several years the
author has used a conventional VHF/UHF varicap TV tuner and 2 G8 selectivity
modules connected to the IF input of a portable TV. This set up was very
effective in separating NZ Ch 3 vision (62.250MHz) from the local Australian Ch 2
vision (64.250MHz). For others not so inclined
to modify and construct TV receiving equipment, a TV tuner unit, known as the
D100, is available. The D100 is a self contained unit, which connects to a UHF
TV and an FM (88-108MHz) radio receiver, to provide VHF/UHF TV DX reception with
pictures and audio of virtually any TV standard worldwide. The vision IF
bandwidth is switchable and variable, allowing reduced or semi-reduced bandwidth
operation for weak signals and full bandwidth, when conditions allow. The
benefit of such a system means that no modifications are required to your
existing TV or radio receiver.
Recording DX TV FM audio
There are several
different methods to record TV DX audio, each of which have wide differences in
sensitivity. The simplest method is to connect a lead to the output jack
(earphone or auxiliary output connection) of a TV and connect it to the input of
either a tape or video recorder. If a video recorder is used, it will record the
audio on to videotape at fair sensitivity. It is worthwhile noting that the
above two methods will not record TV sound signals originating from overseas
countries using different TV systems. The reason being that most video recorders
are only designed to receive the local TV system. In Australia and New Zealand,
the majority of video recorders are designed to receive system B (5.5MHz) audio
signals. The sensitivity of the above two methods are only fair
because of: 1) They both use the inter-carrier audio system, which
relies on both audio and good strength video, being present at the same time on
the TV receiver, which is by no means a certainty with distant
signals. 2) The audio IF bandwidth of a standard video recorder
or TV is typically too wide for weak audio signals (150-250kHz
bandwidth). The method used
to receive extremely weak Australian and overseas TV video and audio signals is
detailed in block diagram Fig A, B &
C.
The antenna is connected
to an optional low noise tunable mosfet preamplifier, which goes to a D100 TV
tuner/converter. The output lead from the D100 at Ch 65 (UHF) is connected to
the RF input of a Toshiba C531 5" colour TV, (or a similar type of TV), for
vision reception. The second lead from the D100 is connected to a scanner at
around 35MHz in wide FM mode for sound. If a scanner is not available, the
second lead can be connected to an FM radio tuner at around 96-100MHz. The other
end of the audio output lead from the scanner (or FM tuner) is connected to the
audio input socket on the video recorder. The other end of the video output lead
from the Toshiba C531 5" TV is connected to the video-input socket on the video
recorder. Once all connected, the Toshiba TV needs to be switched to monitor, to
eliminate the audio mute on the TV. Also, the video recorder needs to be
switched on, so everything is working.
The audio sensitivity will
improve if a narrow IF bandwidth of around 110-150kHz is used by the scanner or
the FM tuner's IF (Intermediate frequency 10.7MHz) circuits. The author uses an
Icom R7000 VHF/UHF scanner with Murata SFE 10.7 MHY-A ceramic 110kHz wide FM IF
filters to improve TV audio sensitivity and selectivity. The advantage of this
approach is that any overseas audio TV signal can be received, regardless of the vision spacing. The scanner or FM tuner acts as a tunable audio IF, thus capable of receiving all TV audio systems.
DVD Recorders
Even though there are several DVD recorders now available at relatively affordable prices, their suitability for recording TV DX signals is yet unknown.
The 2003 Panasonic RDR-GX3 is capable of recording weak TV signals with excellent results. The later model Panasonic GX300 is likely also suitable for recording TV DX.
Video Recorders
There are now a large
variety of VHS Video Cassette Recorders (VCRs) on the market, which have their
own individual advantages and disadvantages for DX reception. One important ability is
the VCR's capability to lock on to weak video signals. This is the same
situation when purchasing a television. If possible, before purchasing a VCR,
try to tune in the weakest signal available, record it, then play it back to see how well
the signal locks horizontally and vertically. It is also important that
the VCR does not mute weak signals. Some VCR's have a switchable mute that can
be turned off, when weak signals are being recorded. Other VCR's have no mute at
all. Any VCR or TV that has a mute that can't be switched off, (a mute will make
the screen totally black or blue on blank stations), is not advisable to
purchase for recording TV DX, as the mute will block most of the wanted DX
signals. Most Panasonic VCRs don't have any picture mute, hence are ideal for recording weak signals. VCRs that don't mute weak signals are as
follows: 1) The National NV-G16 Since all VCR's now have a
VHF/UHF varicap tuner fitted as standard, they can form the basis of a basic DX
receiver system for beginners.
By simply
connecting an outdoor DX antenna to the VCR's RF input and then selecting the
appropriate band, such a recorder may be used to record TV DX signals, when used
in its E to E mode (ie: monitoring the output from the VCR tuner on the TV
receiver. (Fig D). Recordings can also be made directly off-air, but as mentioned
earlier, overseas TV audio, with different systems, can not be
recorded. When a Sporadic E opening is in
progress, the scanner can be used to determine what video frequencies are
present. The next step is to tune in the signals on the VCR and store them in
pre-set memory locations, (eg: Memory 1 - NZ Ch 1 45.250 MHz; Memory 2 - Aust Ch
0 46.250 MHz; Memory 3 - ChE 2 48.250 MHz; Memory 4 - ChC 1/ChR 1 49.750 MHz, etc.
After all the available signals are stored into the memory, it is advisable to
keep a record of memory location/channel pre-sets.
Recently (August 2001), I purchased a Panasonic NV-FJ600 NTSC and PAL VHS VCR. The weak signal locking is excellent. There is no mute on weak signals. I strongly recommend that TV DXers consider this VCR.
HS Publications (7 Epping Close, Derby, DE22 4HR, England, Phone:
0011-44-1332-381-699), sell the D100 TV tuner/converter. This tuner uses a MOSFET varicap tuner covering 45-860 MHz. It also features variable IF bandwidth to improve selectivity. The D100 also converts the TV audio to 95-100 MHz, for use with a FM tuner. TV audio can also be received on 36 MHz using a scanner. A excellent combination is a D100, and Icom R7000 modified with MURATA SFE10.7 MHY-A, 110 KHz filters. The output frequency of the D100 is at UHF. Veteran TV DXer Bob Cooper uses a D100. This tuner is used extensively in Europe, and has received positive reviews from DXers.
HS Publications 2003/2004 TV FM DX equipment catalogue (PDF version).
2) The Panasonic
NV-L20HQ
3) The Panasonic NV-L25HQ
4) The JVC
HR-J635EA
5) The Panasonic NV-FJ600
By using a scanner, it
is possible to detect the presence of TV signals, long before they are strong
enough to produce an image or sound on a TV set.
Copyright © 2005 Todd Emslie