LINE AUDIO INPUT. High level line audio usually from the
same source as plugged into the companion Video input is
plugged into this jack using another RCA phono plug shielded
cable. Minimum level is .1 v pk-pk into a 10K load. The level
is controlled by the line audio gain knob.
LINE AUDIO GAIN control. Nominal input is .1 to 1 Vp-p
into 10K. Increase the line or mic audio gain controls to the
point that the green LED blinks off (indicating that the AGC
limiting at 25 to 40 kHz), and then back off a little. In the off
position, the whole sound subcarrier board is turned off.
MIC jack accepts any low Z dynamic or low Z Amplified
electret camcorder mic in the range of 100 - 600 Ohms with
a mini plug. Mic audio is active at all times and mixes with
the camera or external audio inputs to enable greater pickup,
commenting while running video tapes, etc. Mikes must have
a
shielded cable to prevent RF pickup hum and buzz. Some
electret and amplified mics are very susceptible to RF pickup
and may need the addition of a small 220 pF disc cap (RS
272-124) directly across the mic element. Presently Radio
Shack makes 2 different replacement remote-control dynamic
omnis for portable recorders (33-2001 & -1067) that work
well and some provide the “push to look” plug also. The 33-
2001 has a wind screen which is preferred for portable work.
The unidirectional 33-3015 or 33-3021 is used for full duplex
to minimize speaker feedback.
PTL submini jack. Push To Look is like push to talk only with
video. Grounding the tip keys the transmitter. This jack is in
parallel with the front panel transmit/receive toggle switch.
MIC GAIN control sets the level from the low impedance mic
jack. This audio is mixed with the line audio and its level is
varied independantly. If you connect the audio from your
home VCR or camcorder, you can use the mic input to voice
over comment.
XMIT/REC switch. It is in parallel with the PTL jack. The red
lamp above this switch will light whenever you are in the
transmit mode. In receive, the applied + voltage appears on
pin 4 of the power jack to power an external 70CM ATV
downconverter such as the TVC-4S.
POWER ON switch turns on the applied +12 to 14 Vdc to the
TX70-5. If the green light does not come on, check the
internal fuse and why it to blew before replacement.
OPERATING NOTES: ATV practices are somewhat different
from the other bands and modes. Since we must use
directional antennas to make up for the 26 dB higher noise
floor difference compared to NBFM due to bandwidth (15
kHz vs. 3 MHz), the probability of someone pointing their
beam at you while at the same time you at them and calling
CQ is very low. This is why many ATV contacts are initiated
by calling or listening on an area 2 meter FM simplex ATV
coordination frequency (146.43 in 434.0 areas, and 144.34
in 439.25 transmit video areas due to the 3rd harmonic
relationship).
Two meters, even for FM, has about 9 dB less path loss
than 70CM so that all possible ATVers can be received on 2
meter FM using just an omni antenna. You will find with
experience the correlation between 2 meter simplex and
70CM ATV. It is much easier for all local ATVers to monitor a
squelched 2 meter FM simplex channel than to try tuning
and swinging the 70CM beam looking for sync bars. Once
another ATVer comes up on 2 meters, you can roughly swing
the beams on each other before turning on the ATV
transmitter. Then, if the picture is better than 20% snow, the
video transmitting station can talk on the sound subcarrier,
and all those receiving him can talk back at the same time
on 2 meters (full duplex) to comment on picture content, etc.
Others listening to the 2 meter channel are often hooked
into ATV this way. You can also run full duplex audio and
video with another station on 900 or 1200 MHz bands.
It is more fun as time goes on to have many hams put
their families, other hobbies, and varied interests on the
screen. Let others know your 2 meter ATV freq. by publishing
in local radio clubs newsletters, contact your local ARRL SCM,
or pick a night and time to start an ATV net. The TX70-5 is
portable enough to give a little demo at your local radio club
or hamFest.
3
INTERNAL ADJUSTMENTS - See the individual data sheets
for the Videolynx VM-70X module, FMA5-G and TR-1b
boards.. The VM-70X is best run at 4 Watts pep output if
key down times exceed 5 minutes. There is a pot on the
module for setting this level with a 70cm RF Wattmeter.
Setting is done with no video plugged in. With video plugged
in the average power will read less, but the peak envelope
power (sync tip) will be the same as measured with no video
connected. Make sure that sufficient convection cooling
occurs by having no other objects within 3’ of the unit.
ANTENNA POLARIZATION must be the same in any area
or you could be losing up to 20 dB by being opposite.
Polarization in any area seems to be more of an emotional
rather than technical decision. If most of the ATVers come
from the weak signal or 432 SSB/DX group or using 439.25,
they will push for horizontal. The FMers or those using 434.0
will push for vertical. The main motivation is not to have to
get separate antennas for each mode of interest. Technically
there is little difference between polarization’s above 300 MHz
according to a US Army study. However, below 300 MHz
horizontal is generally better. Vertical polarization is preferred
in areas that have a repeater or want omni directional
coverage for weather radar or other public service
applications due to the fact that there are many manufacturers
of high gain vertical omnidirectional antennas for base station
as well as mobile. Horizontal omni gain takes many more
elements for the same gain as vertical and few are made
commercially. So this is a regional decision that should be
made by the local ATV community. One alternative is for
individual ATVers to use circular polarized antennas, which
works great for all modes. There are many exaggerated
claims for antenna gain and performance. When you select
yours, it should have sufficient bandwidth, and go by the
actual measured gains published from the various VHF/UHF
Conference contests rather than advertisements and
unsubstantiated articles.
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