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EXTERNAL AMPLIFIER SET UP. There are amplifier application
notes on our web site. Basically, they all setup in the same way.
1. With no video source plugged in, turn the pedestal pot in the
ATV exciter/modulator to its maximum power position and RF OUT
pot to minimum. Make sure the amp is connected to a good 50 ohm
dummy load or low vswr antenna (less than 10% reflected).
2. Turn on the amp and transmitter and slowly increase the RF
OUT pot to no more than 90% of the max rated power of the
particular amp - you need that 10% headroom for the sound that
rides on the sync. Never detune the trimmer caps as it can distort
the video or make the transmitter unstable.
3. Then set the blanking pedestal pot for 50 to 60% of the max
pedestal pot power. For example: Downeast Microwave 7025PA.
Pedestal pot at full CCW set the RF OUT pot to 35 W x .9 = 33
Watts pep on the sync tip. Then set the pedestal pot .57 x 33 Watts
= 19 Watts. This sets the final video to sync ratio.
4. Plug the camera back in and have a distant station on two meters
talk your video gain in to just before white clipping or smearing.
That's it. Your peak envelope power on sync tip will be the 33 Watts
regardless of what the power meter reads.
DX vs. POWER vs. ANTENNA GAIN. The 100 mW output of the
TX70-.1s was chosen to provide an easy low cost entry into the
world of ATV, but at the same time give flexibility to all the applica-
tions that hams might put the equipment to. 100 mW connected to
OAL 5L-70cm beams for short distance video up to 5 miles with low
battery drain public service applications is ideal. But for greater
distance or areas of high path attenuation, it's output is matched
for the best linearity drive region of Downeast Microwave 7025PA
amp. The primary design difference between these amplifiers and
others is the addition of various values of capacitors on the transis-
tor bias and collector supply lines to keep the applied voltage con-
stant under the high current swings to 5 MHz of the AM video enve-
lope. Without these caps, the color and sync become distorted.
While it is almost impossible to predict actual ATV DX due
to different terrain and conditions, the line of sight snow free picture
distance can be calculated given all the controllable factors. We
must know the transmitter peak envelope power (p.e.p. - sync tip),
coax loss, and antenna gain over a dipole. At the receive end, we
must also know the system noise figure and bandwidth. See the
ATV DX chart on our web site to find the line of sight mileage.
The purpose of the DX chart is to enable you to better
figure what is needed in your system to have the best chance of
getting good pictures where you want them. This is especially
important to repeater owners or those setting up for a public service
event to figure the expected area of coverage. The DB Products
DB420 is a popular high gain broadband omni exposed dipole
vertical used at single antenna/duplexer inband repeaters - two
Diamond F718x antennas with >20 ft separation is also used.
simple starter antenna for home or portable is the ground plane
you can make yourself - see our application notes web page. For
every 6 dB of increase between two fixed points, you will gain one
P unit or double the distance to a farther line of sight point with the
same picture level.
Obviously, putting most of your time and money into the
antenna system pays off in both transmit and receive. Adding more
power does nothing to improve the receive DX. If you have one of
our TVC-4s downconverters you have a low noise figure (≈1 dB)
and sufficient gain (≈25 dB) to put your receiving system at the
noise floor.
The theoretical noise floor for a 3 MHz wide 70CM ATV
system with a perfect 0 dB noise figure is .8 microvolts (-109 dBm).
So adding another preamp at the shack will do nothing but pump
up your AGC on noise making you more susceptible to intermod
and overload interference without improving the sensitivity. Only
changing to lower loss coax or adding a good quality GaAsfet
preamp at the antenna will give you a little sensitivity improvement.
Since most cases are not line of sight, the distance will be
lessened depending on the amount and type of trees, foliage, hills,
buildings, etc., in the path. On the other hand, there is temperature
inversion ducting, especially in the summer months, or knife edge
refraction that can equal or better the chart estimates. The RF
horizon is about 10 miles for an antenna height of 50 ft - Miles = 2x
sq.root antenna height in feet. If the other station also has an antenna
height of 50 ft then you should get good results over the 20 mile
path in flat terrain. Antenna height is most important at UHF (see
The ARRL Antenna Book pages 1-4) Other sources of ATV
information can be found in the ARRL Handbook.
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.
Ground
Plane
See our app
note web
page
A
Broad band
Diamond
exposed dipole
X510NJ,A
vertical 6-9 dBd
9dBd vertical
omni used at
omni, 17 ft.
inband repeaters
with duplexer.
4
OAL 5L-70cm 8 dBd 420-
450 MHz Yagi Mounts
horzontal or vertical.
31" boom ideal for portable
& public service uses.
DSFO-ATV 25 has 16
dBd and 17 ft boom.
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