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DX vs. POWER vs. ANTENNA GAIN. The >4W output of
the TX70-5s was chosen to provide an easy low cost entry
into the world of ATV, but at the same time give flexibility to
all the applications that hams might put the equipment to.
4W connected to OAL 5L-70cm beams for short distance
video up to 20 miles with low battery drain public service
applications is ideal. But for greater distance or areas of
high path attenuation, you may need to move the antenna to
find a magic spot, increase antenna gain or add an amp.
The primary design difference between ATV amplifiers and
others is the addition of various values of capacitors on the
transistor bias and collector supply lines to keep the applied
voltage constant under the high current swings to 5 MHz of
the AM video envelope. 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 possible 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 are also used.
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.
Monitoring yourself with a TV set to cable channel
58. 59 or 60 in the same room can give erroneous results
due to overload and multipath reflections. Best to have a
close by ham with an outside 70cm antenna describe your
video quality.
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 <1dB NF preamp at the antenna
will give you a little sensitivity improvement.
Since most paths are not line of sight, the distance
will be lessened depending on the density and type of trees,
foliage, hills, buildings, etc., in the way. 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
A simple
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 claims.
See our app
note web
page for
homebrew
antennas
Diamond
X510NJ,A
9dBd vertical
omni, 17 ft.
5
Ground
Plane
OAL 5L-70cm 8 dBd 420-
450 MHz Yagi Mounts
horzontal or vertical.
31" boom ideal for portable
& public service uses.
Broad band
exposed dipole
vertical 6-9 dBd
omni used at
inband repeaters
with duplexer.
DSFO-ATV 25 has 16
dBd and 17 ft boom.
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