PC Electronics tc70-20 User Manual Download Page 4

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DX vs. POWER vs. ANTENNA GAIN. The >20 Watt output of the
TC70-20 is a good practical power level for most all ATV applications.
20 Watts connected to a good beam will easily hit a local repeater if
you have line of sight between the antennas. In addition 20 watts
to an inverted ground plane on the belly of an aircraft will get snow
free pictures to an emergency operations center using a 8 dBd omni
or beam 35 miles away. But for greater distance or areas of high
path attenuation, it’s output is matched for the best linearity drive
region of the Teletec DXP-U150 (150 W) amps. The primary design
difference between this amplifier 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. The chart
below assumes the TC70-20 transceiver, TVC-4G GaAsfet
downconverter connected to a good TV set with 3 MHz IF bandwidth,
3 dB loss in coax at both ends, and snow free defined as a carrier to
noise ratio of 40 dB (about 200 microvolts).

The distances in miles are shown in the order of 20/150

Watts which is the TC70-20 by itself or driving the Teletec DXP-
U150 (150 Watt) linear amplifier. To find the possible DX under line
of sight conditions find your antenna model or equivalent gain across
the top. Then go down to the receive ends antenna or gain. Now
read the miles that corresponds to your transmit power level.

The distance miles are in the order of 20/150 Watts.

XMIT. 3 dBd

8 dBd

16 dBd gain

Antenna

Ground

F718x

FO-25

Plane

5L-70cm

beam

REC.
Gnd plane 7/27

14/52

29/115

5L-70cm

14/52

27*/104

60/231

FO-25

29/115 60/231

133/518

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. A simple starter
antenna for home or portable is the ground plane you can make
yourself - see ARRL Handbook pages 20.55 to 20.57. 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 vertical separation is also
used. If a repeater is running 20 Watts to a DB420 or F718x omni,
it could be snowfree to a station 27* miles away using a 5L-70cm
beam. The distance will double or half with each 6 dB change. For
instance if you mounted a Mirage KP-2 GaAsfet preamp at the
antenna to save the 3 dB coax loss and went to dual beams for 3
more dB gain, you would be able to see a station of the same power
and antenna at the same picture to noise ratio twice as far away, or
one P unit stronger at the same distance. 3dB more gain from dual
beams puts your transmit DX 1.4 times farther.

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. P. C. Electronics
GaAsfet downconverters (TVC-2G, TVC-4G, or TVCX-70) have a
low noise figure (

≈

1 dB) and sufficient gain (

≥

20 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 94-01 ARRL Handbook chapter 12.

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.