MFJ MFJ-8708 User Manual

MFJ-8708

High performance Mini 70cm, frequency agile
Video-Audio ATV Transmitter

I Only to be used by a licensed radio amateur I

Overview: The MFJ-8708 is a high quality 4 channel PLL synthesized mini (3.5

oz.) ATV transmitter. The MFJ-8708 is capable of transmitting on 4

different 70 cm ATV frequencies. Three of the frequencies coincide with the

cable TV channels 58, 59 and 60. An audio sub-carrier generator is

built in. All that is required is an audio source to transmit sound. No
external tuning is required since the MFJ -8708 uses a PLL synthesizer.
Up to 16 custom frequencies within the allocated 70

cm ATV band can be provided.

A internal video test signal and an audio 1 kHz tone is built in for quick
alignment of a receiver.

70cmn Mini
Video/Audio
Transmitter

Notice:
The MFJ-8708 is an Amateur Radio ATV transmitter. The transmitter can ONLY be operated by a Technician
class or higher licensed Radio Amateur in the USA and for legal purposes per 47 CFR part 97 of the FCC Rules.

97.113 of the FCC Rules prohibits Amateur Radio frequencies to be used to further any business purpose
whether profit or non-profit. With few exceptions per 97.111 all transmissions must be directed to at least one
other licensed Radio Amateur. Amateur Radio is intended for personal or hobby non-commercial
communications between licensed Radio Amateurs. With the exception of running less than I Watt for radio
control purposes, Amateurs must identify with their call letters plainly seen in the video every 10 minutes for

extended transmissions and at the end of every transmission per 97.119.

MFJ

-

8708 User Interface

Dip Switch

1

2 3 4

All other combinations of dip switch settings will turn

off

While the MFJ

-

8708

is

only provide

the 4 standard USA AM ATV frequencies. If you are a foreign licensed ham and or have different band

8708. Please note

Frequency Select Dip switches

There are 4 dip switches located on the MFJ -8708 that provide frequency selection.

0 0 1 0 427.25MHz Normal Operation (Cable CH58)

0 0

0 1 0 0 434.00MHz Normal Operation (Cable CH59)
1

0 0

1 1 1 0 Test signal on 426.25MHz

1 1 0 1 Test signal on 427.25MHz (Cable CH58)
1 0 1 1 Test signal on 433.25MHz (Cable CH59)
0 1 1 1 Test signal on 439.25MHz (Cable CH60)

0

426.25MHz Normal Operation

1

0 439.25MHz Normal Operation (Cable CH60)

the RF carrier!

capable of operating on a total of 16 different pre programmable frequencies, stock models

plans, MFJ Enterprises will be glad to provide you with a custom programmed MFJ ­that we will ONLY provide you with custom frequencies that are within allocated ATV ham bands.

Antenna out

A BNC female connector provides RF output at 50 ohms exact. Make sure that the antenna is properly
matched to 50 Ohms. You should also use the lowest loss coax and shortest length to minimize loss.

DO NOT OPERATE WITHOUT A LOAD!

DC Power In

Supply voltage is 9V. The Red power lead is Positive (+) and the Black lead is Negative (-). The

MFJ-8708 draws about 250mA of current. If you plan to use 9V batteries it is best to use two to three 9V
batteries in parallel. You may certainly use a regulated external power supply with the MFJ-8708.

Audio and Video Inputs

The MFJ -8708 accepts NTSC or PAL composite video at 1 V P-P. Audio is standard line level
and a small potentiometer provides audio input level control.

Operation and Setup

The diagram above shows the how the MFJ -8708 needs to be connected to a video and audio source.
Make sure that you have SMPTE standard video at 1 V P-P. Most consumer and commercial camera's
and camcorders deliver composite video, at the "Video Out" connector. If you are not sure if the camera
is delivering SMPTE standard video, you can confirm this by looking at the video signal on a
oscilloscope.

Audio is "Line Level." Line leve l audio is what you would get out of a CD player or VCR audio
output connector. Note: Line level is not the same as microphone level.

Video Signal

Power requirements

The MFJ -8708 requires 9V DC and draws about 250MA of current. There is internal diode reverse
polarity protection. You may use a 0.5A fuse inline to protect the MFJ -8708. If you plan to use 9V
batteries, it is recommended to use two or three batteries in parallel to extend the transmission time. Of
coarse you may use a regulated power supply or 12V battery with a power regulator that reduces the
voltage to 9V. DO NOT supply more than 9V to the MFJ -8708.

Antenna

The transmitting and receiving antenna are probably the most critical items for attaining best range. A
small 1/4 wave ground plane transmitting antenna in combination with a high gain receiving
antenna will do for short range video transmission. Best results are obtained while using both

high gain transmitting and receiving antennas.

Range testing was performed using the following antennas. The transmit antenna was a 14 element
yagi, the receiving antenna was an 8 element yagi . At an approximate distant of 2.5 miles excellent
picture quality was received. The receiver was a consumer grade Sony TV tuned to Cable CH 59. Please
remember, that while a cable ready television would suffice as a good receiver, a sensitive ATV down­converter far outperforms any cable ready television.

Using a 1 /4 wave ground plane (shown below) as a transmitting antenna and using a 14
element yagi antenna for the receiving antenna, excellent picture quality at 1 /2 mile was
received.

An experimental transmit /receive antenna you could build

Ground Plane

5L-

70cm

DSFO

-

ATV25

Some antenna performance characteristics

While some transmit -receive antennas combinations perform differently, the following chart
depicts the theoretical system performance of the MFJ -8708 using various combinations of antennas
and an ATV down converter

Receiver: ATV downconverter
Transmitter: MFJ-8708, with a power output of 50mW
Carrier/Noise: 40 - 45 dB
Picture quality: Snow Free P5
Antenna: Ground plane, 5 element 5L-70cm beam (PC electronics), 25 element DSFO ATV-25

Ground Plane
5L-70cm
DSFO-ATV25

0.25 miles

0.6 miles

1.5 miles

0.6 miles

1.3 miles

3.5 miles

1.5 miles

3.5 miles

8.0 miles

The two diagrams below show a basic and an advanced receiving setup. For most purposes a simple
cable ready TV along with a good high gain antenna would suffice. For best DX performance, a
sensitive ATV down-converter, out performs any standard TV. Remember to keep coax runs from the
antenna to the receiver/down-converter as short as possible. For longer coax runs, an antenna mast mount

LNA may be needed for optimum performance to compensate for coax line loss.

Technical Specification

Spurious emissions

: < 35dB

c

RF System

Operational Frequency range: 425.0 to 440.0 MHz (A VSB filter in -line with the antenna is required
for frequencies below 425.0 MHz with a sound sub-carrier) (Four programmable channels

provided -16 channels possible)

Frequency Control : PLL (Digital Phase locked loop) with XTAL reference
Transmit power to : 50-100mW

RF output connector : BNC female

Video

Modulation
Video Input : NTSC/PAL video 1 V P-P into 75 ohms

Audio

Audio Input (Line level) : 0.01 V to 2V p-p into 10k
Audio deviation (FM) : 25 kHz adjustable (0.01V to 2V p-p into 10k)
Audio sub-carrier frequency :4.5 MHz
Audio Input connector : RCA Female

: AM (Amplitude modulation)

DC power

Input voltage : 9V at 260mA

Mechanical

Size : 1.5"W x.75"H x 3.5"L
Weight : 3.4 oz.