MFJ MFJ-704 User Manual

MFJ-704 LP FILTER INSTRUCTIONS

MFJ-704 In st ructi on Manu al

Thank you for purcha sing the MFJ-704 HF Low Pass

Filter. This low pass filter is designed to accept up to

1.5 K W of power a t a frequency of 1.8 to 30 MHz (with 52 ohm inp u t
and looking into 52 ohms non-reactive impedance).

The purpose of the Low Pass Fi lter is to reduce possible
interference from your transmitter to television or
other services above 30 MHz. This 9-pole Chebyshev filter

has a low insertion loss (not more than .5 dB from 1.8 to
30 Mhz.) and has high attenuation above 30 Mhz.

INSTALLATION

1. Connect the output of your transmitter to the input of
the MFJ-704 Filter. Use as short a cable as
possible between the transmitter and filter. Use 52
ohm coax such as RG-8 or RG-58 depending on the
power level being transmitted. D O NOT USE ANYTH ING
BUT 50 or 52 OHM COAX.

2. Connect the outpu t of the MFJ-704 Low Pass Filter to
a 52 ohm load. This can be a linear amplifier, an
antenna tuner, or a well match ed antenna (52 ohms
with no reactance via coax). See Figures 1, 2, a n d 3
on the following page for vari ous hook-up diagrams.

NOTE: The input and output impedances should be 50 or 52

ohms with no reactance. If the output of the filter
is looking into any reactance, the power handling
capability of the filter is reduced. This is due to
the high reactive voltages which can develop.

NOTE: Care should be taken when constructing RF cables.
CAUTION: Do not operate the filter with a high

SWR.

Cold solder joints can actually cause rectification
harmonics which will defeat the purpose of the
filter. Take steps to insure good solder connections
at all RF connectors.

The filter should be well grounded. If you have a good RF
connection to the transmitter, and the transmitter is well
grounded, that is sufficient.

3. Mount the filter in a convenient location. There are
four mounting holes p rov ided f or mount ing if desir ed or
the MFJ-704 can be placed on a desk behind the
equipment.

Page 1

MFJ-704 LP FILTER INSTRUCT IONS

Figure 1 - MFJ-704 with XMTR and Antenna only.

Figure 3 - MFJ-704 with XMTR, Linear, and Antenna Tuner Page 2