E.F. Johnson Viking 275W Matchbox (250-23)
EF Johnson Viking 275 W Matchbox Antenna Tuner
The "Matchbox" is Left of center with AM-2 SWR Bridge on top
EF Johnson Viking Matchbox Antenna Tuner (Model 250-23)
- GENERAL DESCRIPTION (From EF Johnson Instructions 2/58)
- The Johnson Matchbox is an antenna coupler designed for matching
antenna and transmission lin impedances to a 52 ohm impedance
within the amateur bands from 3.5 to 30 megacycles. The Matchbox
also provides band switching and transfer of the coupler from
transmitter to receiver and in addition mutes the receiver and
grounds the receiver antenna terminals during transmission.
- The Matchox is capable of matching the 52 ohm coaxial output of a
transmitter into loads ranging from approxiamately 50 ohms to 1200
ohms for balanced lines, and 50 to 2000 ohms for unbalanced lines.
Large amounts of reactance can be compensated for by the coupler,
the amount depending upon the frequency, line or antenna resistance.
The Matchbox, or any coupler, installed at the transmitter end of
the transmission line cannot corrrect the standing way on the
transmission line. This standing wave is a matter of impedance
match between the antenna and the transmission line and may be
eliminated in various ways described in antenna and transmission
line literature.
- The coupler circuit consists of a parallel resonant network utilizing
an inductor with a coaxialwound coupling link, a dual tuning capacitor,
and a dual differential matching capacitor as shown in Figure 1.
Impeadance matching for either balanced or unbalanced transmission
lines and antennas is accomplished by the front panel controls of the
dual capacitors without resorting to the customary coil taps or link
adjustments. Adjustment of the Matchbox must be accomplished by the use
of a directional coupler, such as Johnson PN 250-37.
- The matchbox tuned circuit provides additional harmonic attenuation of
the transmitter output by at least 15db. The 52 ohm coaxial line
between the transmitter and coupler provides for the insertion of a
Low Pass (TVI) Filter such as the Johnson 250-20 for improved harmonic
attentuation. A terminal is provided on the terminal board for the
installation of an R.F. probe for use with an oscilloscope, modulation
monitor.
- The Kilowatt Matchboxes (PN 250-30 & 250-30-3) have a switching relay
system designed for "fast make-slowbreak" operation allowing the final
amplifier platae voltage to be cut off before the transmitter load is
removed. The time of delay is fixed between .15 and .25 of a second
which is ample to allow the relay contacts to break without any arcing.
- INSTALLATION
- The Matchbox and its Directional Coupler must be connected to the
transmitter with a 52 ohm coaxial line as shown in Firgure 2. The
length of the coaxial line should be as shourt as practical since
the input reactance varies more rapidly with frequency as the length of
the line increases. If a low pass filter and a TR Switch are used, they
should be installed as shown in Figure 2 using the coaxial lengths as
indicated. The remaining connections to the Matchbox should be made
as follows:
- GROUND- This terminal should be connected to the ground terminal
on the transmitter with a large gauge wire. The lead from the
Transmitter toground should be a large gauge wire and as short as
possible avoiding any length near an odd multiple of quarter-wave
on the operation frequency. For multi-band operation, ground
leads of various lengths should be installed.
- RECEIVER ANTENNA- The antenna leads
- NOTE: Information about other Matchbox Models is included.
- Model 250-23-3 has a directional coupler Indicator (Meter)
installed and a Directional Coupler (PN 250-37) included.
- Industry standard 90 day warranty
-
Knobs to Match the EF Johnson "Viking" Series Equipment
NOTE: The "Matchbox" was an 80-10 meter antenna tuner. The AM-2 is a
Heathkit SWR Bridge. The Receiver Shown is a Collins 75A2. The microphone
is a Turner Desk type from the 1960s. The headphones are R-10 type from
WW2. The Speaker is a counterfeit Collins made from a National Company
speaker.
Last Updated: September 23, 1998
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