A Better Heathkit "Cantenna" - Nostalgic Kits Central

H. C. Sherrod W5ZG
4715 Crockett Blvd.
Gafwston TX 77550
A Better Heathkit "Cantenna"
- improved metering circuit for an old standby
Update your dummy load.
The He a thkit't " Ca n­
te nna" d ummy load,
Model HN·31 . consists of a
fifty-Ohm d ummy load resistor,
R1 , imm ersed in o il,
and an indicating circuit
consis ting of resistors R2
and R3, capacitor C1, and
diode D1. Fig. 1 shows the
schematic d iagram.
The indicating ci rcuit
provides for connection of
a direct current me te r to
the jack marked DC OUT.
This arrangement provides
"'-
a means of indicating
relative power.
Wit h t he c ircuit shown
in Fig. 1, a n am ount of
power at 3.5 MHz applied
to th e dummy load will
prod uce a certain meter
defle ction. If the sa me
amount of power is applied
to the dummy load at
29.7 MH z, the meter deflection
will be considerably
greater.
By modify ing the indicator
ci rcuit to that
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cr
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Fig. 1. Schematic of the Heat hk i~
..
" Cantenna."
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: ~ll I
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C2
" cs
C3
r
' ~·10 . '
,0',,'
R5
C'
T""
'" " 1 °''''
cr R'
shown in Fig. 2. the ind
icating meter can be
made to read the same
va lu e for a given amount
of power whether it be at
3.5 MHz, 29.7 MHz, or at
any freq ue ncy between
these values.
When this has bee n accomplished,
the ind icating
meter may be calibrated in
Watts and will prov ide satisfactory
ind ica tio n of
transmitter output power
at any frequency between
3.5 MHz and 29.7 MHz.
The basic difference between
the indicator circuits
shown in Fig, 1 and
Fig. 2 is that the circuit
shown in Fig. 2 incorporate
s a frequency-compensating
network.
In my case, an indicating
meter tha t wou ld read
200 Watts full scale was
desired.
The first operation was
to modify the c ircuit
shown in Fig. 1 to that
show n in Fig. 2. Note that
in Fig. 2 the va lue of res istor
R3 has been changed
from 1000 O hm s to 2500
O hms. It will be noted that
Fig. 2 incl udes the additiona
l components noted
in Table 1.
All of these additional
components a re installed
within the small metal box
which is attached to the pail
lid of the Heathki~ "Cantenna."
.
The indicating meter
employed had a 2()().microam
pere fu ll -scale movement
with an internal resista
nce of twelve hundred
Ohms. This meter was di-
1.5-7.0-pF glass, piston-type variable capacitor
.01-uF disk ceramic capacit or
1Q()().Ohm mini ature rnrcro-potenucmeter, Bourns trimpot
120-14-E1000
12QO.Ohm, Y. ·Walt resistor, 10% tolerance
4.7-pF disk ceramic capacitor
Fig. 2. Modified schematic of the Heathkit'" " Cantenna." Table 1. The additional components included in Fig. 2.
12'

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