Download A Better Heathkit "Cantenna"

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.
he He a thkit't " Ca nte nna " d um my load ,
Model HN·31 . consists of a
fifty-Oh m d umm y load resisto r, R1 , imm ersed in o il,
a nd a n indicating circ uit
co nsis ting of resistor s R2
a nd R3, ca paci tor C1, and
diode D1. Fig. 1 shows th e
sc he ma tic d iagram .
The indic ati ng ci rcuit
provides for connection of
a di rect cu rrent me te r to
the jack marked DC OUT .
This arrangement provides
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a means of indicating
relative power.
Wit h t he c irc uit shown
in Fig. 1, a n am oun t of
power at 3.5 MHz appli ed
to th e dummy load will
p rod uce a cert ain meter
defl e ction . If th e sa me
amount of power is applied to the dummy lo ad at
29.7 MH z, the me ter deflectio n w ill be considerably greater.
By modify ing the indicator ci rcuit to that
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Fig. 1. Schematic of the Heat hk i~ " Cantenna."
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shown in Fig. 2. the ind icating meter can be
made to read the same
va lu e for a given amount
of power whe th er it be at
3.5 MHz, 29.7 MHz, or at
any freq ue ncy betwee n
these va lues .
When this has bee n accom p lished , t he ind icat ing
meter may be ca librated in
Watts a nd will prov ide satisfactory ind ic a ti o n of
transm itter 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 circ uit
shown in Fig. 2 incorporate s a frequency-compensating netwo rk.
In my case, an indicating meter tha t wou ld read
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C2
C3
R'
R5
C'
Fig. 2. Modified schematic of the Heathkit'" " Cantenna."
12'
200 Watts full scale was
des ired .
The first operation was
to m odi fy the c ircu it
shown in Fig. 1 to that
show n in Fig. 2. Note that
in Fig. 2 t he va lue of res istor R3 has been changed
from 1000 O hm s to 2500
O hms. It wi ll 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 ~ "Ca ntenna."
.
The indicating meter
employed had a 2()().mic roam pe re fu ll -sca le moveme nt with an internal resista nce of twe lve hundred
Ohms. This meter was di-
1.5-7.0-pF g lass, piston-type variable capacitor
.01-uF disk ceramic capac it o r
1Q()().Ohm m ini at ure rnr cr o-potenucmet er, Bourns trimpot
120-14-E1000
12QO.Ohm, Y. ·Walt resistor, 10% tolerance
4.7-pF disk ceramic capac ito r
Table 1. The additional components included in Fig. 2.
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H(.THKIT · CUH .... . · -...J
Ammeter
Reading,
Amperes
.4472
.6325
.7746
.8944
1.0000
1.0955
1.1832
1.2649
1.34 16
1.4142
1.4832
1.5492
1.6125
1.6733
1.7320
1.7889
1.8439
1.8974
1.9494
2.0000
.
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DC OUT
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Fig. 3. Setu p fo r frequen c y-c ompensation ad ju stment.
rectly co nnected to the DC
O UT ja ck of the Heathki ~ dummy load .
An indicato r th at will
sho w the a mou nt of power
a pp lied to t he dummy load
is necessary fo r proper a djustme nt of the frequ e ncyco m pe ns a ti ng netw ork .
Either an ammeter o r a
wattmeter of known a cc uracy m ay be employed .
The se tu p for frequ enc y
a dj us t men t for prop er
c o m pe nsa t io n is shown in
Fig. 3.
The a dju stment pr oced u re is as fo llows :
1. Se t C2 at minimum
c a pac ity and se t R4 a t
ma ximum re sistance .
2. Se t the transmitter o n
3.5 MHz . Gra d ua lly inc re ase the power leve l u ntil the ammeter read s two
Amperes or the wattmeter
read s 200 Watts. De cre a se
t he re sistance o f R4 until
the indica ting meter re ad s
fu ll sc a le .
ing meter by increasing the
ca pac ity of C2 until the indic ating me te r reads full
sca le when e it he r the a mmeter read s two Ampere s
o r the wattmeter reads 200
W atts.
4. Repeat ste ps 2 and 3
in seq uence unti l the ind icating meter reads fuJI
sca le when the a mm eter
read s two Amperes o r the
wattmeter read s 200 Watts,
whether the a pp lie d frequency is 3.5 MH z o r 29.7
MH z.
If a wattm eter was e mp lo yed in the ad jus t ment
se tu p, the ind ic ati ng meter
may be direct ly calibrated
fr om the wattmeter read-
3 . Se t the tran smitter on
29.7 M Hz . G ra d ua lly increa se the power leve l unt il the amme ter read s two
Am peres o r the wattm eter
re ad s 200 Wa tts. Note that
the indi c a t ing meter will
read full sc a le before the
ammeter reads two Ampe re s o r the wattm eter
read s 200 Watts. Redu ce
the re ading of the mdicat-
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If a n ammeter wa s e mplo yed in the adj ustment
setup, the Watts corre sponding to the a mmeter
re ad ing are show n in Ta ble
2, and the ind ic ating meter
ma y be ca lib ra te d from
th is data .•
~
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