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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 T . ---- .,, ,~ • -"' ---,, ,, , "' 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 .- , I ~Il' L cr ,00..' J Fig. 1. Schematic of the Heat hk i~ " Cantenna." "' '00' . T""" ,.--- --., '" , "" ~ll , , 'OIL , : I l ..J . ... "' ," " ' ~·10 .' cs 1 °'''' 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 ec OVT r ,0',,' cr 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. r- "" •• TT"UI. OII .~ ~ ~ ." r- ee 00< " <;0& " Ta T. US"' TT( . ~ ~ © ~ ~ / . ....UI. .'" '''M, ~ ~~ ~~A "'Dl(&fI"G " [T( . ! -' Ioi )-J /' 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 . 1 © o DC OUT OC~ . " . lL 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- The personal Antenna ...Custom created with Hustler components. Make your c hoice of H ustle r c omponents. A luminum and c hrome plated brass mast w ith bumper mounting or st ainle ss stee l 180° adj ustable ball fender/ deck mou nt. Standard 500 wan PEP resonators or S uper 2 KW PEP re so nators wIth large '" H i-O" com put er designed i nducto r and stainle ss steel t ip roo. O pti on al Quic k disconnect sta inless steel reson ator spring . Quality manuf ac tured 50 0hm coax . You want top performanc e . That' s why you sho uld customize with the ... MO/ RM series antennas and compo ne nts from H ust le r. the name recognized f or perso nal qu ality in ante nnas! RSS-2 $5 9 5 • mgs. 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 .• ~ DEALERS De laware Amate ur Supply 71 Meadow Road New Castle. DE 19720 302-32 8-7728 Evans Radio , Inc . Ro ute 3-A Bow J unct io n Concord . N H 03301 603-224-9961 v o-a $2 2 .95 RM -40 S $16 .95 RM-20 S (l#~BB)',: :5 Table 2. Amateur Elect ro nic Supply 621 Commonwealth Ave . Orlando, Fl 32803 305-894-3238 • 0 0 -1 $ 16 .95 Indicating Meter, Watts 10 20 30 40 50 60 70 80 90 100 11 0 120 130 140 150 160 170 180 190 200 RM- 75 S $31 .9 5 ~ H" 3275 Nort h B. Ave. • Kissimmee, Florida 327 41 Clearly the choice o f those w ho know qual ity. MO. ' $22.95 Sunrise Amate ur Radio 13 51 State Roa d 84 Ft. Lauderdale . Fl 3331 5 305-761-7676 Un ion Electro nic Distributors 1601 2 So uth Cottage Grove South H ollan d, Il 60473 312-723-1500 Western Rad io 1415 Ind ia St. San Dieg o . 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