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The Antennascope W. R. Carruthers, VE3CEA 256 Alexandra Avenue Waterloo, Ontario, Canada An E • ectice Tool There are two types of antennas, commercial and amateur. A commercial antenna is generally designed for one frequency, has many acres of ground around it, no obstructions and miles of heavy copper cable buried underground to provide an "effective" ground. These antennas work as designed - very well. The amateur antenna, on the other hand, is just that - an amateur design and construction. This antenna is subject to all ills, roof tops, buildings, trees, TV masts, house electric wiring, telephone wires and what not. It's a wonder they work at all! But they can be made to work and thousands of amateurs make them work. They make them work by pruning or lengthening the feeder cable and by using an antenna coupler. These are always empirical steps, the "let's cut and try and see what happens" method. How much better it would be, and a time saver too, if we tested our antenna systems electrically and knew what was happening and then could take intelligent action to put the whole antenna system into resonance. This fact is well known - an antenna can only accept power and radiate properly when it is operating at its resonant freq uency. This is no problem for the commercial people who operate at one frequency. The amateur, however, wants to "roam the band" and may wish to operate over frequencies hundreds of thousands of cycles wide, even megacycles wide. How can he do this with a fixed antenna system? The answer is, he can't! But he can construct an antenna system for a certain frequency and take the penalty of reduced radiation when he moves far away from it. However this actually works very well, because each amateur has his own particular part of a band in which he likes to operate - and his. friends tend to stay there too. On this particular spot, the amateur works diligently to "put out a good signal" The question arises - how can we make sure our antenna system is radiating well at the particulat frequency we wish to use? One answer is to use electrical test equip- ment to show us what is happening on the whole antenna system, which includes the antenna and the feed line. One of the most useful devices for this purpose is the rf bridge, generally called the Antennascope. Basic circuitry and values were described by WA ICCH in the January 1968 issue of 73 Magazine, page 21 A. It is a simple device, inexpensive to construct and very effective in results. It is usually powered by a grid dip oscillator. Such bridges should be used at the junction of the feed line and the antenna and will show the resonant frequency of the antenna itself and the radiation resistance at the feed point. Making such measurement up in the air is a difficult thing for the average amateur and impossible for those whose antennas are supported at the ends. If we are willing, however, to accept a small degradation in results, we can use the rf bridge at the station end if we have a half wave, or multiple of a half wave, feed cable. At every half wave point on a feeder cable the voltage and current vectors are in phase, which simply means that t he electrical condition seen at the end of the cable is repeated every half wavelength in the cable. We can use the rf bridge then, at the station end of the feed line, if we are willing to agree that the results will not be 100% but reasonably close to it. The results will be affected by all the various factors that affect amateur antenna resonance and these effects may give us some peculiar results, but they can be overcome and the final results may be quite valuable to us. Let me give you an example to illustrate what I'm talking about and to show you how effective the use of the rf bridge can be: - A friend of mine constructed a 40 meter inverted V antenna, held at the feed point 40' up on his beam tower, 66' legs down to supports which held the ends about 8' off the ground. Feed line was 100' of Twin Amphenol cable, velocity factor .68. The antenna was difficult to feed, swr was high, radiation was poor. He asked me to have a 36 73 MAGAZINE
look (electrical) at il. I took my grid dip meter, rf bridge and vlvm. The first thing done was to check the feed line length. 1/2 wave length at 7.1 MHz was 492x .68/7.1 or 47.1 feet. Two 1/2 wave lengths (to get into the station) would be 94.2 feel. The first conclusion was that the feed line was 5.8 feet too long. Next Test No. I was made using the rf bridge with results as shown in Fig. 1, the results being shown in table form and also plotted in graphical form. , .e "OHMS ". '0 .. r., 10 '"' '-' "0 '" ,,"" '00 •• " • 0 so ........ " " --- " ee , , , , / V 1.1 ., I •• ' .0 .r r .e •• •• Fig. 1. 100' Feedline Test No.1 . It was obvious from this graph that the antenna system was resonating outside the band as shown by the dotted lines. This test was repeated and the results were taken down to 6.4 MHz. They showed the system to be resonant at 6.6 MHz. Test No.2 was made next using the feed line cut to 94.2 feet. Fig. 2 shows the results. , . e ' .0 1.0' " t .a '-' II OH"'S ".. 41 ". 000 ·• " •o so / V V II ,oc ' .0 ,., '"' '-' " Ol1MS 4' 41 " " Fig. 3 . • • ~ 000 " ec • 2 , .e 91 .2' Feedline Test NO.3. The resonant point was rising, but not far enough yel. Test No.4 was made using the feed line cut to 88.2 feet long. Fig. 4 shows the results. It was obvious that we were very , .e ' .0 7.05 ,., t.e r.a " 011..5 4' ., ".. " 000 " " ' .0 ' .0 7 .r , .e .2 ~V Fig. 4. 88.2' Feedline Test No.4. close to the resonant frequency of 7.1 MHz which my friend wished to use. Test No.5 was with 85.2 feet in the feed line. Fig. 5 shows the results. , ec r c "t.e '"' ROW""S " " ec .. 000 " •• & 50 ee ., .2 , •• •• BAND I , •
Page 1 and 2: WORLD'S LARGEST INDEPENDENT HAM MAG
Page 3 and 4: MAGAZINE 73 Magazine 11'104 , May 1
Page 5 and 6: .. .. .... .", , , .. ...... ."'"".
Page 7 and 8: Here are several hams who have achi
Page 9 and 10: New 500-Watt 5-Bander from NRCI You
Page 11 and 12: , I ....- , 0 0 ====j .- .'Q ' 0, .
Page 13 and 14: ohm coaxial cable off ce nte r will
Page 15 and 16: Now your transmitter can work . •
Page 17 and 18: e AC and DC SUPPLI ES BU ILT IN! e5
Page 19 and 20: If you're thinking about boning up
Page 21 and 22: y t hose whom I thought were of hig
Page 23 and 24: (Sometimes, when I make like an SWL
Page 25 and 26: -e,-,"' '.-- j - - , . .... .' 1 ,.
Page 27 and 28: preselectors, it is a fine investme
Page 29 and 30: WHAT IS THE BEST ". CO'. TO SET ,,'
Page 31 and 32: I ,~ - Gel more receiver than you p
Page 33 and 34: the boom, shorting all loops so tha
Page 35: The lassie 36 With Patent Approved'
Page 39 and 40: ROHN, as the largest tower manufaot
Page 41 and 42: • ,~, • • • ••• 1>00"
Page 43 and 44: transmission line or the rf voltage
Page 45 and 46: sumed. A signal generator of suffic
Page 47 and 48: may not have enough rf available to
Page 49 and 50: cuitry developed through the earlie
Page 51 and 52: The Super SS Clifford Klinert 520 D
Page 53 and 54: present one, a boornless quad, was
Page 55 and 56: HOWARD W. SAMS Famous Editors & Eng
Page 57 and 58: declared that they " are and have s
Page 59 and 60: and the state regulating agencies.
Page 61 and 62: '0' '" '0. Fig. 2. T he direct r ea
Page 63 and 64: 2 A 7/ I 1/2 ). ----~I/~--- 2 1/ 8
Page 65 and 66: 0000 O' A"'( TE ll " • "'''''' 10
Page 67 and 68: Add This Simple Decimal Readout to
Page 69 and 70: THE STELLAR - CASE STATION IS NOW A
Page 71 and 72: \ 1/ r------- - TO l(lollTlil " P I
Page 73 and 74: Step 4: In actual practice, VBE wil
Page 75 and 76: F ig. 1A (II SPLIC E C E ~ T E R CO
Page 77 and 78: How to Fly YOur Kite M. B. Crowley,
Page 79 and 80: Boulder Antenna 18 28 38 48 58 Tabl
Page 81 and 82: MH z. and consequently less paths a
Page 83: 30 a ~ ~ , I , I ~ V ' - 0 z I .' <
Page 86 and 87:
0 2 1!i0 'O!iV 8+ F ig. 2 . GAG7 Am
Page 88 and 89:
I Telephone Beeper The following is
Page 90 and 91:
the 20's and 30's the phone transmi
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• A Kilowatt Dummy Antenna •
Page 94 and 95:
Mini-Bomb • Bill Brown 11".pSYK 2
Page 96 and 97:
• DX'ing rom Joseph D. Burn ett ,
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Kalashi Nose, KH6IJ/l J0 Cam bridge
Page 100 and 101:
Getting Your Extra Class License Pa
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- stage o f the game, it's a major
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e-radiation produced by the molecul
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and direction usually undergo chang
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Just to complicate things, most aur
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----- ---- o F[EO------- - - ---- -
Page 112 and 113:
corner reflector is sometimes incor
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known that almost any spaci ng (wit
Page 116 and 117:
NEW BOOKS Electronics for Technicia
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Economy Ed Baker, W0EDO 993 7 Truma
Page 120 and 121:
4 x 150 Sockets Larry Jack, WA3AQS
Page 122 and 123:
Alan Shawsmith. V K 4SS 35 Whynol S
Page 124 and 125:
Handbook is easy to use and accurat
Page 126 and 127:
TEST E UIPMENT 'IIi\IATUHE TEST L\U
Page 128 and 129:
SQUEEZE ACTION TOOL A MUST FOR THE
Page 130 and 131:
Adapting A Mobile Antenna System or
Page 132 and 133:
LETTERS This is in response to your
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Also, I thought that the ta x-exemp
Page 136 and 137:
--r..ta U 8H NEW G&G CATALOG! MILIT
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we would retain responsibility for
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LARGEST SELECTION In United State.
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