rf - Free and Open Source Software

More documents

Recommendations

Info

, Measuring Antenna Gain John J. Schultz, W2EEY/1 40 R ossie Street Mystic, Conn. 06355 Some basic methods are described for measuring antenna gain using a reference gain antenna as well as methods that can be used whe n a reference antenna is not available. Even for those who do not plan to use the methods described, reading and understanding them will provide a better insight into the meaning of an antenna gain figure. If one likes to experiment with antennas, either building arrays or experimenti ng with new forms, a continuing problem is how to measure the gain of an antenna. Of course, the proof of any antenna will alwa ys remain in how it performs in actual opera tion. Also, gain is just a number and by itself doesn't convey any information about the overall radiation pallern(exceptto say that it is formed in some directive manner). Nonetheless, it is often handy to be able to talk about some gain fi gure for an antenna. One can estimate gain by using a new antenna in the same mounting position as an antenna of known gain and comparing many signal reports, switching back and forth between the two antennas, to obtain so me reasonably meaningful gain figure for the new antenna. The procedure can be rather tedious, however. This article describes various ways by which the gain of an antenna can be more accurately measured, whether one has an antenna of known gain available or not. Because of physical restraints and the interference produced by atmospheric noise, the described methods work best with VHF antennas. However, with care, the methods ca n be used with well elevated high-frequency antennas. Another way to check the gain capabilities of a proposed high-frequency antenna design would be to first construct a scaled VHF model of the antenna. Such a model is also very useful to study the impedance and matching conditions necessary for best antenna performance. Basic Method Fig. I illustrates the basic equipment set up which is necessary to measure antenna gain. G, POWER TRANSM ITTE D \ ...+t-t1
transmission line or the rf voltage across the line measured and the power calculated. The line itself should be operating as close to a I : I swr ratio as possible. The receiving ant crma power can be measured in essentia lly the same man ner or if the gain of the receiver is accurately known, it can act as a power indicating device. Again , the impedances between the antenna, transmission line and receiver input must be correctly matched . Still another method is possi ble if only the power output of the transmitter can be measured. The transmitting power is adjusted for some convenie nt refere nce level on the receiver (receiver ave is off). The receiver "S" meter, if it is the type that functions with the ave disabled, or an audio output meter, if the transmitter is tone modulated, can be used. The t ransmitter is then connec ted to t he receiver and its output level slowly increased (using an attenuat or network or by varying an operating voltage whic h controls t he output power) until the same reference level is obtained. The power level required will be the same as the received power. If o ne o perates only on a specific VHF band and wishes to construct a sort of " instant reading" gain meter, this can be done by using a dipole as the receiving antenna and placing an rf rectifier circuit and meter directly at its terminals. As long as the distance between the transmitting antenna and t he receiving reference antenna is kept constant and as long as the input power to the transmitting antenna (of unknown gain) is always the same, the meter can be calibra ted directly in tcrms ofantenna gain. It is only necessary to use several antennas at the transmitting end of known gain first in order to establish the calibra tion of the receiving antenna "gain" meter. Such a device can be a great dcal of fun and use during competit ions at field days, etc. for the best antenna designs. Aside from the distance and power co nsiderations mentioned , however, the o nly requirement for the FD " wo nder" antennas tested is that they be capable of producing nea r unity swr in the transmission line to the transmitter. Unless this condition is met, the "gain" meter readings will not be valid in either an absolute or comparative sense . The basis for the calibration of such a meter should become clearer from the following test situations. Gain Using a Standard Reference Antenna If one has co nst ructed an antenna of known directivit y gain and wishes to det ermine the gain of an untried antenna design, the setup of Fig. I can be used . The transmitter out put power and received power are measured and the gain is calculated from th e formula previously given. knowing the antenna separation and the operating frequency (using thc untried antenna as either the tra nsmitting or receiving antenna). For instance. if the reference antenna used is considered to have a gain of 1(0 db), the original formula can sim ply be restated as : G=(4n DrA) 2 Preceived/"transmitted Thus, if an antenna were tried on 2 meters at a distance of I0 me ters and the power received were 1fI 0 of a watt for a 10 watt transm itter out put, the gain would be: G = (4n 10/ 2)2 1/10 = 36 = 15.5 db 1/10 This gain is in reference to the gain of the reference antenna (a ~ A dipole, for instance). In practice, however when one can move the antennas under test about easily, a much more simplified procedure is possible. The tra nsm itter is connected to some available antenna. At a reasonable distance away, the standard or reference antenna is co nnec ted to a recei ver. The transmitter power output and receiver gain are adjusted to produce so me convenient reference level. The transmitter power output is noted. Then, t he antenna under test is substi tuted for the reference antenna. The transmitter power output is readj usted to produce the same reference reading on the receiver. If the t est antenna required only I watt of transmitter power to produce the same receiver reference level as when 10 watts were used with the reference antenna, the gain of the test antenna is sim p ly 10 -i- 1 or 10, whic h also happens to be 10 db. Remember that t he numerical power ratio must be converted using a db power curve for db gain expression. Again, the antenna gains obtained by this method will all be referenced to t he assumed un it y gain (0 db) of the reference antenna. Gain o f Two Identical Test Antennas Suppose that o ne had two iden tical antennas and did not know the gain o f eit her nor had any reference antenna of known gain ava ilab le. Surprisingly enough, t he gain of the test antenna design can still be easily found. If the gain of both antennas in the . test set up shown in Fig. 1 is the same, the original gain formula is re-arra nged in the form: " received/Ptransmitted The received power and transmitted power can be measured with some specific antenna separation and the formula will yield the gain G=4n OrA MAY 1969 43
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 and 36: The lassie 36 With Patent Approved'
Page 37 and 38: look (electrical) at il. I took my
Page 39 and 40: ROHN, as the largest tower manufaot
Page 41: • ,~, • • • ••• 1>00"
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
Page 92 and 93:
• 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 ,
Page 98 and 99:
Kalashi Nose, KH6IJ/l J0 Cam bridge
Page 100 and 101:
Getting Your Extra Class License Pa
Page 102 and 103:
- stage o f the game, it's a major
Page 104 and 105:
e-radiation produced by the molecul
Page 106 and 107:
and direction usually undergo chang
Page 108 and 109:
Just to complicate things, most aur
Page 110 and 111:
----- ---- o F[EO------- - - ---- -
Page 112 and 113:
corner reflector is sometimes incor
Page 114 and 115:
known that almost any spaci ng (wit
Page 116 and 117:
NEW BOOKS Electronics for Technicia
Page 118 and 119:
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
Page 134 and 135:
Also, I thought that the ta x-exemp
Page 136 and 137:
--r..ta U 8H NEW G&G CATALOG! MILIT
Page 138 and 139:
we would retain responsibility for
Page 140 and 141:
LARGEST SELECTION In United State.
Page 142 and 143:
HAMFEST sponsored by Lancaster & Fa
Page 144 and 145:
JOHN MESHNA JR. 19 ALLERTON. ST., L
Page 146 and 147:
LIBERTY PAYS MORE! LIBERTY OFFERS M
Page 148:
NEW DRAKE MODEL • TR·B 6-M SIDEB
show all

rf - Free and Open Source Software

Create successful ePaper yourself

Delete template?

Save as template?