More about Antennas - VU3NSH Pattern part 2.pdf · MORE ABOUT ANTENNAS By N.S.Harisankar VU3NSH, ... feeder. If the VSWR of a system shows 3:1, ... you can check these problems of

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MORE ABOUT ANTENNASBy N.S.Harisankar VU3NSH, Phone : 0491-2576102

Sankar Nilayam, Ambikapuram, Palakkad - 678 011, Kerala, INDIA

The purpose of antenna is to radiate the signal or to receive the signal as effectively

as possible. It is a passive component of radio broadcasting, navigation and other systems.

Physically, an antenna is an assemblage of conductors. A receiving antenna is like an

electric analog of the eye capable of “Radio Vision”. For antenna, it is important to maintain

a high standard of efficiency. In radio communication, we deal with electromagnetic waves

travelling through the earth’s atmosphere. Some knowledge of wave characteristics and

how the radio waves are influenced by propagation conditions are worthwhile. Such

knowledge is an asset to successful DXing on HF / VHF. (It is important to know the time

when conditions are good, and an antenna having top performance). In the last issue, it

was explained about the vertical antennas. Ref. “Angle of Radiation and You” by

N.S.Harisankar, VU3NSH. http://www.hamradioindia.org/circuits/radiation_pattern.php

An antenna will always have some “Ohmic Resistance”, which is the first element.

In ohmic resistance, power is dissipated as heat and is lost, like in a resistor. The radiation

resistance is the second element, which determines what portion of TX Power is coupled

to space and radiated. The third element is called “reactance”, in which there are two

types (a) capacitive [notation (-J)] , (b) inductive [notation (+J)] (See Figure A). A perfect

antenna is a 50 ohms resistive load to the transmitter, which gives an excellent

performance. Radiation resistance is defined as the ratio of power radiated by the antenna

to the square of the current at the feed point. Radiation resistance is that value of resistance,

if replaced, the radiation mechanism of antenna would dissipate exactly the same amount

of power that the antenna radiated. In Fig. A-1, typical radiation patterns for a 2-element

beam are shown, and its horizontal field over 3600 and its vertical angle of radiation. It is

necessary to be able to physically rotate antennas in order to get maximum power in a

required direction. In many amateur systems the requirement is for a highly directive pattern

to achieve increased “gain and interference reduction”. However, for such systems like

repeaters and mobile communication, it is often necessary to radiate energy in an omni

directional pattern, in order to provide a broadcast type of coverage.

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With amateur antennas, which normally have radiation patterns of a simple shape,

the important characteristics of the patterns can be specified by the beam width and side

lobe level in the two principal planes, usually taken as the V (vertical) plane and the H

(Horizontal) plane. The beam width in a principal plane of the radiation pattern is defined

by the pattern’s angular width at a level which is 3dB down from the beam maximum. Refer

Fig. A2. This is also known as the “half-power beam width”. Side lobes (radiation in any

direction other than that required) and the front to back ratio (F/B ratio) with directional

beam antennas (the ratio of the energy radiated in the required direction to that radiated

in the diametrically opposite direction - 1800, Ref. Fig. A2) are both expressed as the

number of decibels (dB) down with respect to the maximum radiation of the main beam.

The directional antennas have directivity gain. This useful degree of gain (Fig. A3) is very

much useful for DXing with minimum TX power.

The gain of an antenna is usually expressed as a power ratio, either as a multiple of

so many “times”, or in dB units. l / 2 dipole ( a l / 2 wave long antenna is called Hertz

Antenna, the l / 4 is called Marconi Antenna) is the hypothetical isotropic radiator is 1.64

times or 2.15 dB power gain, it is generally accepted as a basis of comparison. For

example, a 13dB antenna’s multiplier is 19.95 times say 20, in such antenna, the input

power of 50W, the effective radiated power should be 20 x 50 W = 1000 W ERP !!!. In the

case of a l / 2 dipole with 50W input, it is 1.64 x 50 = 82 W !! In two types of vertical

antennas with same height and same TX power, illustrated in Fig. A5, low gain antenna

works as an alligator with high power TX, big mouth, no ears !!!. High gain antennas are

needed for good communication.

Consider an antenna after proper matching which shows a VSWR of 2:1 (Sometimes

written as 2/1 or, 2 to 1). Then, in 50 ohms transceiver, the load (antenna) impedance on

that frequency is 100 ohms or 25 ohms; or if the antenna shows 3:1 VSWR, i.e., the

impedance are 150 ohms or 16.5 ohms. This is a mismatch level. The 50 ohms impedance

are convenient and easy to use, input and output of a device matched to 50 ohms allows

an easy transfer of other replacement units. The reflection product is the power “bounced

back” from the load due to mismatched impedances at the load (antenna) or from the

feeder. If the VSWR of a system shows 3:1, then the matching efficiency is 75%, so the

loss is 25%, i.e., for a 100W, TX power fed to such a system, 25W is the reflected power.

For 1000W, the reflected power from the system is 250 W!. The cable will melt !!! .......

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The PA stage of your TX will be damaged. For this kind of system, if it receives a signal,

the above mentioned loss is also applicable. So a weak signal can’t be received properly

(reciprocal characteristic of the antenna). In the case of 50 ohms TX system a 75 ohms

cable, itself will have a VSWR of 1.5:1. In this 75 ohms cable, if a 3:1 load is connected,

then the total VSWR is 4.5:1 and also cable’s own loss as per manufacture’s data according

to the frequency / length. The total loss in the system will be including mismatch cable,

antenna mismatch, cable attenuation and inferior connector loss.

In some conditions, highly mismatched cables show low VSWR. Due to reflected

power from the load to the VSWR meter, absorbed (self loss) or attenuated by the cable

itself. It can be identified by testing the system by the next TX high power. So this will show

the reflective level. This means testing with a low power cannot be accurate. Test should

be done with different TX power levels. The TX power from the rig to the antenna should be

efficiently transferred as much as possible by the feed line without radiating. If it radiates,

it is no longer a feeder line (feeder cable), it becomes a part of the antenna. Due to feed

- line radiation, any flow of current on the outside of coax feed line is known as “parallel

standing waves”. This current can also flow into VSWR meter, resulting in false readings

combined with the normal VSWR readings. Placing the VSWR meter at different places

in the feed lines, you can check these problems of cable radiations or by using a field

strength meter.

A cable can be tested for confirming its characteristics, by connecting a 50 ohms

dummy load instead of antenna, and a VSWR / power meter in between TX and dummy

load. If it shows 1:1 VSWR, then the cable is OK, if it sows 1.5:1, then the cable is a 75

ohms type. The loss of such cable is 4%. Cable attenuation at operating frequency / length

is also added with the 4% loss. It is essential to check whether the full amount of power

reaches the antenna from the transmitter and that equally full level of received signal is

being transferred from antenna to receiver. Cable into which moisture has seeped in, the

outer conductor becomes tarnished due to oxide formation and each lead acts as individual

conductor. This will change its own impedance level. The physical length of an antenna

should be checked for resonance, it means they must be electrically tuned to the operating

frequency. Radio frequency current travels slower in metal conductors than in a free space.

Its electrical length is less than the length in free space.

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The feeder cable length is critical. If a shorted line is less than a quarter-wave long,

it will have “inductive” reactance. If it is beyond that, it is a “capacitive”. So odd multiple of

a quarter wave length of the operating frequency is the matched line. See Table 1. If the

reflected waves arrive from ground or from some point, the result could be a strong signal

(due to the wave arriving in phase); if a weak signals received, it is due to the waves

arriving out of phase (because in phase, signal add-up and out-of -phase signals cancel

each other or distorted by delayed reflected signal). In practice, VHF / UHF wave

propagation is more seriously affected by shadowing due to high buildings, heavily wooded

ground, hills, or by diffraction and absorption losses. To avoid this kind of problems, while

using a beam antenna system, the beam direction can be changed. See Fig. A4 and A6.

Toroidal radiation pattern (Omni) is used for radio communication, radio navigation

and radio broadcasting. Pencil beam radiation pattern is mostly used for radar, long

range communications, etc. Fan beam patterns are also used for radar and communication

radio systems. The cosecant beam patterns are preferable in air-borne ground surveillance

and ground-based air surveillance radars. It is established that an antenna system according

to the use should be chosen.

I thank Dr. T.K.Mani VU2ITI (Principal, College of Engineering, Cherthala), VU2DX,

R.S.RAMA RAO VU2RID (IBS) for the proof reading and advice, and SWL Rejeesh for

making the illustration and Sajeesh for the data processing for my article.

Reference :

1. Micro wave Circuits and Antennas - MIR - Russia, ISBN-5-03-001411-X1990-By D.M.SAZNOV

2. ARRL Antenna Hand Book - 18th Edition - US, ISBN-0-87259-613-3

3. VHF UHF Manual - RSGB - UK, ISBN-0-900612-63-0-1997

4. Radio Measurements - MIR - Russia, 1978 - By F.KUSHNIR

5. Practical Wireless - August 1980 - UK, Power Gain from TX Aerials - By G2BCX

6. How to Make Walkie Talkies - BP 43 - BABANI - UK

7. All About VHF Amateur Radio - W6 SAI - RAC - US, ISBN-0-8230-8705-0

8. Lew Mc Coy on Antennas - CQ - US, ISBN-0-943016-08-8

9. Two Meter Antenna Hand Book - FC Judd - G2BCX - UK, ISBN-0-408-00402-9

10. 73 Amateur Radio Today - 1995 - US, Above and Beyond - By WB6IGP

11. Out of Thin Air - PW - UK, IPC - Magazines Ltd.

12. Vertical Antenna Classics - ARRL - US, ISBN-0-87259-521-8

13. Electricity and Magnetism - MIR - Russia, 1986 - By A.N.MATVEEV

14. ARRL Hand Book - 1968, 1978, 1989 - ARRL - US

15. Networks Lines and Fields - G.S.Bijoor - Hi-Tech Series - Subghas Publications, BLR2

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