G8MNY TECH 27.08.19 05:45l 82 Lines 3054 Bytes #365 (0) @ WW
BID : 19079_GB7CIP
Subj: Bent Dipole with GAIN!
Path: OK0NBR IR2UBX DB0RES DB0OVN DB0GOS ON0AR GB7CIP
Sent: 190827/0844Z @:GB7CIP.#32.GBR.EURO #:19079 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

The antenna commonly used within the VHF/UHF domain generally consists of a number of straight lined single dipoles. There is little to gain with symmetrical dipoles as long as their length does not considerably exceed one wavelength. With all current elements approximately in phase, there is a distinct radiation orthogonal to the dipole axis. Antenna gain is 2.1 dBi for the halfwave & approximately 3.8 dBi for the fullwave dipole.
For dipoles longer than a wavelength the situation changes drastically for the phase reversal within the current distribution it becomes evident that radiation normal to the dipole axis will be poor. As a consequence, dipoles which are considerably longer than one wavelength are scarcely used.

If properly shaped, however, a 1.5 wavelength dipole can be a very efficient radiator, the basic is to compensate the phase differences within the current distribution by time delays. Due to a special shaping, wave originating from various sections of the antenna arrive at different times at far-off point in the direction of main radiation (parallel to the plane of symmetry), the time delays correspond to phase shifts.

For optimum shape of the antenna structure, the resulting phase shifts will approximately compensate the phase reversals within the current distribution & the field strength phasors in the far field will add almost in phase.

This is only true,however, in the direction of main radiation but will in general not hold for other directions. As a consequence, relatively high values of directivity can be expected only in the main direction.

The optimum shape of a dipole which is long 1.5 wavelength is a Gaussian shape. The Gaussian shape can be approximated by five straight lined sections

*
*
* A
*
*
*
*
*
* A
*
* B ------------------------------------ MAIN DIRECTION
*
* A
*
* A=.35 wavelength
* B=.05 wavelength
* G= 7 dBi
* F/B = 3 db
* A
*
*

Originated by another ham C.S. lost. -------------------------------------------------------------

Hi,
I made an analyses of the above dipole on 14MHz in MMANA soft.

Freq(MHz) R Ohm jX Ohm SWR(75.0) Ga(dBi) F/B(dB)
13.85 103.94 -55.93 2.01 7.45 -0.66
13.90 104.8 -41.89 1.77 7.31 -0.66
13.95 105.8 -27.92 1.59 7.16 -0.65
14.00 106.94 -13.97 1.47 7.03 -0.67
14.05 108.23 -0.06 1.44 6.88 -0.67
14.10 109.66 13.84 1.5 6.72 -0.66
14.15 111.23 27.72 1.64 6.56 -0.66
14.20 112.94 41.54 1.83 6.41 -0.68
14.25 114.79 55.39 2.05 6.25 -0.67

so, F/B only 0.7dB!

best regards, 73's de Ryszard, SP5RH
ax25 address: SP5RH@SR5DPA.WA.POL.EU
e-mail://sp5rh@qsl.net


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