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TWO ELEMENTS VERTICAL PHASED ARRAY FOR 40 METERS BAND
Here is the description of my vertical array. The main idea is to provide phasing alteration in the travel of the RF signals. In one case, the 2 signals coming from the antennas added because they are in phase. In the other case they cancel while opposite phase.
LET'S BEGIN WITH SOME THEORY
RF TRAVEL IN FREE SPACE
Imagine a 7 MHz wave travelling in the space.
At point A, a antenna receiving the signal induce a RF current. Let's say that this current is our reference or 0°.
At point B, placed on a imaginary line source of wave / antenna A, a second antenna 10,7m away from A. The same signal that induced a 0° signal in A, travel 10,7 m more or Lambda/4 @ 7 MHz, reach antenna B for inducing a current dephased by -90°. Minus sign show the delay due to the lamba/4 delay.
90° lambda/4
180° lambda/2
270° 3 x lambda/4
360° lambda
MODIFYING THE SEPARATION BETWEEN THE ANTENNAS MODIFY THE PHASE
RF TRAVEL IN COAXIAL
As the distance in free space adds phase delay, the same occurs in coaxial cable. Due to the velocity factor of the cable, the real lenght that provides 90° is lamba/4 X VF.
Example :
At 7 MHz lambda/4 = 10,7 m
For RG58 VF = 0,66 for 7MHz lambda/4 = 10,7 m x 0,66 = 7,07m
MODIFYING THE LENGHT OF COAXIAL LINE MODIFY THE PHASE
RF TRAVEL IN A 90° HYBRID COUPLER
The device used for the array as 4 x 50 Ohms ports. The power applied to PORT IN is equally split between PORT 0° and PORT -90°. A 4th port LOAD absorb power if Z desadaptation occurs.
The property of this coupler is reversability.
Each signal entering in PORT 0° and PORT -90° are combined et selon la relation de phase apparaissent en PORT IN.
Suppose a signal of 0° on PORT -90°. It travels the coupler and appears in PORT IN with a phase of -90°. A signal of same frequency but dephased by -90° is applied on PORT 0°.
Having the same phase, the two signals are added by the coupleur.
If we revert the 2 incoming signals, they cancel due to their opposite phase relation.
REVERSING INPUT TO THE HYBRID COUPLER SUMS OR SOUSTRACTS SIGNALS
CONSTRUCTION OF THE LAMBDA/4 COAXIAL LINES
CONSTRUCTION OF THE COUPLEUR
Based on theory, I wind the required number of turns and soldered the required capacitors.
I plugged a RF generator to PORT IN and a scope on PORT 0° and PORT -90°. Verifie d'abord que les 2 sondes n'apportent pas d'alteration.
Here the result :
As you can see, same amplitude and 90° phase difference. The BIG PROBLEM is that frequency is not 7,025 MHz but 8,5 MHz.
Due to dispersion of C and L, the frequency of the coupler was not the one I like. After modification of the winding and some capacitor added, the result becomes the same at 7,025 MHz.
Hybrid coupler :
Being satisfied, I decided to know how works the coupleur :
ATTENUATION PORT IN – PORT 0° 2,994 dB
ATTENUATION PORT IN – PORT -90° 3,179 dB
Return loss PORT IN 27 dB, all others ports loaded with 50 Ohms
Return loss PORT 0° 27 dB, all others ports loaded with 50 Ohms
Return loss PORT LOAD 25 dB, all others ports loaded with 50 Ohms
I mesured between PORT IN and PORT LOAD 26 dB of attenuation. With 50 dBm on PORT IN (100W) with all ports reasonnably loaded, 24 dBm (250 mW) are present on PORT LOAD.
A 50 Ohms load with 30W power rating will be good enough.
