Need tutorial for VSWR meter

[royalkhan89]

Hi

I need to know about the about dB, Gain and fine Knobs of VSWR meter. How and When the are use. In Lab we set gain and fine knob at mid position and dB knob at 50 dB but i do not have any idea why. waiting for a positive response.

Thank You.

 

[E Kafeman]

A basic tutorial about VSWR meters:
https://www.circuitstoday.com/working-of-standing-wave-ratio-swr-meters
Actual meter in above article is using two meters in one housing. VSWR value is read where the indicators are crossed.
That way is it no need to calibrate for actual TX power level at a separate knob.
It is also possible to use just one meter, and then with a knob for calibration of TX power level.
A simple design with knob can be seen here: **broken link removed**

 

[chuckey]

To measure VSWR, you need two RF probes, one detects forward power, the other reverse. So some way you have to calibrate it so for a constant forward power, the level of the reverse power relates to the VSWR. There are as many ways of doing this as there are equipment manufacturers. The simplest is the "Bird Throughline", which is a power meter with a reversable power pick up probe. one way it reads forward power, reverse the plugin pick up and it reads reverse power. You then have to calculate the VSWR. No doubt there will be some one who makes one with a micro in it so it so it is entirely automatic, giving a direct readout!
Frank

 

[royalkhan89]

Thanks.
I have Tesca Technologies VSWR meter that directly calculates VSWR (loss in dB as well). I am curious to find out about its Range(dB) knob. Do you have any idea why these knobs are there for? we usually set Range dB knob at 50dB position, but i do not have any idea why

 

[chuckey]

VSWR is normally expressed as something like 1:1.06, which is the ratio of the Vmin to Vmax on a line. From memory this, equates to a reflected power of 3%, which can be expressed in dB.
When you twiddle the 50 dB knob, what happens? I looked at the TESCA website for details, the site is absolutely useless!
Frank

 

[royalkhan89]

Ya I know, I have manual and even i don't have any idea but if we move the prob to 60dB it Usually say over range

 

[chuckey]

60 dB = 1000 : 1 or .001, so the reflected wave is greater then .001 times the forward wave, seems too low to me. 50 dB = .003.
Frank

 

[royalkhan89]

Wow! man thank you.

- - - Updated - - -

Dear i was performing the Lab with slotted line in which i have to measure wavelength but i have lots of difficulties in that i.e. Observe and calculated frequencies are not same (minor difference can be neglected but difference was about 1 GHz ). Other problem is that when VSWR is very close to 1 VSWR meter shows "Over range" after some time. I am sending you a copy of the Lab task i am performing.

- - - Updated - - -

(4) Set the components and equipmentsas shown in block diagram.
(5) Initially set the variable attenuator for maximum position.
(6) Keep the control knobs of Klystron Power Supply as below:
Meter Switch - ‘OFF’ Mod Switch - AM
Beam voltage knob - Fully anti-clockwise
Reflector voltage - Fully clockwise AM- amplitude knob - Around fully clockwise AM- frequency knob - Around mid position.
(4) Keep the control knob of VSWR meter as below: Meter Switch - Normal Input Switch - Low Impedance
Range db Switch - 50 db
Gain Control knob - Mid position
(5) ‘ON’ the Klystron Power Supply, VSWR meter and Cooling Fan
(6) Turn the meter switch of power supply to beam voltage position and set
beam voltage at 300V with the help of beam voltage knob.
(7) Adjust the reflector voltage to get some deflection in VSWR meter.
(8) Maximize the deflection with AM amplitude and frequency control knob
of power supply.
(9) Tune the plunger, reflector voltage, and probe for maximum deflection in
VSWR meter.
(10) Tune the frequency meter knob to get the ‘dip’ on the VSWR scale and
note down the frequency directly from frequency meter.
(11) Replace the termination with movable short, and detune the frequency
meter.
(12) Move probe along with the slotted line, the deflection in VSWR meter
will vary. Move the probe toa minimum deflection position, to get
accurate reading, it is necessary to increase the VSWR meter range db
switch to higher position. Note and record the probe position
(13) Move the probe to next minimum position and record the probe position
again.
(14) Calculate the guide length wave as twice the distance between
successive minimum positions obtained as above.
(15) Measure the wave guide inner broad dimension ‘a’ which will be
around 22.86 mm for X- band
(16) Calculate the frequency by following equation.
f = C / λ= C √1/ λg
2
+ 1/ λc
2
where C = 3 X10
8
m/s i.e velocity of light.
(17) Verify with frequency obtained by frequency meter.
(18) Above experiment can be verified at different frequencies.
http://obrazki.elektroda.pl/7071013800_1397115172.jpg
http://obrazki.elektroda.pl/8724748100_1397115173.png