how to use vswr meter

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ratbag

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my vhf is receiving ok but not transmitting,i have a altair av 40 meter but no manual & no joy with google, its a twin needle type i know how to set it up but have no idea on what readings to expect , any help peeps. ta
 
Unplug the aerial from your VHF.

Plug the aerial lead into one socket on the VSWR meter, then connect a coax patch lead from the VHF to the other socket on the meter.

Press the PTT.

One needle will measure forward power, the other needle will measure reflected power.

Where the two needles intersect, you read off the VSWR.
 
ProDave said:
Press the PTT.

One needle will measure forward power, the other needle will measure reflected power.

Where the two needles intersect, you read off the VSWR.
Click to expand...

While you are there... presumably, ideally, you would want to see 25w forward and zero reflected? What is a good result in real life and what would the corresponding VSWR reading be?

Just curious.
 
ratbag said:
thanks prodave,that makes sense now, the needles dont even cross over so as i suspected i need a new antenna,
Click to expand...

You might just have a bad connection somewhere.could be at the aerial, or at the VHF, or at a join in between, if there are any. I had to dismantle a connector and resolder it, after which the improvement was quite amazing!
 
The needles don't always cross over on twin meter VSWR meters. On some you adjust the sensitivity so that forward power reads max ie full scale deflection. The other reading then shows actual reflected power or VSWR. If it ony has one meter then you get the reflected power in the other switch position. So you get 2 meter readings if the lower one is less than 15% of the higher one you are getting to be OK. Ideal is no reflected power.
As said it may just be a bad connection in the cable. Check starting with the connector at the radio then the connectors at any deck gland or joint and finally at the antenna itself.
good luck olewill
 
Sorry, that's confused me even more, 3 answers and all vastly different:

1:1 Means 25w forward, 25w reflected?
1.5:1 Means 25w forward, 17w reflected?
15% Means 25w forward, 4w reflected?

So who's right?
 
1:1 is perfect, but impossible. Up to 1.5:1 is good, very acceptable, 2:1 is okay-ish. 3:1, getting dodgy, could some corrosion be setting in, antenna coil distorting under transmit, mismatch somewhere? Over 3:1 your radio may start giving you an "antenna" signal, meaning it's getting a bit worried about potential damage, you have a problem somewhere. 9:1 will wreck your radio, don't transmit!
 
alahol2 said:
Sorry, that's confused me even more, 3 answers and all vastly different:

1:1 Means 25w forward, 25w reflected?
1.5:1 Means 25w forward, 17w reflected?
15% Means 25w forward, 4w reflected?

So who's right?
Click to expand...

The first two of those are completely wrong.
VSWR is voltage standing wave ratio.
When you have reflections on a cable, you get the reflected wave adding and subtracting from the forward wave.
VSWR = 1+r/(1-r) where r is the reflection coefficient in linear terms
so 15% power reflected is 85% power transmitted,
15% power is 38% volts (square root)
VSWR=( 1+0.38)/(1-0.38) =1.38/0.62= 2.25:1
There are online calculators to help.
 
Thankyou Salty John for some real life interpretation of the figures.
Thankyou lw395 for the explanation (which I sort of half understand), VSWR is obviously not just the ratio of power transmitted to power reflected.
 
Here is a typical VSWR curve for a marine vhf antenna:
View attachment 34473

You will see that this antenna is tuned to 158 MHz at which point it has its best SWR - about 1.25:1. As you move away from the tuned point the SWR deteriorates so that by the time you reach 155.5 on one side and 160 on the other the SWR exceeds 2:1. This antenna would be OK for radio, not much good for AIS which is at 162 MHz. An AIS optimised antenna would be tuned to 162 MHz so the best SWR would be at that point - if it had the same shape curve as this antenna it would be useless for radio frequencies on the left hand of the scale.
Really good quality antennas might have a flatter curve so that they stay within, say, 2.5:1 even at the extremes of the frequency band. and some AIS optimised antennas have an asymmetric curve so that they give good performance at 162MHZ as well as further down the range, but not further up it.
I hope this gives you an idea of what VSWR means to antenna performance.
 
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