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SWR meters, some usefull facts

 
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SWR meters, some usefull facts
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pcs
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Joined: 18 Jan 2002
Posts: 3063
Location: Radio Land

Post SWR meters, some usefull facts Reply with quote
I notice many of our customers have a problem with this:

1.) SWR meter will only show correct power when loaded with 50 ohms.
Even than it might be 10% - 20% or so off, especially at low powers
around 1W or less. For example, when you disconnect antenna, needle
jumps up like crazy. It does not mean that you're suddenly pumping out
1000W instead of 100W.

2.) CB SWR meters are made for CB. When used at around 100MHz:
- they don't show correct power, could be 10-20% off or even more,
depending on design.
- they are not perfectly 50 ohms at 100MHz meaning they influence SWR
reading. This means that you can set your SWR to be perfect, but once
you remove the meter this will change. You have in fact tuned a
combination of your SWR and antenna to be 50 ohms.

3.) 1.8-150MHz SWR meters perform poorly at MW AM (540-1710KHz)
As the frequency drops they tend to seriously underestimate power,
meaning they can show 50% or even less power than is in fact transmitted.
They're more/less accurate around 1400-1710 MHz.

Any questions or comments?

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Wed Apr 06, 2005 12:23 am View user's profile Send private message Visit poster's website
aaeceo
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Joined: 01 Apr 2005
Posts: 1

Post swr meter question Reply with quote
i have the sx-200, but when i try to calibrate it the needle isn't moving at all. i have the function set on cal as im suppose to but i'm not getting a response out of it. do i need a new one or can i use a cb swr meter from radio shack? i am trying to broadcast on 90.7 out of texas. please help. thanks
Wed Apr 06, 2005 6:37 pm View user's profile Send private message
Fission7x
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Joined: 03 Jul 2004
Posts: 21

Post Reply with quote
So is there a way, or even a need, to measure SWR for a transmitter putting out 250 mWatt?
Wed Apr 06, 2005 9:47 pm View user's profile Send private message
pcs
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Joined: 18 Jan 2002
Posts: 3063
Location: Radio Land

Post Reply with quote
A few mode things about SWR meters:

SWR meter just tells you whether your antenna is tuned or not, it does not
tune anything for you.
There are typically two types: The first, more expensive type has two needles,
one shows the power, the other shows reflected power. Reflected power should
be minimal or zero.

The other type of SWR meter has just one needle and is usually cheaper. First
you're required to set the switch into cal/tune mode, set the cal knob until the needle
shows cal on the meter, than set to swr and read reflected power or switch to PWR
to read output power.

Again, the power shown on the meter depends on the impedance of the antenna.
In order to accurately measure power it is best to use a 50 ohm dummy load, and
even in this case keep in mind that the shown power can be 10-20% or so off.

CB SWR meters typically show too much power when used at 88-108MHz and
contribute their share of capacitive or inductive reactance resulting in worse SWR
measured than under normal conditions with normal SWR.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Thu Apr 07, 2005 4:17 pm View user's profile Send private message Visit poster's website
pcs
Site master
Site master


Joined: 18 Jan 2002
Posts: 3063
Location: Radio Land

Post Reply with quote
Fission7x wrote:
So is there a way, or even a need, to measure SWR for a transmitter putting out 250 mWatt?


There often is a need. The best way to do this is to use one of the
specialised chips, available for the purpose.

AD8307 is a nice example:

FEATURES
Complete Multistage Logarithmic Amplifier
92 dB Dynamic Range: –75 dBm to +17 dBm
to –90 dBm Using Matching Network
Single Supply of 2.7 V Min at 7.5 mA Typ
DC to 500 MHz Operation, 1 dB Linearity
Slope of 25 mV/dB, Intercept of –84 dBm
Highly Stable Scaling over Temperature
Fully Differential DC-Coupled Signal Path
100 ns Power-Up Time, 150 A Sleep Current
APPLICATIONS
Conversion of Signal Level to Decibel Form
Transmitter Antenna Power Measurement
Receiver Signal Strength Indication (RSSI)
Low Cost Radar and Sonar Signal Processing
Network and Spectrum Analyzers (to 120 dB)
Signal Level Determination Down to 20 Hz
True Decibel AC Mode for Multimeters

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Thu Apr 07, 2005 4:20 pm View user's profile Send private message Visit poster's website
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