Vswr Meter
ashwanthh
Hi all,
For my RF project, I want to measure vswr readings, in order to
that I was planning to construct a VSWR meter. The concept of VSWR is to
calculate the ratio of Voltage transmitted to the Voltage reflected. So
my basic question is,
1. What sensor/instrument is used to measure the voltages from the coax
cable(in my case)?
Please some one help me with some ideas
--
ashwanthh
david
say:
Google is over there --->
KBa
> Hi all,
>
>
> For my RF project, I want to measure vswr readings, in order to
> that I was planning to construct a VSWR meter.
>
> cable(in my case)?
There are several means depending on frequency used, transmission lines,
sampling transformers and other like resistive bridges.
One useful source could be to look at N2PK web page, he has done
measurements for HF range bridge.
gl kba
Andrew VK3BFA
wrote:
Dont know where you are, but get yourself a copy of the ARRL handbook
- doesnt matter the age - and all will be explained....
Andrew VK3BFA
raypsi
Hey OT
:
Most so called VSWR meters will give you a reading but is it a true
reading? Not likely, by the time the reflected wave comes back down
the cable it has lost some voltage because of cable losses.
A true method would be to measure the voltage at the antenna,
telemeterize the voltage reading and send it back to the VSWR meter to
compare to the output voltage of the tx. And use a dual cross meter
so the ratio is calculated by the meter scale with no adjustment
needed. Except to calibrate it just once.
73 OT,
de n8zu
On Nov 9, 3:58 am, ashwanthh <ashwanthh.71ee...@radiobanter.com>
wrote:> Hi all,
highlandham
>
> On Nov 9, 3:58 am, ashwanthh<ashwanthh.71ee...@radiobanter.com>
> wrote:> Hi all,
>> >
>> > For my RF project, I want to measure vswr readings, in order to
>> > that I was planning to construct a VSWR meter. The concept of VSWR is to
>> > calculate the ratio of Voltage transmitted to the Voltage reflected. So
>> > my basic question is,
>> >
>> > 1. What sensor/instrument is used to measure the voltages from the coax
>> > cable(in my case)?
>> >
>> > Please some one help me with some ideas
>> >
>> > --
>> > ashwanthh
Is it really important to KNOW the actual VSWR? I don't think so.
I use a swr meter to be able to adjust the Antenna Matching Unit (I
don't call it a tuner) for zero or minimum reflected power ,just to
'keep the transmitter happy'
If the swr meter shows a different level of reflected power at a
specific setting of the AMU for a specific frequency,using a specific
antenna ,then something is lilely wrong with the antenna or feeder.
When adjusting the AMU for minimum/zero reflected power ,for final
adjustment I set the swr meter at maximum sensitivity.
Currently I only use a 'doublet' = a wire dipole with twin lead feeder.
Frank GM0CSZ / KN6WH
Paul Keinanen
<ashwanth...@radiobanter.com> wrote:
>For my RF project, I want to measure vswr readings,
The first question is why ?
If you want to tune some HF antenna to resonance within a specific
ham band, just use a noise bridge and adjust the antenna dimensions
accordingly.
For VHF/UHF, the antenna dimensions are even more critical, i.e. the
antenna feedpoint impedance should be the same as the feedline
impedance. Due to the cable losses, any ATU at the lower end would be
completely unacceptable.
K7ITM
wrote:
What is your "RF project"? What frequency or range of frequencies?
What power level?
Pretty much all SWR meters don't actually measure the forward and
reverse voltages directly, but deduce them from measurement of the
voltage and the current on a transmission line at a point along the
line. Then, knowing that the forward voltage divided by the forward
current (including phase) equals the line impedance, and also equals
the reverse voltage divided by the reverse current, you can deduce the
forward and reverse voltages (and forward and reverse currents). You
use the line impedance in the calculation, and if you've assumed a
line impedance different from the impedance of the line you're
actually measuring, you'll get an answer that's not correct for your
actual line. The "calculation" is commonly done with analog
electrical parts in typical ham SWR meters, and there is generally a
way to adjust the parts so that the meter reads correctly for the line
impedance you want to use--but you must actually do the check/
calibration or you won't know. (The calibration may involve just
adjusting a variable part, or it may involve replacing a fixed-value
part with one of a different value...not nearly so easy to get it
right.) Also, the accuracy of many ham-type SWR meters suffers if you
don't operate them at the right power level, because of non-linearity
in the detector diodes; there are ways around that (e.g. using non-
diode power detectors or operating the detector at a fixed full scale
range).
It's also possible to measure just the voltage at a few appropriately-
spaced points along a line and deduce the SWR more directly from those
readings, but this is very seldom done in practice. See "slotted line
measurements" for more details.
Cheers,
Tom