W5DXP wrote in message ...
Dr. Slick wrote:
I didn't think you could tell us. I've never seen an SWR meter
that you could "calibrate" to 50 or 75 ohms, or less.

The calibration of the SWR meter is controlled by the internal sampling
load resistor, the 'R' in Peter's V + IR equation. I have a home-brewed
SWR meter that measures SWR on both balanced 450 ohm feedlines and on
300 ohm feedlines simply by changing the internal load resistors.


BTW, how do you know the accuracy of your homebrew SWR meter?


Slick

Dr. Slick wrote:
BTW, how do you know the accuracy of your homebrew SWR meter?

I have a bunch of 50W 600 ohm non-inductive resistors that I use
for calibration purposes. And I really don't know the accuracy.
An SWR of 20:1 looks the same as an SWR of 25:1 on the scale.
I have an upper and lower acceptable limit to the SWRs with the
matching method I use. The SWR meter tells me if the SWR is
outside of that acceptable range.
--
73, Cecil http://www.qsl.net/w5dxp



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W5DXP wrote:
Dr. Slick wrote:
BTW, how do you know the accuracy of your homebrew SWR meter?

I have a bunch of 50W 600 ohm non-inductive resistors that I use
for calibration purposes. And I really don't know the accuracy.
An SWR of 20:1 looks the same as an SWR of 25:1 on the scale.
I have an upper and lower acceptable limit to the SWRs with the
matching method I use. The SWR meter tells me if the SWR is
outside of that acceptable range.

It's interesting to see an example of an SWR meter for a Z0 that isn't
50 ohms, because it helps to confirm that they all work in basically the
same way.

If [V] is a sample of the line voltage, and [i] is a sample of the line
current, then the forward reading is the sum of two RF voltages, [V] +
[i]R, where R is the resistor that converts the [i] sample into a
voltage.

The reverse reading is the difference, [V] - [i]R. The "calibration to
Z0" procedure consists of terminating the line in the design value of
Z0, and then adjusting R so that the reverse reading [V] - [i]R is zero.

The RF voltages are either summed or subtracted, and then the resultant
is detected by the diode.

Just one small point, though... it is not necessary that R = Z0. The
value required depends on the sampling factors kV and kI that relate the
voltage and current in the line to the sampled values [V] and [i]. In
full, the instrument is calibrated to Z0 when:

kV*V - kI*I*R = 0

In a typical bridge, two out of the three constants kV, kI and R are
fixed, and the third is adjustable. In a Bruene bridge, kI is fixed by
the number of turns on the current-sampling toroid, R is fixed, and you
calibrate the bridge by adjusting the kV factor in the voltage divider.
However, it would be equally good to build-in fixed values of kV and kI,
and balance the bridge by making R a small trimpot. So R really does not
have to equal Z0... and in most published circuits, it doesn't.

This can also be shown in a different way, by thinking of it as a
Wheatstone bridge, with Z0 as one arm. The requirement for balance is
only that Z0/R2 = R3/R4. It is not necessary for any of the other
resistors R2, R3 or R4 to equal Z0 in order to achieve balance.

AFAIK, the only situation where the "terminating" resistor truly needs
to equal Z0 is in parallel-line couplers for microwaves, when the
sampling line approaches a quarter-wavelength long and its own
characteristic impedance is Z0 too.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

Ian White, G3SEK wrote:
The RF voltages are either summed or subtracted, and then the resultant
is detected by the diode.

I know you know this, Ian, but let's make sure that everyone understands
that the summation (or subtraction) is a phasor (vector) summation. It is
the phasing between the total voltage and total current that allows the
forward wave to be separated from the reflected wave, and vice versa. The
directional coupler designer assumes that the ratio of Vfor/Ifor = Z0
and that the ratio of Vref/Iref = Z0. If that assumption is incorrect,
the SWR meter will still assume that the assumption is correct.

Just one small point, though... it is not necessary that R = Z0.

That's true and is just a habit on my part. I set R=Z0 and then adjust the
voltage accordingly for calibration purposes. For awhile, I was using a
450 ohm load for ladder-line with a measured Z0 of 388 ohms. It still
worked pretty well.

AFAIK, the only situation where the "terminating" resistor truly needs
to equal Z0 is in parallel-line couplers for microwaves, when the
sampling line approaches a quarter-wavelength long and its own
characteristic impedance is Z0 too.

In most of the slotted line pickups that I have seen, the internal load
resistor is equal to the Z0 of the slotted line. I don't know if that
is necessary or not.
--
73, Cecil http://www.qsl.net/w5dxp



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