(Dr. Slick) wrote in message . com...
(Tom Bruhns) wrote in message om...

This could be a whole 'nuther thread. For a reference, I don't have
any difficulty making a 50 ohm load with 40dB return loss out to a GHz
or so for less than $10, and most of that is the connector. Does
someone need to document how to do that? But I submit that if you
want accurate SWR measurements on a particular line, you should
calibrate your SWR meter to that line, and that doesn't take any
reference except the line itself.

Cheers,
Tom


But can you make a 50 Ohm dummy load with those specs that can
handle
300 Watts? The stays at 50 Ohms out to at least 200 Megs or so?


50 +/- what? What return loss are you shooting for in this 300W dummy
load? Do you really need 40dB, or is 30dB good enough? I believe
it's possible to bootstrap yourself into measurements that are far
more accurate than you'll need for what you are doing, and do it quite
economically if you don't count your time. But you ought to first ask
yourself just what accuracy you really need, and understand why.

Probably one can make a quite reasonable broadband power load, at
least to your 200MHz limit. There have been some good construction
articles on making tapered shields for power load resistors, for
example, to get good high frequency performance. But if you can make
just a good low-power one, you can bootstrap your way to accurate
measurements at high power. Use the low power one to insure your
directional coupler is good to some tolerance, and use that to tune
your load at whatever frequency you wish to check. Even your cantenna
should be low enough Q when tuned with an L network that it would be
acceptable over the whole of the 2-meter ham band. It's tedious, but
you can re-tune for any spot frequency inside or outside the ham band.
And given one accurate load, you can determine what impedance your
long, lossy coax is and then use that as a dummy load (probably quite
broadband). For example, if your cantenna is good enough through
30MHz, and pretty good at 54MHz, and under 2:1 SWR at 150MHz, then
perhaps enough RG-58 to give you 10dB loss at 150MHz (150 feet or so),
feeding that cantenna, would work fine from 1MHz to 1GHz. Just beware
of power dissipation in the line itself at higher frequencies. You
can even cascade large coax, small coax and the cantenna, to insure
power handling.

Beware of harmonics messing up your readings!

Also, you'd probably do well to consider how small a change in power
results from a modest load change, for various source impedances.
What's the worst case? What's the best? Is there a reason it might
be nice if an amplifier output was "reasonably close" to 50 ohms, or
doesn't it matter at all? If you think all this through, you may
realize that if your dummy load is even only 20dB return loss, it will
be just fine for the measurements you need to make. But YOU should
convince YOURSELF of that, or of what you really do need.

And i'm still not sure what you mean by "calibrate your SWR meter
to the line". All the SWR meters i have seen are all for 50 Ohms.

Could you tell us the exact procedure?

Of course not; I know nothing about YOUR SWR meter. If you understand
how yours works, you should be able to see how to adjust it, though it
may not be worthwhile. To a close approximation, practically all of
them work by sampling the line current and voltage at a point. The
current is somehow turned into a voltage, and if you adjust either the
voltage sampling percentage or the voltage produced by a given
current, you will have adjusted the calibration impedance. At the
high frequency end, you may need to worry about reactive or
phase-shift effects. If it's not adjustable, don't you at least want
to know WHAT impedance it's calibrated for? Or at VERY least, what it
reads with a 50 ohm load?

Cheers,
Tom

(Tom Bruhns) wrote in message om...
(Dr. Slick) wrote in message . com...

But can you make a 50 Ohm dummy load with those specs that can
handle
300 Watts? The stays at 50 Ohms out to at least 200 Megs or so?


50 +/- what? What return loss are you shooting for in this 300W dummy
load? Do you really need 40dB, or is 30dB good enough? I believe
it's possible to bootstrap yourself into measurements that are far
more accurate than you'll need for what you are doing, and do it quite
economically if you don't count your time. But you ought to first ask
yourself just what accuracy you really need, and understand why.


+/- 5 ohms or so. Something that doesn't swing up to 65-70 ohms or
40 ohms.




And i'm still not sure what you mean by "calibrate your SWR meter
to the line". All the SWR meters i have seen are all for 50 Ohms.

Could you tell us the exact procedure?

Of course not; I know nothing about YOUR SWR meter. If you understand
how yours works, you should be able to see how to adjust it, though it
may not be worthwhile.


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.


Slick

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.
--
73, Cecil http://www.qsl.net/w5dxp



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"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.
--
73, Cecil http://www.qsl.net/w5dxp
Cecil,

When you do this, does the scale still display the correct SWR for
conditions other than 1:1 ?

Tam/WB2TT

Tarmo Tammaru wrote:

"W5DXP" wrote:
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.

When you do this, does the scale still display the correct SWR for
conditions other than 1:1 ?

The scale is calibrated using known loads so yes, it displays the
correct SWR up to 5.83:1. Above 5.83:1 the resolution of the scale
is poor because the full scale SWR is infinity. I have marks at
5.83:1 and 10:1. 5.83:1 is where the reflected power equals half
the forward power. Incidentally, the Z0 of my '450' ohm ladder-
line is closer to 388 ohms.
--
73, Cecil http://www.qsl.net/w5dxp



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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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