On Nov 22, 7:09*pm, exray wrote:
Stray Dog wrote:

On Sat, 22 Nov 2008, exray wrote:

Date: Sat, 22 Nov 2008 16:13:37 -0400
From: exray
Newsgroups: rec.radio.amateur.homebrew
Subject: SWR meter

I've got a cheapo CB swr meter which I thought might be suitable for
some HF QRP work. *Problem is that it is very insensitive on the HF
freqs. *Can't even get more than about 20% of the scale with 5 watts
on 40 meters.

I've used those cheapo CB SWR meters, too, and yes at low HF they are
not so sensitive even with the sensitivity turne as high as it can go.

What is more important is the meter reading on "reflected" compared to
"forward". As long as the reflected is much lower (roughly 1/4 or less)
than forward, you should be OK. Most scales I've seen will have a
calibration such that reflected at about half the scale as forward means
an SWR of about three to one. One of my meters has this point at 2/3 of
full scale. I have always felt satisfied if I can get reflected down to
one tenth, or less, of forward by manipulating the antenna tuner.

If you want to be a purist, then you might also consider locating a more
sensitive meter, say 20-50 microamps DC and using that. Most of those
internal meters are 100-200 microamps DC. Or, make the modification you
describe below.

It uses a stripline for pickup and I'm wondering if maybe I should
replace that with a broadband toroid configuration. *Any specific
recommendations? How about adding some gimmick capacitance across the
stripline to increase coupling?

I'm not sure it is worth the work to try this, but I don't have a good
recommendation or preference to emphasize.

TIA,
Bill WX4A

Well, this little dilly has several wattage ranges, calibrated at 27 MHz
of course, as well as the 'adjust to full scale, flip to REV' for SWR
readings. *I could not get anywhere near full scale with the setting.

I did add some additional capacitance off of the stripline and am now
within range of the thing. *It took about 10pf, a gimmick wasn't enough..
* Celebrating that success I reset the lowest ranges of the wattmeter (5
and 50 watts) to read accurately at 7 MHz. *I have no fantasies about
the linearity of the circuit now nor any fantasy that the other ham
bands will read anywhere even close wattage-wise but it seems just the
ticket for 40 meters.

If I get enthusiastic I might try a ferrite transformer and just see how
much flatter that might be across the bands.

73,
Bill

Operation of an SWR meter depends on monitoring the line current and
line voltage in the proper ratio. If you simply add capacitance to
increase the pickoff of voltage, you'll screw up the ratio and it
won't come close to reading SWR correctly. If, on the other hand, you
make a directional coupler with two identical toroid transformers, you
should be able to get decent directionality as well as good flatness
across the whole HF range. A coupled-line hybrid (directional
coupler) has maximum response where the coupled section is 1/4 wave
long (or 3/4, or other odd quarter), and the response drops off as you
go to lower frequencies. Where the line is short compared with 1/4
wave, the response in volts is very nearly linear with frequency, so
at 2.7MHz, your 27MHz coupler will most likely have about 1/10 the
voltage response (1/100 the power response) as at 27MHz. A
transformer coupler can be flat within a dB over more than a 10:1
frequency range, but it is important to use the right permeability
ferrite in the design. You do not want too much inductance--imperfect
coupling between primary and secondary causes a rolloff at high
frequencies--higher inductance means higher leakage inductance for
given coupling, and that in turn means rolloff at lower frequency.

Cheers,
Tom