Re: 70cm reflectometer?

Hello Tom

My responses**

"K7ITM" <k7itm@xxxxxxx> wrote in message
news:5ab713e4-1597-4b6d-941b-a02907953b52@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jan 30, 2:08 pm, "Suzy" <not@valid> wrote:
"K7ITM" <k7...@xxxxxxx> wrote in message

news:bed95710-4606-4c9b-8e71-170223ac284a@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

On Jan 30, 9:57 am, "Suzy" <not@valid> wrote:
...

I'd like to thank all you kind fellows for your assistance, but
unfortunately a lot of it is rather above my head. I need to be
pointed
at
an article that spells out exactly what dimensions, how to build etc.
(eg
is
there one in the ARRL handbook?) And Owen, no in this case Suzy if
not
short for Susan, so I'm not the person you knew.

OK, back to the first posting I made here. How about if I build one,
test it, and post the design and results somewhere? As I noted in one
of my postings, if I were to make one, I'd first consider how much
power I wanted to read, full scale, so if my offer is appealing, let
me know how much power you want to measure, max. Don't tell me a
kilowatt if you really are going to use it at 10 watts, because if I
design for a kilowatt, 10 watts will be low enough that you won't be
able to read it very well. In fact, I'd propose 10 watts as a
reasonable full scale for a lot of ham uses.

Cheers,
Tom

Hi Tom

That's a kind offer, but you needn't go to all the trouble of building
it.
Just a pointer at the practical design will do (but no complex theory --
over my head!).

I want to investigate various 70 cm antennas (central frequency in
Australia
435 MHz). TX is switchable 5 10 20 watts. I want to standardise on BNC,
and
have readouts on analogue meters (probably 1 mA movements)

OK, I gave this some thought last night. I see a couple problems...

Though you could use 1mA meter movements, that puts you at a detected
power level high enough that the meter scale won't be linear in power,
assuming Schottky or germanium diode detectors.

**Pardon my ignorance, but isn't it just a case of using an op amp or
whatever to suit whatever meter movement I have? ANyway, I have now sourced
a 100 microamp meter (MU65) with a 3.9K resistance (sounds strange as the 1
mA one has a 210 ohm resistance.

BTW, I'n not bothered about the linear issue. I will be having two meters to
show forward and relected powers simultaneously. I don't want to clculate
actual SWR.

To me, having a
linear power scale is a big advantage, because then you can reasonably
accurately figure SWR without having to worry about temperature
compensation of the detectors. There's a reason that Bird power
meters use a sensitive microammeter movement. (I think I've heard
30uA full scale, but I'm not sure about that.) Anyway, that's why I
suggested using a DVM for readout.

The second problem is, if you want to implement a microstrip design,
how do you get the trace width right? If you're afraid of surface
mount parts, how will you control the trace width to +/- a fraction of
a millimeter? On 1.6mm thick PC board, assuming FR4 with a relative
dielectric constant of 4.75, you'd like to have a trace width about
2.78mm to get a 50 ohm line. If your trace is 3.5mm wide, you get a
bit under 44 ohms, and if your trace comes out 2.0mm wide, you get a
line that's almost 60 ohms. If you can do the PC board
photographically and have confidence that you can control the trace
width to within 0.1mm, that would work. If you're doing it by
scribing the copper and pulling up unwanted copper, I think you'll
have to be working under a pretty good microscope to get to much
closer than a mm of the desired width-- or maybe cut it on a milling
machine.

I suppose there's still the possibility of cutting the trace a bit
narrow on purpose and adjusting the impedance by adding a grounded
plate above the board. It could be spaced an adjustable distance away
by mounting it with threaded rods (long screws), and adjusted to make
the traces 50 ohms.

**But how do you check that in a workshop with no test gear?

But there's still the problem of making the two
(or three) all the same width. Not knowing how you might be able to
do this, I'm rather discouraged about how this would come out. Maybe
there's a better way to make the coupled lines that's easier for a
typical ham with minimal shop facilities to handle. Or maybe if there
was enough interest, someone could make some boards with guaranteed
performance.

On the positive side, I did find BNC jacks that edge-mount on PC
boards, so that part of it becomes easy at least.

Cheers,
Tom

Cheers


.

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