Tom,

Thanks for the suggestion. I have used RFsim and come up with dimensions
for a 915 MHz version.
I would like to put a coupling strip each side of the centre to pick up
forward and reverse power at the same time. The description in RFsim
don't mention this - Do you know if I need to make any alterations in
dimensions to accommodate this or should I have a second 1/4 in series
with another coupler that picks up reverse power ?

I would like to use the AD8302 Gain/Phase device to measure the incident
and reflected voltages. This thing will has an input range of -60dBm to
0dBm which will be great if I feed the directional coupler with 0dBm and
the coupling is -20dB. It provides measurement for +/- 30dB and Phase
orange over 180 degrees. It also will operate to 2.7GHz so should be ok
for the 915Mhz.

I thought I would make this gain/phase detector in a separate enclosure
so I can unplug it from the directional coupler and use it for Amp
gain/phase etc. With additional of a PIC micro and LCD display it could
be programmed to provide various measurement types.

There are construction details in the ARRL Antenna book for a fixed gain
antenna that looks good for a reference.

Hopefully this will give me a suitable arrangement for setting up 900MHz
band antennae using the sig gen power levels.

Regards

David

K7ITM wrote:
I would expect the outer is a Teflon or Teflon-like material, with
probably low loss and fairly low relative dielectric constant, but it's
still going to require that you shorten that lower section quite a bit
to get the quarter-wave stub to reflect a high impedance. In addition,
the dielectric between the outer of the coax and the inner of the
sleeve is pretty thin, and the impedance of the resulting coaxial
arrangement is pretty low. That means that it won't ever reflect a
very high impedance. You can use a tapered sleeve that bells out at
the bottom, to good effect. That will also lower the feedpoint
impedance and match better to 50 ohms. Think: ground plane with
drooping radials. But you can also wind the coax just below the
antenna into a small coil (I'd make the axis of the coil coincide with
the axis of the antenna) that's self-resonant near your operating
frequency, and it will very effectively choke off antenna current
(current on the outside of the coax) at that point. Put one such coil
an inch or so below the bottom of the sleeve, and another about a
quarter wave further down the line.

SWR meter: Get RFSim99--do a Google search for it. Build a coupler,
per toolsdesigncoupler. I'd suggest a microstrip version, if you
can make a little PC board reasonably accurately. Design it for 50
ohms. Terminate each coupled port in 50 ohms (e.g. 49.9 ohm 0805 SMT
part). Using vanishingly short leads, connect a simple diode detector
to each of those two loads. Use a calibrated attenuator to calibrate
at least the relative response of those detectors. Use those two
outputs to calculate SWR. You can read the diode detector outputs with
a DVM that has good resolution (10uV sensitivity preferred; 1uV is even
better). I'd recommend about a 20dB coupler for the power level you're
using, though even a 30dB coupler would work. Try terminating the
through line in a 49.9 ohm (or a parallel pair of 100 ohm) 0805 parts
to check that you see essentially no reflected, and try a 100 ohm load
and a 25 ohm load to check that you get the expected reflected. -- To
have the coupling right according to the RFSim99 directions, it needs
to be 1/4 wave long, but it's a pretty broad peak. Coupling drops to
zero at 1/2 wave, and at DC. So you could make one for 900MHz, and it
would work OK at 450MHz, you'd just get lower coupling. The ratio for
SWR would still be OK. The 450MHz version on FR4 board
(fiberglass-epoxy) would be roughly four inches long, if my mental
arithmetic is right, and half that for 900MHz.

Cheers,
Tom


David wrote:

Tom,

Thanks for the information. The inner coax is the smaller RG174 coax.
The sleeve is made of earth braid pulled from RG58 cable. The dielectric
between the sleeve and inner cable is therefore the outer sheath of the
RG174 cable (Not sure what this is, the RG174 I have is Teflon inner
dielectric and stranded conductor. The utter sheath is a very strong
heat resistant material - I therefore have no ideal of the dielectric
constant to calculate Vp for correct electrical length). If I use a
copper tube and strip off the sheath from the inner coax, I can
calculate correct length as it will have an air dielectric.

Do you know where everyone is getting the dielectric constants for
various materials ? I noted people using small metal tubes as sleeves
and quoting these magic numbers even for copper tubes of certain diameters.

From my discussion with Telonic, they say the Rho_Tector was designed
as an in-house tool for measuring inputs of amps and filters. They
suggest SWR meter would probably be best for antenna adjustments.
Do you happen to know where I might find details for a low power SWR
meter for 915 MHz ? I need one that will operate with only 20mW applied
power. The only SWR meter I have has min. FSD of 3W

Thanks

Regards

David

K7ITM wrote:

Hi David,

You wrote, "The decoupling is via a 1/4 wave sleeve that provides high
impedance for
RF returning along outer coax and also as the second 1/2 of the
dipole. "

EXACTLY how is this built? The details of construction make a BIG
difference in performance! (There's a lot of BAD info about it out
there...)

It's not a bad idea to ALSO put some additional decoupling further down
the feedline.

If your spectrum analyzer/field strength meter is far enough away from
the antenna you are testing, then it should provide a reasonable
indication of relative antenna radiation performance. The SWR
indication, if properly calibrated and given that you are apparently
exciting the antenna with a source whose output impedance matches your
feedline, should also be a good indication of power actually absorbed
by the antenna. That is, lowest SWR represents maximum power absorbed
by the antenna. Presumably that power is being radiated as RF, mostly,
and not dissipated as heat. But where the RF radiation goes depends on
the pattern of currents excited on the conductors that compose the
antenna, and nearby conductors as well (such as the feedline). What
you probably want is standard resonant half-wave dipole currents on
your vertical dipole, and no (very little) antenna current on the
feedline and on support structures. By the way, whether the antenna is
resonant or not is of little real importance, so long as you can
efficiently feed power to it and the antenna currents are in the right
places and not the wrong places. But it happens that with your
antenna, if things are working properly (properly decoupled feedline,
etc), you probably will see lowest SWR at half-wave resonance. If you
have no other matching going on, the lowest SWR will probably be about
1.5:1 with 50 ohm feedline. You could add parts to get a better match
if you wished.

And as you can probably tell from all that, I'm suspecting that your
decoupling sleeve, with associated dielectrics in that area, probably
isn't doing a very good job...

Also...Joe noted that your coax feedline may well be a length that
accounts for the SWR peaks and valleys. (I think it may be about twice
as long as Joe wrote...but same idea.) Do you see the peaks and
valleys when you terminate the line in the precision 2:1 load? If you
do NOT, then it's a further indication that the feedline has antenna
currents on it, because the flat 2:1 is an indication that your
transmission line is matched to the calibration impedance of the SWR
bridge, and if that's the case, the SWR bridge should be giving at
reasonably accurate estimate of the actual line SWR. If you DO see the
SWR ripples vs frequency with just the precision load, either the load
isn't "flat" or the line is not the same impedance as the SWR bridge is
calibrated to, and the differing impedances is by far the most probable
explanation if the line length is right.

Cheers,
Tom

You should probably have a look at http://users.adelphia.net/~n2pk/.
Paul has done a nice job on a vector network analyzer project, with
some good additions from others, and you can probably pick up quite a
few nice ideas from that design, maybe even use a lot of it.

The single parallel strip really does pick up both forward and
reflected, and if you can terminate it properly at both ends
(especially the forward end) so that the load there does not reflect
back down that line segment, you can sense both forward and reflected
on the single parallel strip. Making the load really be 50 ohms isn't
easy when it also has to be a detector, though, so two strips may make
more sense for you. It should work fine to have them just be
symmetrical, one on either side of the main line.

If you really care about phase detection, be aware of the phase shifts
through line segments. Such couplers are sometimes called quadrature
hybrids. The sampled forward output will be in phase with the applied
input, but the through-line output will be delayed 90 degrees (a
quarter wave in the transmission line, obviously). The sampled
reflected output will be in-phase with the reflected that arrives at
the through-line load port--by symmetry. But that means you'd measure
a 90 degree phase difference between the forward and reflected, for a
case where they were in phase at the through-line output port. Hope I
have that all correct...if not someone will probably jump in and
correct it. It should be easy to calibrate out. Since the phase
shifts are frequency-dependent, if you want to cover much of a
percentage frequency range, be prepared to handle frequency-dependent
calibration. The coupler goes out of quadrature, and you probably
won't have exactly the same length lines from each coupler to the
detector.

Since the parts are really pretty cheap and replicating PC boards is
straightforward, you might think about the advantages of NOT having to
disconnect things, and the expense of putting good RF connectors on the
coupler, and maybe just build the detector into the coupler, and make a
separate one for other uses. A couple good SMAs and the hassles of
connecting and disconnecting a few times should pay for just buying
another $20 part. Half a tank of gas these days, huh?

Cheers,
Tom

Tom,

Thanks for the link. This unit only goes to 60MHz so needs a lot of mod
to get it up to 900MHz where I am working.

I have found this one that does go up there
http://www.geocities.com/robert_laco...us_designs.htm

Some of the parts used in his design are getting difficult to obtain so
I am thinking of replacing the Cypress device and prescaler for a
LMX1600 PLL and driving the generator ramp via a PIC micro controller
instead. The VCOs, Splitters and Amp are all still available.

Before I start on this project I will make a manually adjustable version
(The directional couplers will not be redundant)

The reason the detector head is a separate item is that this will be
inside a small shielded area inside the main box that also has the micro
controller, LCD display, keypad and serial port.

Then I can simply swap the 900MHz coupler for the 433MHz coupler to
change bands.

I have completed the artwork for the Bi_Directional coupler and will
make the PCB today. Using RFsim99 I was able to play with combination of
track widths and gape between mainline and coupled lines to get the 20dB
coupling and 50 Ohms.

K7ITM wrote:
You should probably have a look at http://users.adelphia.net/~n2pk/.
Paul has done a nice job on a vector network analyzer project, with
some good additions from others, and you can probably pick up quite a
few nice ideas from that design, maybe even use a lot of it.

The single parallel strip really does pick up both forward and
reflected, and if you can terminate it properly at both ends
(especially the forward end) so that the load there does not reflect
back down that line segment, you can sense both forward and reflected
on the single parallel strip. Making the load really be 50 ohms isn't
easy when it also has to be a detector, though, so two strips may make
more sense for you. It should work fine to have them just be
symmetrical, one on either side of the main line.

If you really care about phase detection, be aware of the phase shifts
through line segments. Such couplers are sometimes called quadrature
hybrids. The sampled forward output will be in phase with the applied
input, but the through-line output will be delayed 90 degrees (a
quarter wave in the transmission line, obviously). The sampled
reflected output will be in-phase with the reflected that arrives at
the through-line load port--by symmetry. But that means you'd measure
a 90 degree phase difference between the forward and reflected, for a
case where they were in phase at the through-line output port. Hope I
have that all correct...if not someone will probably jump in and
correct it. It should be easy to calibrate out. Since the phase
shifts are frequency-dependent, if you want to cover much of a
percentage frequency range, be prepared to handle frequency-dependent
calibration. The coupler goes out of quadrature, and you probably
won't have exactly the same length lines from each coupler to the
detector.

Since the parts are really pretty cheap and replicating PC boards is
straightforward, you might think about the advantages of NOT having to
disconnect things, and the expense of putting good RF connectors on the
coupler, and maybe just build the detector into the coupler, and make a
separate one for other uses. A couple good SMAs and the hassles of
connecting and disconnecting a few times should pay for just buying
another $20 part. Half a tank of gas these days, huh?

Cheers,
Tom