Hi Steve,
If you are interested I had really good results recently with a homebrew
RLB, in fact so good that I doubted the results and had to redo them several
times before I was comfortable I hadn't made a mistake, and I probably still
have but anyway it works heaps good enough for me. Basically using the non
lab standard test equipment I had available it shows a directivity of better
than 40db from about 1 to 500MHz. I found the best results by using a more
or less current balun with coax, and a balancing extra bit of coax, and
paying close attention to keeping everything balanced/symmetrical and having
the reference connected with a connector identical to the unknown, rather
than just a 50ohm resistor directly connected. See writeup in December issue
of Nerg News
http://www.nerg.asn.au/NERGNEWS/NN200512.pdf

73 Paul VK3DIP.



wrote in message
oups.com...
I have just built an HF return loss bridge according to the info in
"Solid State design for the Radio Amateur". While it works ok, I was
hoping for a bit better performance. Clearly the "balun" doesn't have
enough inductance to operate well below 10 MHz or so (10 turns bifilar
#30 AWG enamelled wire on Amidon T23-43, as specified in the book -
twisted pair, which isn't specified one way or the other in the book).
This can probably be largely fixed by using a larger ferrite core
(T37-43's available in junkbox). I am interested primarily in the
1.8-50 MHz range, though I wouldn't complain if it worked on 2m too.

But even at 30 MHz a reasonably good microwave 50 ohm load gives only
about 28 dB apparent return loss...Not bad, but I might have expected a
bit more.

And an open and short give about 2 dB different signal levels at 30
MHz. That is with big pads (20 dB attenuation) on both the signal
generator and detector. The detector is an HF receiver with a step
attenuator used to maintain a constant S-meter reading.

The circuit is built on a small PC board using construction techniques
typical for the UHF or low microwave range (except that the test port
connector is an SO-239), and is enclosed in a shielded box. Each of
the three 50 ohm resistors is made of two 100 ohm 1206 chip resistors
in parallel and measures between 50.0 and 50.3 ohms at DC.

Has anyone with experience with this circuit any suggestions for how to
tweak it for best performance ? What accuracy level have you achieved
? Do you know where I might find an error analysis for this circuit ?
Or if I am to think about errors myself, does anyone know how to model
the balun in SPICE ?

73,
Steve VE3SMA

Paul:

Thanks for the info....that's an interesting balun approach. I was
thinking about the two-connector approach and using a good microwave
load for the termination, but decided it wasn't likely needed for 50
MHz and below. Though with the results you are getting it's tempting to
build another one for VHF/UHF. But I need a good signal generator
first !

I have since obtained much better results than I was initially getting,
with two main approaches to improvement:
(1) I changed the balun to 12 bifilar turns on an FT37-43 core, which
now has enough inductance for reasonable measurement accuracy at the
lower frequencies (though the degradation is still pretty noticeable
with good loads that read 35 dB return loss at 10-30 MHz showing as 25
dB at 1.8 MHz). A few more turns would probably be better.
(2) I spent lots of time chasing down leakage paths and sensitivity to
leakage...grounding cables, reducing attenuator on signal generator,
scraping paint from inside front panel of signal generator, removing
microphone from transceiver in use as a detector, etc.

I don't really understand the leakage paths, but I was able to get
their impact down to a much more usable level. Clearly a high standard
of shielding on all components of the test setup is a must. I'm now
getting around 0.5 dB change from open to short at 30-50 MHz
(insignificant at lower frequencies) and my good loads read 31-34 dB
return loss at 50 MHz, which is probably about what should be expected
due to the adapters to PL-259 plug. I am not yet quite fully confident
when measuring antennas though, due to somewhat greater leakage effects
into the receiver.

73,
Steve VE3SMA

Steve,
Yes the connectors can make all the difference, as you would have seen
from the write-up I used BNC's and even though they were reasonably good
quality at the extremes just wiggling the connectors slightly to vary the
contact resistance ( and probably capacitance etc.) a bit can make dB's of
difference to the measured directivity. Thankfully this is not a big problem
under normal usage. Your PL259's would probably have similar effects though
possibly not quite as noticeable at the directivities and frequencies you
were looking at.

Paul. VK3DIP