Return Loss Bridge Accuracy Questions
 

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

Return Loss Bridge Accuracy Questions
 

Return Loss Bridge Accuracy Questions
 

On 18 Mar 2006 05:09:52 -0800, wrote:

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 ?

I don't have the book and haven't used the design. However, the
minute you say "SO-239", all bets are off. You must have used an
adapter to connect your "reasonably good microwave load" to the
SO-239. You measure 28 dB RL, that sounds optimistic to me, other
things considered.

I have a link to a must read paper at:

http://www.k6mhe.com/n7ws/

Note No. 11.

Make particular note of the part, "The Curse of Adapters."

Return Loss Bridge Accuracy Questions
 

CA wrote:

My homebrew RLB uses a current balun. Some turns of thin 50 Ohm coax on a
ferrite core. The core is from a scrapped Siemens inductor. I have
acheived good performance between 0,1 to 200 MHz with this technique. I
first tried the twisted pair winding approach but the current balun was
far better.

Chris SM6PXJ

wrote:
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

If you could give more details of how to build your circuit, or a photo of
it, I'd appreciate that.

Chris

Return Loss Bridge Accuracy Questions
 

Chris:

Thanks....I have been wondering about this option. At least you know
that the differential-mode impedance is 50 ohms. I am not sure my box
has room for the much larger core needed to wind coax on, though.

73,
Steve VE3SMA

Return Loss Bridge Accuracy Questions
 

Wes Stewart wrote:

the minute you say "SO-239", all bets are off.

I did some SPICE modelling of an ideal bridge (with resistive detector)
and came to the same conclusions....that the higher-than-50
ohm-impedance UHF connector and adapter may be a limiting factor in the
performance on a good SMA- or N- connectored load. It has virtually no
effect on the difference in level between open and short terminations
though.

I have a link to a must read paper at:

http://www.k6mhe.com/n7ws/

Note No. 11.

Make particular note of the part, "The Curse of Adapters."

Thanks...the original Wiltron app note has been a favourite of mine for
many years, but have never seen the updated version.

73,
Steve VE3SMA

Return Loss Bridge Accuracy Questions
 

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

Return Loss Bridge Accuracy Questions
 

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

Return Loss Bridge Accuracy Questions
 

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

Return Loss Bridge Accuracy Questions
 

Christer,
It looks like Rockby has sold out and removed them from their catalogue, the
one you pointed out is indeed not the correct part. The ones I used are as
far as I know a standard/common 14mm balun former as used in TV baluns etc..
The Jaycar (www.jaycar.com.au), or Electus
(http://www.electusdistribution.com.au) part number LF1220 looks basically
identical to those I used, and there are probably lots of equivalents out
there. Electus has some data on the M7 material used here
http://www.electusdistribution.com.a...d/ferrites.pdf
I hope this helps, and I would be interested in how you get on if you end up
building one, let me know at the email address shown below.

73
Paul VK3DIP




"CA" wrote in message
...
Paul VK3DIP wrote:

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...

I have read your article with great interest.
Do you have a specification for the "balun formers" or maybe a Rockby
Electronics stock no.?
I suppose it is www.rockby.com.au. The only 2-hole ferrite core I could
find is stock no. 10967 but I believe this one is too small.

73
Christer SM6PXJ