How do you tune a multi-section helical bandpass filter?

[mike]

A friend has asked me for help re-tuning a 4-section helical bandpass
filter for the 2-meter ham band. It has no adjustments, so there
won't be too many chances to recover from botched cutting.

As I recall, there's considerable interaction between the sections.
And since he's been bending on the resonators, the bandpass is
quite out of whack. He wants to move it up 2 MHz. and restore
the shape.

A couple of decades ago, I had a procedure that involved measuring
return loss.
You disable (short out) all but the first resonator and set the dip
of the curve here.
Then enable the second section and put that dip there.
So on till you get to the end.

AS I recall, it got you very close in one pass.
The second dip moved the first one, but the end result
bandpass came out close to what you want.

Problem is that I've misplaced the details of where "here"
and "there" are. I've also misplaced much of my memory.

Anybody remember the details of that technique?
Or any technique that might be practical on a filter
with
no tuning adjustments and little chance to recover from
mistakes?

Thanks, mike

 

[Jamie]

A friend has asked me for help re-tuning a 4-section helical bandpass
filter for the 2-meter ham band. It has no adjustments, so there
won't be too many chances to recover from botched cutting.

As I recall, there's considerable interaction between the sections.
And since he's been bending on the resonators, the bandpass is
quite out of whack. He wants to move it up 2 MHz. and restore
the shape.

A couple of decades ago, I had a procedure that involved measuring
return loss.
You disable (short out) all but the first resonator and set the dip
of the curve here.
Then enable the second section and put that dip there.
So on till you get to the end.

AS I recall, it got you very close in one pass.
The second dip moved the first one, but the end result
bandpass came out close to what you want.

Problem is that I've misplaced the details of where "here"
and "there" are. I've also misplaced much of my memory.

Anybody remember the details of that technique?
Or any technique that might be practical on a filter
with
no tuning adjustments and little chance to recover from
mistakes?

Thanks, mike

Service monitor? an all in one tool!

Jamie

 

[Joerg]

A friend has asked me for help re-tuning a 4-section helical bandpass
filter for the 2-meter ham band. It has no adjustments, so there
won't be too many chances to recover from botched cutting.

As I recall, there's considerable interaction between the sections.
And since he's been bending on the resonators, the bandpass is
quite out of whack. He wants to move it up 2 MHz. and restore
the shape.

A couple of decades ago, I had a procedure that involved measuring
return loss.
You disable (short out) all but the first resonator and set the dip
of the curve here.
Then enable the second section and put that dip there.
So on till you get to the end.

AS I recall, it got you very close in one pass.
The second dip moved the first one, but the end result
bandpass came out close to what you want.

Problem is that I've misplaced the details of where "here"
and "there" are. I've also misplaced much of my memory.

Anybody remember the details of that technique?
Or any technique that might be practical on a filter
with
no tuning adjustments and little chance to recover from
mistakes?

http://www.macomtech.com/static/PDFs/TechnicaArticles/Bandpass_Filter_tutorial2.pdf

Design instructions but no tuning info per se. You could contact them
and see if they have more. Then there is a book by Zverev. The best
"McGyver style" alignment instruction I ever saw was in an ARRL handbook
but I can't recall what year. You might want to ask in a ham radio group
if someone still has it.

http://www.rfcafe.com/references/electrical/helical-resonator.htm

Not sure how far the Zverev book goes into tuning, I don't have it here.

 

[Joerg]

It does and thanks for jogging my failing memory. I it here.
"Handbook of Filter Synthesis" by Anatol I. Zverev. (1967)
9.4 (P513 to 521) is on "Alignment of Helical Resonators" with
references to Dishal's method. It's too much to type here, but I can
scan the pages and post them if needed.

Google finds quite a few hits for Dishal's Method.

In case Mike has IEEE library access:

http://www.rfcurrent.com/wp-content/uploads/2010/12/Milton-Dishal-Publications-on-Filters.pdf

Unfortunately I only have access to ultrasound and to aerospace
publications.

 

[Joerg]

Dishal's method. For high-Q filters, mistuned series sections look like
opens, and mistuned parallel sections look like shorts.

If you know what the resonant frequencies of the sections should be, you
can do that with a return loss bridge from each end.

I tuned my last one using the "wet finger and a beer" method. Worked

 

[[email protected]]

Is this out of a commercial radio?  If so, the maker and model please.
Perhaps a photo.  I can't imagine anyone shipping something like that
without tuning adjustments.


That's an understatement.  It's nothing but interactions.  Change one
thing and all the adjacent resonator tuning changes.


2MHz / 146MHz = 1.4%
That should be easy without unwinding turns.
Any clue on the expected bandwidth?


Well, I'm not sure this is the correct procedure, but when I was
making 902-928Mhz solid cavity duplexers, I put a reflection
coefficient bridge on the input (no network analyzer available) and
swept for lowest return loss and best looking sweep.  As I vaguely
recall, it was 5 sections, with a total of 9 adjustments for tuning
and coupling:
<http://www.qsl.net/n9zia/wireless/pics/rtrn_loss_bridge.png>
<http://www.qsl.net/n9zia/wireless/rtrn_loss-pics.html>
<http://www.eagle-1st.com/eagle1st.nsf/17100001!OpenFrameSet>
<http://www.eagle-1st.com/eagle1st.nsf/10201000!OpenFrameSet>

I might have shorted out downstream sections to reduce the display
clutter, but there were no provisions for that inside the cavity.  So,
you start at the input, tune until the curve looks best.  Move on to
the next stage, and tune again.  Because the insertion loss of the
first stage is presumably very low when it's properly tuned, the 2nd
stage will have a big effect on the pattern.  However, if the
insertion loss through the first stage is high for some reason, tuning
the 2nd stage will have minimal effect on the displayed pattern. After
you're done making the return loss look pretty, check the overall
response.  If the return loss looks good, the bandpass response will
also look good.  However, don't bother trying to tune it for best
bandpass response.  It just doesn't work, which is probably what your
friend is struggling with.

If you have access to the helical resonators, you can see which stage
is responsible for which dip by waving a ferrite or brass rod near the
coil.


Yep.  That's it.  Everything affects everything else, but when you're
done tuning for best looking return loss, the bandpass response will
magically be perfect.


Try googling for repeater cavity duplexer tuning procedures.  It's
very similar, except that you're tuning a notch instead of a peak,
with the added bonus of overly critical, white knuckle, adjustments.
<http://www.eagle-1st.com/eagle1st.nsf/17100001!OpenFrameSet>
<http://www.amtronix.com/duplexertuning.htm>

This Agilent app note looks interesting, but I don't have time to read
the whole thing:


A digital camera is your friend.  Take lots of photos so you at least
have a chance to put Humpty Dumpty back together again.

I dunno if it counts as an official method, but I've used a spectrum
analyzer+tracking generator. Each resonator makes a "bump" in the
passband; you can locate any given bump by grossly mistuning it (with
a hand-held cap, for example), and watching the bump jump back and
forth. Tune, repeat.

 

[mike]

Dishal's method. For high-Q filters, mistuned series sections look like
opens, and mistuned parallel sections look like shorts.

If you know what the resonant frequencies of the sections should be, you
can do that with a return loss bridge from each end.

Cheers

Phil Hobbs

Thanks, guys for the inputs.
Dishal's method was what I was looking for.

I'd found the HP article very interesting. I'll try the time-domain
stuff next time I get an opportunity.

I'm trying to do this remotely with someone with limited equipment.
We'll see how it goes... ;-)


The filter is
http://www.dci.ca/pdf/DCI-145-2H.pdf

Thanks, again, mike

 

[Simon S Aysdie]

Thanks, guys for the inputs.
Dishal's method was what I was looking for.

I'd found the HP article very interesting.  I'll try the time-domain
stuff next time I get an opportunity.

I'm trying to do this remotely with someone with limited equipment.
We'll see how it goes... ;-)

The filter ishttp://www.dci.ca/pdf/DCI-145-2H.pdf

Thanks, again, mike

Yeah, the (Dishal) method is the classic one for coupled resonator
filters.

Vizmuller has a book on helical filters, if you want something in
addition to the Zverev material.