Newsgroups: rec.radio.amateur.homebrew
From: gary@ke4zv.atl.ga.us (Gary Coffman)
Subject: Re: Newsgroup rec.radio.amateur.homebrew, Articles 8368-8397RE: Duplexors
Reply-To: gary@ke4zv.atl.ga.us (Gary Coffman)
Organization: Destructive Testing Systems
Date: Sat, 3 Jun 1995 14:24:49 GMT
In article <3qovbi$1br@huron.eel.ufl.edu> afn17891 writes:
>I am looking for the alignment procedeurs commonly used when setting up
>4 & 6 pack cavities for 2 meters... Either the down an dirty quick ways
>or the step by step methods usually used in tweaking in a set of duplexors.
>Any info would be appreciated
Here's a quick and nasty method. All you need is a pair of HTs, one with
an S meter, a step attenuator, an inline pad, and a terminator.
Terminate the Tx port, put a HT on the antenna port through the inline
pad, and hook the other HT to the Rx port through the step attenuator.
Now transmit with the HT on the antenna port at the repeater's *transmit*
frequency. Adjust the Rx notches for minimum signal on the Rx port HT,
adjusting the step attenuator as necessary to keep a midscale reading
on the HT S meter. Now switch to the repeater Rx frequency and peak
any bandpass elements of the Rx cavities for max signal to the HT on
the Rx port, again adjusting the step attenuator as necessary to keep
the reading at midscale (note, you'll probably want to operate the
HT on the antenna port at it's lowest power setting during this
adjustment). Iterate back and forth between these two sets of
adjustments for the best Tx frequency rejection combined with
the best Rx frequency pass.
Now switch the HT and attenuator from the Rx port to the Tx port
and terminate the Rx port. Go through the same procedures with
this set of cans, except reverse frequencies, IE peak the bandpasses
with the repeater Tx frequency and dip the notches at the repeater
Rx frequency. Iterate as needed for the best combination of notch
and pass.
You can now measure the insertion loss of the pass and reject
elements by noting the setting of the step attenuator for a
midscale reading on the HT then replacing the cavities with
a jumper cable and readjusting the step attenuator for the
same reading. The difference in step attenuator readings is
your insertion loss. Combining the numbers for insertion
loss for the Tx port to antenna and antenna to Rx port at
the receiving frequency of the repeater gives the isolation
in dB between Tx and Rx. It should be 100 dB or better to
prevent desense. The insertion loss from Tx to antenna on
the repeater Tx frequency should be low, under 3 dB, and
similarly, the insertion loss from antenna to Rx port at
the repeater Rx frequency should be low, under 3 dB.
Now you would be finished if the antenna presented a 50 ohm
non-reactive termination at the two frequencies, and if the
Tx and Rx presented 50 ohm non-reactive impedances at their
respective frequencies. But of course they won't. So more
work is left to be done. The reactance presented by the antenna
feedline must be absorbed into the Tx and Rx cavities immediately
adjacent to the antenna connection. The reactance presented by
the transmitter must be absorbed into the cavity immediately
adjacent to the Tx port. And the reactance presented by the
receiver must be absorbed by the cavity immediately adjacent
to the Rx port. Note: if the *real* part of the impedance on
any of these ports strays far from 50 ohms, you'll need to
work on the antenna, Tx, or Rx as appropriate to solve that
problem. Tweaks to the cavities will only absorb the reactive
part of the impedances.
So hook up the repeater in operating configuration and put an
inline wattmeter at the antenna port. Key up the transmitter
and make slight adjustments to the pass element of the Tx cavity
nearest the Tx port and the Tx cavity nearest the antenna port
for maximum indicated forward power.
Now hook up a SINAD meter to the receiver output and feed its
tone to one of the HTs on low power and terminated in a dummy load,
and probably shielded in a metal box as well. What you want is a
noisy signal in the receiver. Inhibit the repeater Tx and carefully
adjust the pass elements of the Rx cavities nearest the antenna and
the Rx port for best SINAD.
Now let the transmitter key up. Unless you're very lucky, the
SINAD reading will drop. Now slightly tweak the reject notches
nearest the Tx port and Rx port for best SINAD. Now slightly
tweak the notches nearest the antenna for best SINAD. If you've
done everything right, you shouldn't see any drop in SINAD now
with the repeater Tx on or off. You're done.
Gary
--
Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems | we break it. | emory!kd4nc!ke4zv!gary
534 Shannon Way | Guaranteed! | gary@ke4zv.atl.ga.us
Lawrenceville, GA 30244 | |
From: bobw@col.hp.com (Bob Witte)
Newsgroups: rec.radio.amateur.homebrew
Subject: Re: Newsgroup rec.radio.amateur.homebrew, Articles 8368-8397RE: Duplexors
Date: 4 Jun 1995 19:17:33 GMT
Organization: HP Electronic Measurements Division
NNTP-Posting-Host: la.col.hp.com
X-Newsreader: TIN [version 1.2 PL2]
Gary Coffman (gary@ke4zv.atl.ga.us) wrote:
: In article <3qovbi$1br@huron.eel.ufl.edu> afn17891 writes:
: >I am looking for the alignment procedeurs commonly used when setting up
: >4 & 6 pack cavities for 2 meters... Either the down an dirty quick ways
: >or the step by step methods usually used in tweaking in a set of duplexors.
: >Any info would be appreciated
: Here's a quick and nasty method. All you need is a pair of HTs, one with
: an S meter, a step attenuator, an inline pad, and a terminator.
I've used the Step-attenuator-plus-receiver-S-meter measurement technique
and it can be really useful for a variety of applications. I find
that a couple of step attenuators gives me some very powerful measurement
capabilities if I just calibrate a point or two on my receiver's S meter.
However, there are a few very serious caveats that must be considered,
especially when measuring something like a duplexor.
1. How good is the HT receiver? I bet Gary is thinking of a commercial
grade HT, not the typical amateur transceiver. Many modern ham HTs
will respond to signals away from the indicated receive frequency.
Many times this will not be a problem, but trying to make 80 to 100
dB dynamic range measurements (like when tuning a duplexor) with a
a leaky receiver will be difficult. (And if you do this "at the site"
are there any other transmitters on?)
2. There are enough interactive adjustments on a typical duplexor,
its really easy to get them off into the weeds, frequency response
wise. A simple single-point measurement technique will often not
be enough to get all the cavities tuned to where they need to be.
So there is a big difference between, say, tweaking up a duplexor
that was already tuned to the desired frequency and just needs to
be adjusted slightly and one that is tuned for, say, 463 MHz and
is being moved down to the ham band.
So my suggestion is find someone with a network analyzer, or at least
a spectrum analyzer with tracking generator.
Bob Witte / bobw@col.hp.com / HP / EMD / (719) 590-3230 / KB0CY / 7J1AUE
Newsgroups: rec.radio.amateur.homebrew
From: gary@ke4zv.atl.ga.us (Gary Coffman)
Subject: Re: Newsgroup rec.radio.amateur.homebrew, Articles 8368-8397RE: Duplexors
Reply-To: gary@ke4zv.atl.ga.us (Gary Coffman)
Organization: Destructive Testing Systems
Date: Sun, 4 Jun 1995 21:22:15 GMT
In article <3qt0sd$70i@nonews.col.hp.com> bobw@col.hp.com (Bob Witte) writes:
>
> So my suggestion is find someone with a network analyzer, or at least
> a spectrum analyzer with tracking generator.
Bob, that's fine for rough tuning, but those pieces of equipment
just aren't good enough for final tuning. That's got to be done
at the site with the duplexer hooked up the way it's going to
operate. I've never yet met a real installation where all the
terminations are 50.0+j0.0. The SINAD meter and the inline
wattmeter are the final tuning tools you have to use in every
case to get the duplexer working correctly with the real sources
and loads with which it has to contend.
Anything that will roughly get the duplexer in tune for that
final tweak is good enough, and a couple of HTs, etc are lots
cheaper than a network analyzer or spectrum analyzer and tracking
generator. Sure, I'll use them if they're available. I love high
buck test equipment with lots of knobs and buttons, and they do
make tuneup quicker, but they aren't essential. This is definitely
a case where you can substitute experience for bucks.
Gary
--
Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems | we break it. | emory!kd4nc!ke4zv!gary
534 Shannon Way | Guaranteed! | gary@ke4zv.atl.ga.us
Lawrenceville, GA 30244 | |