We might *like* measuring antennas to be low-Q, but remember the bicon
shorting bars that were added some years ago? The skeleton bicons
lacking them inserted a rather sharp notch that had to be accounted for
in transducer tables. Baluns might go off from impact damage, too. And
so on. I gave an ebay 50 MHz comb good past 2 GHz to a former employer
who later gave all their own EMC lab gear to an NRTL.
But you still have to check.
For that matter, I've seen coax factors programmed with GAIN at a narrow
range at microwave frequencies; that was caused by reflection from a
crimped cable*. So we do need to be able to spot them.
*This was due to the ignorance of operators collecting the data, IMO. I
spoke to them and likely, they won't do it again. But they believed the
SA/TG reading because they didn't know what they were measuring. That's
another thread.
Though the >sweep< may be continuous, if you can read amplitude only by
marking pixels, you need to insure .5 BW or less per pixel. That's
straight sampling theory, right? If your analyzer has a 1024 pixel wide
screen you can't SEE less than about .1% of the scan width and can rely
on less. Does it matter? Often it doesn't.
Fun, isn't it?
Cortland
KA5S
On 12/6/2011 5:11 PM, Ken Javor wrote:
This discussion has gone far a field from the original post. I'm
assuming some sort of scan has been run and a continuous sweep is
available. The question is, how densely does that sweep need to be
digitized? Not as a practical matter, but as a matter of compliance
with standards or standard practice.
What manner of antenna would have performance as cited below where at
100 MHz the antenna factor is 10 dB and at 120 MHz it is 12 dB, but at
110 MHz it could be 20 dB? Let's generalize the question to any
closely spaced frequencies.
Let's look at the types of antennas available.
30-200 MHz: A half-wave tuned dipole is nowhere near that sharp.
Neither is a biconical.
200 -- 1000 MHz: A half-wave tuned dipole is nowhere near that sharp.
Neither is a logperiodic, log-spiral, nor a Yagi.
1 GHz+: Logperiodic, log-spiral nor pyramidal horns act the way surmised.
My conclusion, and the point here is to invite discussion, not close
it out, is that no test-type antenna is a high "Q" device. Antennas
can have arbitrarily high gains, depending on construction, but the
high gain is a geometrical quality, not a high quality factor in the
frequency domain.
-
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