Hi Jim,
This is hard to explain without showing you a graph of impedance vs.
frequency, but I noticed that Mr. Cebik explained it quite well in a
message dated 7:40 AM on 1/30/98. I hope I get my point across as well as
he does.
I'm not an expert, but here's my try:
A 1/4 wavelength long stub at frequency (x) will show the opposite
impedance from one end of the line to the other. If it's open at the far
end, it'll show a short. If it's shorted at the far end, it'll show an open.
A 1/2 wavelength (wl) stub, at frequency (x), shows EXACTLY the
impedance of the far end. If a 1/2 wl line is open at the far end, it'll
show an open. If shorted at the far end, it'll show a short. If
terminated with 50 ohms it'll show 50 ohms. If it's terminated with 5 ohms
of resistance and 45 ohms of capacitive reactance, it'll indicate that at
the feed end. Sort of like a mirror, but without any reversal. For this
to happen it must be a 1/2 wl line at frequency (x) or close to it.
For example:
A 1/2 wl long open stub for 28 MHz, exhibits a high impedance and is
passive on that band. It acts like a bandpass filter for 10 meters. Now,
that same exact stub is a 1/4 wl long on 14 MHz, and presents an electrical
short on that band.
Here's some more. When open stub or feedline exhibits an electrical
short at the feed end, it must be a 1/4 wavelength long, or multiples of
1/4 wl, at that frequency. That electrical short will repeat every 1/2 wl
down the line from that point, or a stub length of 3/4 wl, 5/4 wl, 7/4 wl,
etc. For example, if you were to sweep an open stub with an Autek RF-1,
sweep upward from a low frequency of, say, 1.2 MHz, and look for the first
broad minimum or null in impedance (Z). Once the low Z point is found,
switch from the Z readout to the frequency readout, and read directly.
This would hopefully be the frequency of initial electrical 1/4 wavelength.
It could be verified by then shorting the stub at the far end, and
sweeping like before. A short, or minimum Z, will now appear at the 1/2
wavelength point, or twice the frequency of the initial short found on the
open stub. My Autek manual says these minimum and maximum Z measurements
are quite accurate, and only a slight discrepancy might be found due to
second-order effects.
These shorts and opens occur every 1/4 wl or 90 degrees. And, it seems as
though the "Q" of the stub will determine the shape, or rate of change, in
impedance between these 1/4 wavelengths.
If you try measuring these things like I have over the past couple of days,
don't forget to take into account the velocity factor of the coax.
Good Luck. It's fascinating. de Steve, KL7DC
At 09:11 AM 1/31/98 -0500, you wrote:
>There still seems to be some confusion about the 1/4 wave versus 1/2 wave
>stubs for harmonic suppression. The confusion seems to be if the length is
>related to the band you are operating or the band you are trying to
>suppress.
>
>
>I think the problem for dummies like me is what you are referring to, is it
>that the stub on the 20 meter antenna line is a 1/2 wave long at 20
>meters............that is where the confusion is starting....or is it that
>that length of line is attahced on the ten meter antenna's line.........and
>is it still a half wavelength at 20 meters now or a 1/2 wavelength at 10
>meters.......
>
>With the ARRL DX around the corner and a few of us interested in multi-op
>and SO2R options this info is currently "in demand"
>
>
>If I understand correctly, to attenuate a 20 meter harmonic in the ten
>meter band, the 20 meter antenna line has a T-connector in it and off of
>that t-connector is a 1/2 wavewlength (at 20 meters) open stub........
>
>Did I get it right?
>
>(And I guess, the 40 meter feedline has a t-conn with a 1/2 wave 40 meter
>open stub on it to minize 20 meter QRM..........)
>
>DId I get it right?
>
>73 - see you in the FQP
>
>de
>
>Jim White, K4OJ
>k4oj@ij.net
>a proud member of THE Florida Contest Group
>...visit our website @
>http://www.4w.com/deemer/fcg.htm
>
>What's an FQP?
>Check out the new Florida QSO Party (its April 25 & 26, 1998) @
>
>http://home1.gte.net/wd4ahz/fcg/ssqp.htm
>
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