On Mon,7/18/2016 12:08 PM, Kenneth Wolff wrote:
Jim right of course -- the exception being the old Alphas with Pi-L
outputs.
There are many amps with Pi-L outputs. There's a table of those I've
been able to learn about (with help from friends) at this link
http://k9yc.com/Coax-Stubs.pdf
At the the old K1AR M2 we ran from my house we used a lot of stubs because
we only had two towers.
Stubs made with hard line have less loss and are deeper, but narrower
nulls.
Yes, and they can be TOO narrow, locking you into a narrow range of
operating frequencies.
Fixing 40 into 15 is a little trickier because it as a third
harmonic. I used an open 1/4 stub on 15m with a compensation stub to tune
out the imaginary part of the leftover conductance on 40.
Double and even triple stubs slightly offset across the band can be used to
make a broader null.
Yes.
The most important thing to do is to measure the coupling from each
transmit antenna into each other antenna using 10w transmit and either a 50
ohm scope or a calibrated receiver.
Yes, a very good move. But when we're talking stubs for harmonic
suppression, we must measure that coupling at the frequency of the
harmonic.
I made a spreadsheet of all the coupling (transcontuce) between antennas to
plan my defense between bands. Some bands needed only a receive filer.
1500W on 160m used to couple over 50W into the 80m vertical array. You can
smoke receivers if you don't watch out!
Great advice. A vector network analyzer or a calibrated spectrum
analyzer and signal generator can also do this well.
73, Jim K9YC
- Ken K1EA
On Mon, Jul 18, 2016 at 12:04 PM, Jim Brown <k9yc@audiosystemsgroup.com>
wrote:
Your logic is exactly backwards, Jukka, and it applies ONLY to optimizing
the relationship between the stub and the power amp. When the output
circuit element is a capacitor, the stub should be 1/4 wavelength from the
amplifier at the harmonic it is intended to suppress. When the output
circuit element is an inductor, the stub should be directly at the
amplifier output, OR a halfwave up the line at the harmonic.
But the relationship between the antenna and the stub also matters a LOT.
A stub works by shorting the line at the frequency of the harmonic, and
will be most effective when applied at a HIGH Z point on the line AT THE
HARMONIC FREQUENCY. Most single-band antennas present a HIGH Z at the
second harmonic, so the stub should be some multiple of half waves from the
antenna.
Note also the advantage of double stubs in portable setups where length of
the line between antenna and stub cannot easily be controlled. Even if the
first stub (nearest the antenna) is poorly placed, the second stub, a
quarter-wave nearer the transmitter, provides its full attenuation.
All of this is covered in the applications note for which I posted a link
last night.
http://k9yc.com/LocatingStubs.pdf
The next link shows how we implement stubs in the trailer that W6GJB put
together for or CQP and 7QP expeditions. Note how close the 80 and 40
antennas were to each other in the last setup, and the 40M dipole and C3SS
were to each other in the daytime setups. At 600W from an Elecraft KPA500,
we had zero problems with harmonics with CW stations on both bands.
http://k9yc.com/7QP.pdf
73, Jim K9YC
On Sun,7/17/2016 10:37 AM, Jukka Klemola wrote:
Placing a stub at output connector helps a tad in case your output circuit
that shows a low impedance to ground at the harmonic. That is typical for
nearly all our equipment that have a low pass circuit with a capacitor
going to ground as the last component of the output circuitry; a Pi
circuit
output.
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