What happens if you have a single stub, and it is placed right at the
output connector?
Joe WB9SBD
Sig
The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com
On 10/5/2015 12:07 PM, Jim Brown wrote:
On Mon,10/5/2015 7:20 AM, Jukka Klemola wrote:
Jim,
I thought placing double stubs on the TX line is not so critical.
The two band stopping stubs are supposed to be about quarter wave
apart; on the harmonic band; and that should effectively remove the
need for extended optimization.
Also, in my experience, if you get approx 30 or 35dB attenuation
using one stub, you will get more than 45 dB attenuation over the
whole band when measured in a 50 ohm system.
Sure -- using my DG8SAQ VNWA, I measured peak attenuation of these
double stubs of 55 dB for the 80M stub pair and 59 dB for the 40M
pair. BUT a monoband antenna is NOT a 50 ohm system at the harmonic
frequency, and the transmitter is NOT a 50 ohm source at the harmonic
frequency.
Most monoband antennas present a very high Z to the line at their 2nd
harmonic, which establishes a very high SWR for the harmonic, so the Z
varies over a wide range along the line. A stub works by placing a
short on the line at the harmonic frequency, and is most effective if
placed on the line where the Z is high. It is MUCH less effective when
at a place in the line where the impedance is much less than 50 ohms.
SO -- with double stubs separated by 90 degrees at the harmonic, the
second stub will always be at a high Z spot on the line, but the first
will depend on where it is along the line.
There is a second issue. By their nature, the output stages of modern
power amps, both tubes and solid state, produce 2nd harmonic that is
only about 6 dB below the fundamental, and must be filtered by the
amplifier's output network. Most of these networks are designed for 50
ohms, and that is how they are tested. If the last element of that
network is a capacitor, it will be most effective if it sees 50 ohms
or more as a load AT THE HARMONIC, and least effective if it sees a
short. Likewise, if the last element of the filter is an inductor
(Pi-L), it will be most effective if it sees Z of 50 ohms or less.
I rigged two power amps, a Ten Tec Titan and an Elecraft KPA500 with a
voltage tap at their output terminal into a dummy load and into a
double stub network in line with my 40M dipole. With the stub feeding
either the antenna or the dummy load, the second harmonic at the
output of both power amps varied by +/- 10 dB as I added short
sections of coax to vary the length of the line between the amp and
the stub over more than 180 degrees at the harmonic frequency.
SO -- if we put the stub in the "right" place for the power amp, we
get full performance of it's harmonic suppression network plus the
suppression of our stub(s), but if we put it in the "wrong" place we
can lose as much as 20 dB less of the 2nd harmonic suppression of the
output network.
At least my measurements show these results:
-placement not critical for double stub
As you can see from above, that's only true if the antenna is near 50
ohms at the harmonic.
-attenuation almost doubles in dB compared to one stub
Yes.
--> I have been building only double stubs nowadays.
Me too.
Another point. When you have done this amount of suppression, you will
likely begin to find other sources of 2nd harmonic, as W3LPL recently
observed (perhaps on another reflector), AND you may also find leakage
paths in your antenna switching system. I recently replaced a vintage
Six Pack with a 6x2 sold by 4O3A. I measured crosstalk in the 4O3A
unit before installing it and the Six Pack after removing it and found
the 4O3A unit had more than 20dB better isolation on 20M.
In hopes of picking up another 6-10 dB of isolation, I am also
planning to replace some of the random vintage patch cables in my
station with newly made jumpers using high quality RG213 and Amphenol
83-1SP connectors.
73, Jim K9YC
73,
Jukka OH6LI
2015-10-05 9:57 GMT+03:00 Jim Brown <k9yc@audiosystemsgroup.com
<mailto:k9yc@audiosystemsgroup.com>>:
On Sun,10/4/2015 2:59 PM, Jeff AC0C wrote:
That's a great way to start. Especially as the solar cycle
fades and 10/15 are not open. Then in the day, you run 40/20,
and in the night you run 80/40.
Yes, but there's a LOT more to it if you want to maximize the
effectiveness of the stubs. It can matter a LOT (20-30 dB) where
along the line stubs are placed, both with respect to the antenna
and to the power amp.
See my piece in NCJ one issue back, or download it from my website.
For our CQP expedition, I made up double stub packs for the 40M
and 80M CW stations only. Each pack was two stubs cut to kill the
second harmonic, with a quarter wave (at the harmonic) connecting
them. I didn't have time to optimize their placement, but using
two stubs insures that you'll get at least 25-30 dB, and with
optimized placement, another 25-30 dB.
Stubs are less important on SSB because the likely operating
frequencies are not directly harmonically related, whereas the
harmonic of 3525 hits 7050, and 7025 hits 14050.
73, Jim K9YC
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com <mailto:CQ-Contest@contesting.com>
http://lists.contesting.com/mailman/listinfo/cq-contest
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
|