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
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