Just wondering, why use stubs at first place? What's the advantage compared to
LPF or BPFs?
Rudy N2WQ
Sent using a tiny keyboard. Please excuse brevity, typos, or inappropriate
autocorrect.
> On Jul 17, 2016, at 12:11 PM, Joe <nss@mwt.net> wrote:
>
> 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
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>> 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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