[CQ-Contest] Coax Stubs for SO2R

[Jukka Klemola]

Nobody has answered this Joe's question as the pileup roars on thing that
are easier to comment than ideas about improving the station.


2016-07-19 16:26 GMT+03:00 Joe <nss at mwt.net>:

> OK,
> We all have here been discussing harmonics.
>
> While I have heard harmonics, and these stubs help greatly with that
> problem.
>

Excellent !


I personally get more results from protection from ummm, wide band noise?
>

That is additional good news.


Maybe that isn't the right term,


That can be called whatever phase, oscillator, or some other more creative
noise. They all are under the headline wide band noise.

The stub attenuates RF energy from the transmitter on the harmonic while
the stub passes the energy on transmitter's actual band.

There is the harmonic signal, many kinds of oscillator or mixing products
or other phenomena your transmitter creates on the harmonic band.

Using a stub, they all are attenuated on the harmonic band.



> but say someone in on 40 CW and you are on 20 any mode,
> You are not hearing the direct harmonic, but more of think a 2 meter
> repeater without duplexors. a either sucking of the band where each time
> the 40 station transmits, the 20 station looses it's sensitivity, like it's
> being desenced.
>

Not many have ever seen a 2 meter repeater, not to mention to be able to
recognize there is a stub as a part of the 2m repeater. That stub pair is
called duplexer.
Many know triplexer.
It is basically the same thing. But, instead of being a two radio duplexer
the triplexer is a three radio device as most cq-contest readers know.

In our case, 40-20, 20-10, 40-15 40-10 interaction are harmonics.
But there are also other  effects we attenuate by the stubs. Those are the
same as in the 2m repeater case, where the filtering is used for cleaning
up the signals for other reasons. Like 20-15 interaction; there is no
harmonic but there are other kinds of phenomena we are avoiding with the
filters.


So, this tells me there might be another symptom you write about.
Having a really big signal on the band may cause sensitivity degradation
throughout the band.

This phenomena is not only the dynamic range of the receiver, but it may be
also something else.

In case the harmonic from TX on 40 sends a signal that is S9+55dB on the
20m receiver at your multi-radio station.
The 20m radio can became less sensitive so much you really notice it.
This phenomena is not noise from the transmitter but is a feature of the
receiver.

The dynamic range is the separation of the receiver noise level to the
maximum signal the receiver can accept without compromising the receiver
properties. This is, the dynamic range can become a problem with all the
phenomena it brings in.

There is more, however. I try to simplify so you get the idea:
Our receivers align themselves to accommodate these big signals.
This automation is called AGC all of us have seen on the receiver front
panel.

AGC is used to equalizing the sound pressure level we get into our ears to
accommodate nice and easy reception of small signals like S3 signals while
the signals at S9+10dB sound pretty much the same despite the difference is
a whopping 40 deciBels.

By this automatization, we do not need to adjust gain knobs by 40 dciBels -
the AGC adjusts that 40dB for us. AGC was patented by Harold L. Beverage
himself if I am not mistaken.

So, if there is a really big signal outside the band you hear but is within
the receiver front end filtering, the AGC may adjust the RF gain so your
receiver's sensitivity drops on the actual signal you are trying to copy.
This same thing can happen to a DSP radio as well. There is analogue
circuitry as the front end and it can limit the input based on strong
signals the receiver gets.

The AGC is largely the same phenomena if you are driving during dark hours.
You see all kinds of dim and medium level lights,
Then someone has really bright driving lights.
Your eyes adjust the sensitivity so the dimmer lights disappear and you see
only some of the brighter medium level lights and that one huge light
source,


**Alternatively:

The receiver in this case may be such there is no tuned front end filtering.
It means, the AGC may get the gain adjusting triggering signal from the
transmitter frequency that is on the other band.
This phenomena was a part of earlier Ken K1EA message under this same topic.
You can see several watts or even tens of watts at the 20m antenna
connector when there is a transmitter on 40m; even if the 40m transmitter
has been cleaned of 20m energy..
Now, we place a 40m rejecting stub on the feedline of the 20m radio; of
course with the 20m receiver.
This stub attenuates the 40m signal and it saves the 20m sensitivity
degradation. Or at least improves the situation.

Such 40m reject 20m pass stub is the lowest possible cost, while being
simplest to build, to improve the 20m operating at a multi radio station if
the 40m transmitter has already been cleaned by 20m filtering.

For those interested in cutting some coax and soldering:
http://www.k1ttt.net/technote/k2trstub.html
and
http://audiosystemsgroup.com/Coax-Stubs.pdf
  -low end of page 11/13


There is more:
Not many have noticed the NB; noise blanker; may also get the other radio
at the multi-radio-station as the triggering input.
That is, the stubs help NB as well.



> OR
>
> not a de sensing, but an elevated noise floor pulsing with the 40 CW think
> 2 meters aurora contact.
>
> Are the stubs really helping with these two wide signal problems ,, or am
> I dreaming that they are.
>

You have to dream harder.
The stubs help with at least four kinds of problems at a multi radio
station.

Nobody want to contest at a multi radio station without band2band filtering.
Stubs are the best cost per dB band filters.

When increasing station performance, after double stubs, if you still need
something better, you need to build actual resonating filters.
This was brought up by Tom W8JI on this same technical topic some time ago
on this same cq-contest email reflector.


Joe WB9SBD
>

Jukka OH6LI