[Trlog] SO2R and Front end Protection

Tree tree at kkn.net
Fri Oct 6 12:08:57 EDT 2006

On Thu, Oct 05, 2006 at 08:51:24PM -0400, WG0M at aol.com wrote:

> I need input on how to protect the rigs front end during SO2R  operation when 
> each rig is running a kw.

Well - I might be able to offer some tips based upon my experience, but would need
to know a bit more about what your station looks like and what bands you expect to
be operating at the same time.

Some protection circuits are horrible for generating harmonics - which can result
in more interference.

At my station, I typically use some 1/4 or 1/2 wave stubs to take care of 
problems.  However, I did create a new situation I didn't think out ahead 
of time, and ended up blowing up my 20 meter radio during the CW Sprint 
when I had my KL7 beam in the mix when transmitting on 40 with only 20 
feet between the antennas.  :-)

Here is one practical example on how to use stubs - to help with one rig 
on 40 and the other on 20:

If you have a 1/4 wave stub cut for 40 meters (mine your velocity factors) 
with the far end shorted will exhibit a high impedance on 40 meters, but a 
low impedance on 20 meters.  This can be put on your 40 meter tranmission 
line to prevent 20 meter energy from getting to your 40 meter radio.

If you take the same stub and open the far end, it will be a low impedance 
on 40 meters and open on 20 meters.  Put this on your 20 meter coax. 

With this setup, you can likely operate without any damage between the two
rigs - unless your antennas are very close.

You can use this same concept to help when operating on 80 and 40 at the
same time - and if you think about 1 wavelength on 20 being very similar
to 1/2 wavelength on 20, you can make 80/20 meters work as well.  20 and
15 are a bit more of a challenge, but I typically just make a low 
impedance for the band I want to attenuate and not worry too much about 
losing a db or two on receive for the band I am operating on.

There are some combinations that are harder to deal with using stubs - and
you might need to think about some bandpass filters.  However, with my 
antenna situation, I have been able to solve most of my problems without 
resoting to this costly alternative.

If you want to really get into this, W2VJN "wrote the book" on this stuff
and you can find his book on the inrad website:


One final note - use good coax for these stubs - and they quickly start to
lose their effectiveness if you are using lossy coax.  Probably more of an
issues > 10 MHz.  Most of mine are made out of RG8X.

If you don't understand velocity factors - go brush up in the ARRL handbook.
Signals travel slower in coax than air, and you have to correct for that
when computing the length of a stub.  I use a SWR meter to verify mine, often
cutting them a bit long and pruning them to have their max or min impedance
on the target frequency/band.


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