[CQ-Contest] LP SO2R with no filters?

halken halken at comcast.net
Tue Jun 16 09:20:49 EDT 2015

Hi Mike - Pete and others have made some good suggestions.  SO2R without 
filters is certainly possible.  Many have done it, including myself.  
You can proceed risk-free with a simple test.  Transmit on one rig and 
receive on the other - same frequency.  Slowly turn the power up on the 
transmitter, starting as low as it will go. Blocking for an FT1000MP is 
around 80 dB over S9.  The damage threshold (conservative number) is 
around 90 dB over S9.  You can't see 90 dB over S9 on the S-meter on 
most rigs, and the damage threshold will vary from rig to rig, but by 
slowly raising the power on the transmitter you can get a feel for where 
90 dB over S9 is. You can also calculate this.  With a measurement or 
two you can calculate the isolation between antennas - every antenna 
combination for every band.  Using the isolation number and the 
transmitter power, you can calculate the power at the receiver.  A good 
rule-of-thumb puts the damage threshold around 50 mW or +16 dBm. For a 
100 W transmit power, you will need -35 dB of antenna isolation.  At 
1500 W you need -45 dB.  -35 dB is not an unrealistic number.  I have 
seen -50 dB and more in some cases.  Multi-band antennas will have the 
worst isolation.  If you are transmitting on a 20 meter dipole and 
receiving on a tri-bander, isolation may not be very good, but even that 
case can fool you and be better than you might guess.

If you see you will be getting into trouble, either by casual testing or 
calculations, there are at least three inexpensive ways to proceed.
One is as Pete suggested - on receive use the remote antenna jack with a 
receiver limiter in line.  Array Solutions makes one, as do others.  Or 
you can build your own.
Another approach is to build some simple stubs.  Stubs are good for at 
least 20 dB of rejection and may bring you not only below the damage 
threshold but also below the blocking level which is where you would 
ideally like to be.  A great resource is George, W2VJN's book: "Managing 
Interstation Interference."  It covers this subject in detail.  It's 
available from INRAD and is one of the best $20 I ever spent on the hobby.
The third option is to simply keep the transmit power down to where the 
power at the receiver yields something below 90 dB over S9. It's an easy 
way to get started...
Hal N4GG

On 6/16/2015 6:18 AM, Pete Smith N4ZR wrote:
> I did this for a couple of years - my main antennas were on a tower 
> about 250 feet from an HF9 vertical.  I was running over 1 KW to them, 
> and 100 watts to the HF9.  Did no damage - what's that adage about God 
> protecting fools and...?  If I were to do it again, I would put some 
> protection in the RX antenna lines - if your transceivers have 
> provision for bringing those lines out the back and back in again, 
> it's easy and cheap to build something - like 
> <http://pvrc.org/~n4zr/Articles/Simple%20Protection%20for%20the%20Fledgling%20SO2R%20Station.pdf>.
> 73, Pete N4ZR
> Download the new N1MM Logger+ at
> <http://N1MM.hamdocs.com>. Check
> out the Reverse Beacon Network at
> <http://reversebeacon.net>, now
> spotting RTTY activity worldwide.
> For spots, please use your favorite
> "retail" DX cluster.
> On 6/15/2015 5:53 PM, Mike Smith VE9AA wrote:
>> Has anyone ever done SO2R with no bandpass filters?
>> I am looking to enhance my contesting experience, especially for those
>> Sunday afternoon doldrums or times when condx really stink (no good 
>> bands to
>> be on) and nothing at all is happening on my "run' freq,. but am 
>> hesitant to
>> put out the cash. I have a very small stn (with even smaller cash flow)
>> consisting of mostly a HF9V vertical, some very low inverted L's on 
>> 80/160,
>> and low 10m yagi on a 20' pipe etc. Ie: No towers.
>> Looking around the property yesterday while mowing , there are a couple
>> spots 75-100' away I could located a low ZS6BKW (G5RV) antenna for 
>> the mult
>> radio away from the main cluster of antennas for the run radio.
>> I am especially hesitant as a well known local VE9 invested several 
>> thousand
>> bucks in SO2R (he could afford it, but I can't really) and tried it a 
>> few
>> times and ended up selling it all at a loss.(not to me! - I missed 
>> the boat
>> on that deal by a couple yrs)
>> I'd like to try it a few times. but not commit - if that makes sense.  I
>> find myself reading books, watching TV, looking outside, talking to 
>> family,
>> etc. while bored calling CQ.
>> I'd be OK using just 100w.  Maybe even get some surplus coax and make up
>> some stubs for some small amount of isolation if I thought I could 
>> gurantee
>> safety.  I made some measurements with a scope some months back but I 
>> was
>> "just at the edge of danger-maybe even into danger---but it was using a
>> closer antenna, now gone.  Maybe one farther away would do it?
>> Too risky..Ideas?
>> Mike VE9AA
>> Mike, Coreen & Corey
>> Keswick Ridge, NB

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