Doing the SO2R Math

The Tough Part of SO2R is Isolation

Gearing up for SO2R involves a 2nd rig, some switching and software facilities, and a lot of isolation work.  The isolation comes in a combination of antenna coupling, band pass filters, traps, preslectors and other filtering.  Just how much is needed?

For the transmitted signal to reach the receiver's input, it needs to pass through several items which have loss.  These include:

Item	Typical Isolation
Antenna to antenna coupling	15-40 db
W3NQN type BPF	40 db (25-60 db)
Stubs or traps	20-30 db
Receiver preselector	0-40 db

You can see from the values shown that the typical ranges are so large, we can't really assume any of these as a given.  They are best measured.  There are a number of ways to check the coupling using the rig's s-meter or VNA.  And they are well documented in W2VJN's book Interstation Interference.

[ UPDATE:  The actual values have been measured following the Gen 5 Array updates - see this LINK ]

There are two categories of attenuation to be considered.  Adjacent band and in-band.  Let's look at each now...

 Isolation to Prevent Rig Overload and Damage

Say rig A is listening on 15m.  And rig is transmitting on 20m.  Depending on the amount of signal that reaches the 15m listening rig from the 20m transmitted signal, we may overload or even damage the listening rig.

What do the numbers look like?

A conservative rule of thumb to avoid damage would be to limit the rig's exposure to 0 dbm signals.  And for overload, that depends on the rig.  But for out-of-band signals, I am going to assume that any rig is able to withstand a -33 dbm (S9+40) signal level.  So for the purposes of my SO2R setup, I am targeting 0 dbm in the worst case, and -33 dbm as a goal.

How does the math work?

Item	Level	Net
1.5 KW	+62 dbm	+62 dbm
Antenna coupling	-36 db	+26 dbm
W3NQN BPF	-40 db	-14 dbm

OK.  No problem!   The BPF will see around 1/2 watt of exposure in this case which is well within the capabilities of the 5B4AGN BPF modules.  And the overall antenna signal level (with the BPF) is clearly in the safety zone from a rig damage standpoint.  

From a redundant standpoint, I would hate to accidentally bypass one of the BPF (or encounter an unplanned failure) - the result of which would let the +26 dbm signal level hit the rigs front end.  So I will plan on adding stubs onto any of the elements measuring under -60 db of coupling which will limit exposure of the rigs to RF under any circumstances to around the 0 dbm level.  

 Isolation to Prevent In-Band Noise and Signals

Say rig A is listening on 15m.  And rig is transmitting on 40m.  The transmitting rig will have some level of 3rd harmonic energy output. And that gives our 15m rig problems in two ways - first is a in-band signal of some amplitude - which may be very strong - at 3x the 40m transmitting frequency. And secondly, the transmitting rig's phase noise will be present in the harmonic as well - but it will be much more wide-banded. Both of these interference products, being in-band signals, are not affected at all by the receiver side filtering.

So what signal level is acceptable? Here, in the ideal case, I would like to have the harmonic signal to be no stronger than a typical signal on the band - that is S9 or -73 dbm. So that becomes the target value. Of course, higher levels may not provide any problem - but that will depend on the rig's relative performance. For a baseline, -73 dbm is a good target.

The math is similar to the rig protection case in the prior section. But the target signal levels are much lower.

The task is not as hard as it first sounds in that the harmonic output of the rig is already significantly reduced giving us a big head start.   And since the target level is now -73 dbm instead of 0 dbm or -33 dbm, we will need it. Unfortunately, when an amp is part of the chain, the harmonic attenuation level can again be a problem...

Here's the math:

Item	Level	Net
1.5 KW	+62 dbm	+62 dbm
Rig + amp harmonic attenuation (on transmitter side)	-30 db	+32 dbm
W3NQN BPF (on transmitter side)	-40 db	-23 dbm
Coax notch trap (E.g.:  15m trap on 40m ant)	-20 db	-43 dbm
Antenna coupling	-22 db	-65 dbm

So even though we are targeting a very low absolute level of -73 dbm, our quick rough estimate of the problem is one that is not difficult - assuming we add in one harmonic notch filter onto the transmit antenna.  

Note that I have listed the harmonic content here as -45 db.  Based on the QST product reviews of the FT-2000 and FT-5000, the actual suppression levels are about -60 db or better.  That means the signal levels may be closer to -80 dbm (around S8) - and it may take away our reliance on the coax notch trap. 

Note that all these calculations assume a "linear" linear amplifier. The impact of the amp may in fact be a richer level of harmonic content than the rig itself.

However, building coax notch traps is pretty easy so that will be part of the plan from the start.  And making coax notch traps is another cool use of the N2PK VNA!  See here for more details on trap construction.

Note:  For an assessment of the FT-2000 and FT-5000 preselectors, see this page.