Topband: Bias Tee Measurements (Test Data)

[Tom W8JI]

> Tom, you said "Any test is meaningless", and I don't understand where you
> are coming from on that statement.  If you look at my test, I tested with
> and without the bias Tee, and I tested with a well regulated bench top
> linear power supply, and the results are similar so I don't see how you 
> can
> say you can't do that.  I then went and tested with a wall wart power
> supply that produced 1.5 volts peak to peak ripple when under my 100 ohm
> load on the Bias Tee DC port, and it did indeed produce noisy data as you
> said would happen.  I believe my test shows that you can indeed make valid
> measurements as long as you are using a well regulated supply.

I understood your test to be an indictment of using an unregulated supply, 
and that a well regulated supply cures issues. Of course what you intended 
to convey and what I thought you intended to convey might be two different 
things. If you only meant the filtered dc (unrelated to regulation or lack 
of regulation) allowed a reading in that case, I agree. It allows a reading 
in that case, BUT it is still dangerous to use a low voltage diode test 
device on that line to test things.

Because it can damage test equipment, I do not think it is a valid public 
test protocol for the general population.

The issue is not regulation. The issue is noise or ripple making it into the 
analyzer port. We can run well regulated dc with ripple, or even unfiltered 
unregulated dc into the line and just clean it up at the relay end so the 
relay does not chatter, and get a valid dc test as long as test equipment is 
not sensitive to low frequency noise.

We could never test ac, irrespective of filtering, regulation, or waveform, 
without skewing measurements. This means testing a four-way system with a 
bias T and ac, or rippled + or - dc, or with cable ground loop ac voltage 
offsets on the shield, can result in false readings. It won't affect the 
receiver at all, but we might think the system has a problem or damage our 
test gear.

The best way to improve the test method and increase reliability is to make 
the measurement device insensitive to offset on the output. It is better to 
remove ground loop low frequency bias or coupling through the capacitor by 
making it a highpass filter for low frequencies, although I probably still 
would not switch the relays with my test equipment connected unless I 
confirmed no transients first. :-)

The problem actually comes from the reactance of the series coupling 
capacitor and the sensitivity of many cheap measuring devices to out of band 
voltages. A solution that reduces low frequency offset from external ground 
loops and allows ac or unfiltered dc operation, is adding a shunting choke 
on the RX port.

So to clarify, I am saying:

1.)  the problem is not regulation, it is noise or ripple
2.) a filtered or regulated supply does not solve the ac mode test issue
3.) switching can result in a high voltage transient that can damage test 
gear
4.) low frequency ground loops might still inject ripple on long cable runs, 
or with poor shield connections
5.) regulation will still not allow an ac switch test

There is a second caveat I have about switching high impedance lines. We 
have to be very careful about relay contact and wiring capacitance. Just 10 
pF of contact capacitance is 8k ohms on 160 meters. We only have a coupled 
load to leakage path ratio of 10 times if we switch an 800 ohm line. That 
same leakage path to load becomes a 100 ratio if we use a transformer to 80 
ohms at the switch point.

Now I absolutely understand we will see an empirical "good F/B ratio" with 
some pretty dismal relay isolation, and of course it will not show as an SWR 
issue at all. I'm not disputing switching high Z lines will still make many 
people happy. When I build a system, I probably look at it differently. If I 
could have 15-20 dB of relay isolation when switching or 30-60 dB of relay 
isolation, and the difference only costs a couple dollars, I'd probably give 
up a cheeseburger and buy the piece of mind from switching low impedance 
points. If the transformers were $25 dollars each, I might not do that. My 
cheapness might take over. :-)

Simplicity is a wonderful thing as long as the saving a few pennies does not 
cost us dollars of joy over time.  When all the little mistakes we don't 
notice are added, we might be worse off than we assume. I'm simply offering 
a more reliable or accurate way to do things.

73 Tom