Topband: Bias Tee Measurements (Test Data)

[Don Kirk]

Hi Tom,

Thanks for the clarification, and I don't disagree with your most recent
posting except I consider a well regulated supply to be one that has low
ripple, but that might be where you and I went astray on this posting
(sorry about that).  My posting was not to tell folks that they should
repeat my same test since my antenna analyzer has diodes that likely have a
higher voltage rating than some of the antenna analyzers commercially
sold.  I was just trying to follow up on the postings earlier this year
regarding Petes Bias Tee problems in an attempt to shed some light on the
topic.

P.S. another ham thought it would be a good idea if I repeated my test
using a battery, so here are the results (same feedline with 200 ohm RF
load and 100 ohm DC load), and the battery yielded the same results as my
well regulated DC supply (well regulated meaning low ripple and stable
average voltage).

Freq : 1.505 Mhz
Bias Tee (+12 Vdc regulated supply) : R = 30, X = 32
Bias Tee (-12 Vdc regulated supply) : R = 30, X = 32
Bias Tee (0 Vdc regulated supply) : R = 30, X = 32
Bias Tee (no power supply, no battery) : R = 30, X = 32
Bias Tee (+13.06 Vdc SLA battery) : R = 30, X = 32
Bias Tee (-13.06 Vdc SLA battery) : R = 30, X = 32
No Bias Tee : R = 26, X = 27

73, and thanks for the lively discussion.
Don


On Sun, Aug 24, 2014 at 10:52 AM, Tom W8JI <w8ji at w8ji.com> wrote:

> 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
>