Topband: Bias Tee Measurements (Test Data)

[Don Kirk]

Hi Tom,

I also just went and changed the RF load resistor to 200 ohms and checked
readings at various frequencies where the impedance was part reactive.  I
also added a 3rd part to the test in which I physically removed
(disconnected) the DC power supply from the Bias Tee.  All tests done using
my well regulated powder supply set at 12 Vdc for the Bias Tee DC voltage
and 100 ohm load on the Bias Tee DC output port.  Impedance readings are
identical with the Bias Tee connected to the DC power supply versus not
connected to the DC power supply and slightly different when the Bias Tee
is removed from the circuit (all of which we should expect).

Freq : 5.672 Mhz
Without Bias Tee : R = 50, X = 45
With Bias Tee : R = 46, X = 39
With Bias Tee (but power supply connections removed from the circuit) : R =
46, X = 39

Freq : 1.501 Mhz
Without Bias Tee : R = 26, X = 27
With Bias Tee : R = 31, X = 32
With Bias Tee (but power supply connections removed from the circuit) : R =
31, X = 32

The Bias Tee has a slight impact on the impedance but it has nothing to do
with the DC power supply that powers the Bias Tee (as long as it's a well
regulated supply), and the slight change in impedance is what we would
expect with a well designed Bias Tee (the impedance change is real and due
to the component selection within the bias tee).

Therefore I say (based on my tests) that valid impedance measurements can
be made on a Bias Tee using an antenna analyzer as long as the DC power
supply feeding the Bias Tee is well regulated.  I understand your concerns
about the voltage ratings on the detector diodes used in the MFJ antenna
analyzer not being adequate for measuring Bias Tee impedance, but that's an
entirely different issue.

Just FYI, and hope you agree.
Don (wd8dsb)


On Sun, Aug 24, 2014 at 2:06 AM, Don Kirk <wd8dsb at gmail.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.
>
> Please explain in more detail why you said it can't be done?
>
> Thanks,
> Don (wd8dsb)
>
>