Late last year Pete (N4ZR) reported problems with his home brew Bias Tee
that included radical shifts in measured impedance when DC voltage was
applied to his Bias Tee. Today I decided to make impedance measurements on
a simple Bias Tee circuit I recently proposed for Dwight (NS9I) who was
looking for a method of switching pennants via the feedline.
I was not able to duplicate the problems that Pete reported, but I did
notice unstable (noisy) impedance measurements when using a DC power supply
on the Bias Tee that had a lot of ripple when under load, and below is my
test data. My proposed Bias Tee schematic for NS9I is on my Pennant
website at http://sites.google.com/site/pennantflagantennas/
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*Test Data Using Resistor as the RF load *
66 foot of RG58U coax (measured Zo = 56 ohms)
Test Frequency = 4.545 Mhz (frequency where the coax was an electrical 1/2
wavelength)
RF Load = 50 ohm resistor
No Bias Tee (Bias Tee bypassed) :
R = 49, X = 0
Bias Tee (with 100 ohm 10 watt resistor connected to the Bias Tee DC output
port = 120mA load when 12 volts is applied), and well regulated DC supply :
0 Vdc R = 51, X = 0
+12 Vdc R = 51, X =0
-12 Vdc R = 51, X = 0
Note : when using a DC supply that had 1.5 volts peak to peak ripple the
measured R was jumping around between 46 and 53 ohms
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*Test Data Using Transformer with the RF resistive load *66 foot of RG58U
coax (measured Zo = 56 ohms)
Test Frequency = 4.545 Mhz (frequency where the coax was an electrical 1/2
wavelength)
RF Load : Transformer BN-73-202 Binocular core (Primary = 3 turns,
Secondary = 12 turns) with 1K resistor connected to secondary
Bias Tee (with 100 ohm 10 watt resistor connected to the Bias Tee DC output
port = 120mA load when 12 volts is applied), and well regulated DC supply :
0 Vdc R = 55, X = 0
+12 Vdc R = 55, X =0
-12 Vdc R = 55, X = 0
Note : when using a DC supply that had 1.5 volts peak to peak ripple the
measured R was jumping around between 49 and 58 ohms
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Note : For the above tests I was using an antenna analyzer that I designed
and built last year, and the detector is based on the VK5JST antenna
analyzer. The diodes used in the detector are germanium which have a high
enough voltage rating to allow impedance measurements on the Bias Tee
without the concern that W8JI had about detector diodes being damaged due
to high voltage exposure when measuring Bias Tees.
I repeated the 1st test shown above (Resistor as the RF load) between 1.4
and 12.5 Mhz and the results were similar (no change in measured impedance
between 0, -12, and + 12 Vdc applied Bias Tee voltage).
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*Conclusion*
The Bias Tee (concept based on an AD5X Bias Tee design) measured impedance
does not change between an applied voltage of 0 and +/-12 volts DC when
using a well regulated supply (based on the 120 mA load used in my test)
when tested between 1.4 and 12.5 Mhz.
Just FYI,
Don (wd8dsb)
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