Topband: Bias Tee Measurements (Test Data)

[Tom W8JI]

We cannot test impedance that way with a DC coupled impedance meter and get 
good data with any noise in the supply. Any test is meaningless. The .1 
capacitor will just couple any distortion or ripple in the AC to the 
analyzer, where it would show as jitter or false readings.

It would be a valid test if a moderately low resistance 200-500 uH RF choke 
shunted the analyzer (receiver) port.



----- Original Message ----- 
From: "Don Kirk" <wd8dsb at gmail.com>
To: "topband" <topband at contesting.com>
Sent: Saturday, August 23, 2014 10:48 PM
Subject: Topband: Bias Tee Measurements (Test Data)


> 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/
>
> ----------------------------------------------------------------------------------------------------------
> *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
> ---------------------------------------------------------------------------------------------------------
>
> *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
> --------------------------------------------------------------------------------------------------------
> 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).
> ---------------------------------------------------------------------------------------------------------
>
> *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)
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
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