## Characterizing the AntennaIt's done with a signal generator, oscilloscope and impedance bridge. See "Building an Impedance Bridge" below. - Connect the test equipment to the antenna as shown below. Connect the UNK test cable ground to the antenna grounding system (ground radials) or the counterpoise in the case of a balanced antenna (dipole). If the antenna includes a feedline, connect the test equipment to the feedline at the feedpoint (the near end) where the transmitter would normally be connected. (The signal generator, oscilloscope and REF are grounded through the test cable shields and case of the RF bridge.)
- Adjust the signal generator to the operating frequency at maximum or near maximum output amplitude (avoid clipping).
- Adjust the reference potentiometer for a null. The null will be quite sharp and narrow. You may have to vary the signal generator frequency to find it. (A nearby AM broadcast station can really mess up this test. See "Optional Bandstop Filter" below for a fix.) If a null can't be found, replace the 100-ohm potentiometer with a 1000-ohm potentiometer and try again. Repeat as necessary to obtain the deepest null. Do not disturb the potentiometer setting after the null has been found.
- If you obtain
*total*nulling, with no residual signal present (ie, flatline), then the antenna impedance is non-reactive - purely resistive - at that frequency. Measure the resistance of the potentiometer to obtain the impedance. Z = R where: Z is the antenna impedance R is the measured resistance of the potentiometer in ohms.
- If you are not able to obtain a total null, leave the potentiometer undisturbed and proceed to the second pass.
#### Second Pass - find the reactive component (capacitive)Antennas shorter than ¼ wavelength (½ wavelength dipole) tend to have residual capacitive reactance. - Connect a variable capacitor (~500 pF)* in series with the potentiometer, as shown below (test lead grounds remain connected):
*Tuning capacitor from an old radio (sections wired in parallel).
- Adjust the variable capacitor to obtain the deepest possible null. You may have to slightly readust the potentiometer.
- If you obtain a flatline, the antenna impedance includes a negative reactive component (capacitive). Measure the potentiometer to obtain the resistive component. Measure the test capacitor to obtain the capacitance. The reactive component is calculated as follows:
X_{c}= 1/2πFC where: X_{c}is the capacitive reactance in ohms, F is the operating frequency in Hz, and C is the measured capacitance in farads.
The antenna impedance is: Z = R - jX_{c} where: Z is the antenna impedance R is the resistive (real) component in ohms and jX_{c}is the reactive component.
The antenna's absolute impedance is: |Z| = √R^{2}+ (X_{c})^{2} where: |Z| is the absolute impedance in ohms.
- If you are not able to obtain a total null, leave the potentiometer undisturbed and proceed to the third pass.
#### Third Pass - find the reactive component (inductive)Antennas longer than ¼ wavelength (½ wavelength dipole) tend to have residual inductive reactance. - Reconfigure the test setup with the test capacitor connected between the antenna and the UNK bridge port, as shown below (test lead grounds remain connected):
- Adjust the test capacitor to obtain the deepest null. You may have to slightly readjust the potentiometer.
- If you obtain a flatline, the antenna impedance includes a positive reactive component (inductive). Measure the potentiometer to obtain the resistive component. Measure the test capacitor to obtain the capacitance. The reactive component is calculated as follows:
X_{L}= 1/2πFC where: X_{L}is the inductive reactance in ohms, F is the operating frequency in Hz, and C is the measured capacitance in farads.
The antenna impedance is: Z = R + jX_{L} where: Z is the antenna impedance, R is the resistive (real) component in ohms and jX_{L}is the reactive component.
The antenna's absolute impedance is: |Z| = √R^{2}+ (X_{L})^{2} where: |Z| is the absolute impedance in ohms.
## First pass - find the resistive (real) component## Building an Impedance Bridge## The Bridge CircuitThe ground connections for REF (reference), UNK (unknown) and oscilloscope are not shown. The circuit's aluminum enclosure forms a common connection point for all grounds.
## Bridge InsideThe windings are made by first twisting the three wires together into a cable, then winding 5 turns of the 3-wire cable on the core. White = GEN, brown = REF, green = UNK, brown/green (twisted) = SCOPE. [This method of winding is called "trifilar".] ## Bridge ExternalEquipment grounds are interconnected via the common case. ## Test LeadsThe test leads for REF and UNK, constructed with RG-58 coax, are made the ## Optional Bandstop FilterIf your antenna is within a couple of miles of an AM broadcast transmitter, signal pickup from the transmitter can obscure the test signal on the scope, making it difficult to see a null. Including a bandstop filter in the test setup, tuned to the broadcast transmitter's frequency, will make life much easier. ## 1340 kHz Bandstop Filter ExampleCircuit assumes 1 megohm scope input impedance.
Design calculations: L ≅ 400/(2πF) C ≅ 1/(2π400F)
## As-built 1340 kHz Bandstop Filter
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