Article 2: XS-800 Antenna Measurement Bridge
By Phil Anderson, WØXI
If an RF generator and scope are part on your bench, then the Hybrid-Coil RF Bridge shown in Figure 1 can be used to measure the capacitance and resistance of your end-feed broadcast band (BCB) antenna. With known antenna impedance, your crystal set can be optimized. While a Wheatstone bridge is often suggested, the configuration usually mentioned requires that headphones or meter with diode be used and that either must be floated (not connected to ground), given that the generator is referenced to ground. Our bridge utilizes a trifilar-wound broadband transformer feed, enabling an oscilloscope to be used as the meter and referenced to ground.
Figure 1: Antenna Measurement Bridge.
The bridge consists of three sections: the transformer, T1, your antenna to be measured, and an RC series circuit with known values. T1 consists of twenty trifilar-wound turns on an FT-140-61 ferrite core. Black, yellow, and red #26 hookup wire were used. One end of the black lead was grounded, leaving the other for generator attachment. Since the bridge requires a center-tap, the red lead on one end of the core was connected to the yellow lead on the other end of the core, as noted by the “dots” on the transformer in the schematic. A 1 mH choke was added at the tap, connection “S,” to eliminate any 60-cycle buildup. The antenna connects to one end of the transformer, position “ANT.” The known components make up the other branch of the bridge. C1 is a calibrated 365 pf air variable capacitor, and C2 is a 240 pf mica capacitor. Switch S1 is closed to add additional capacitance. R1 is a 100 ohm ceramic potentiometer. S2 is opened (after antenna measurement) to measure R1 with a VOM. The bridge, of course, must be attached to antenna ground, shown at right.
My bench, for this application, consists of the following: a B&K Precision 4017B 10 MHz signal generator, a Tektronix 2445 150 MHz scope, the bridge, and a TES 2360 LCR Multi-meter. My antenna is 100 feet in length: 30 feet up to the eve of the house, 50 feet horizontal to the other end of the house, and a horizontal ninety-degree dog-leg of another 20 feet above the garage. A 500-watt AM station at 1320 kHz is five miles off.
Measurements are taken as follows:
- Set the generator frequency, e.g. 500 kHz, and voltage output. I usually use 10vpp. Generally, be quick and use the minimum voltage necessary to take the measurement, so as not to interfere with other listeners.
- Attach one channel of the scope to the transformer tap, point S. Set the scope timing such that a couple of RF cycles at the frequency of interest can be displayed.
- Close both S1 and S2.
- Attach the antenna.
- Adjust C1 and R1 for a minimum reading on the scope.
- Log the capacitance of C1 and C2, and open S2 to measure R1. C1 & C2 represent the effective capacitance of your antenna. R1 denotes the resistance and ground-return of your antenna system.
- If the variable capacitor has not been calibrated, open S1 and S2 and measure the capacitance with a meter, such as the B&K-810C.
Figures 2 and 3 display the results obtained at 1 MHz. Figure 2 shows the voltage obtained with C1 fully meshed. Figure 3 denotes the minimum voltage with C1 & C2 and R1 adjusted to match the antenna. Note at the minimum that my local station signal, 1320 kHz, is all that is left; and, it’s value is less than 100 mvpp. No 60 cycle energy appears in either case, as L1 was added to the bridge.
Figure 2: Scope Picture of C1 Fully Meshed.
Figure 3: C1 tuned to balance with antenna.
The following measurements were taken for my 100-foot end-fed antenna.
|Frequency kHz||Fixed C pf||Var-C pf||Total C pf||Rant ohms|
*local flame thrower.
It is interesting to note that the resistance of the antenna, Rant, averaged about 32 ohms. It rained heavily the night before the measurements. Normally, Rant is in the high 40s range. It will be interesting to see what happens over the winter. The above data was taken October 14, 2007.
(sidebar) Trifilar Winding.
N-filar winding of a coil means to wind N pieces of wire simultaneously on a form. To reduce capacitance between the wires, they are twisted together. It’s handy to use different colors for the wires, in order to ease identification after winding. My technique is to attach equal lengths of wire to the bench vice at one end and to the bit of a slow-speed drill at the other. I then run the drill at a slow speed and wind the wires together. The twisted combination is then wound on the form with the number of turns called for in the transformer.