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.

bridge schematic

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:

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.

scope picture

Figure 3: C1 tuned to balance with antenna.

2nd scope picture

The following measurements were taken for my 100-foot end-fed antenna.

Antenna Capacitance Versus Frequency
Frequency kHz Fixed C pf Var-C pf Total C pf Rant ohms
250 240 139 379 32
500 240 160 400 32
750 240 196 436 32
1000 240 261 501 30
1250 240 306 546 34
1320* 240 335 575 33

*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.