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ANTENTOP- 01- 2017 # 021

Another Approach to Hidden HF Antennas



Figure 4


Balun for Grasswire Antenna



How much wire?


A general rule of random wire antennas is to get as much wire in the air as you can - longer is better.  Does this still hold for the Grasswire?  The answer is no.  Measurements show that anything over a wavelength does no appreciable good.


My first measurement program parked a car on a dirt trail, with a spool of 18 gauge insulated wire unwound, one end tied to the bumper and the rest run on down the trail.  The dirt was average stuff, mostly clay and loam on top of granite.  At the car the wire was untied from the bumper, passed through a small RF toroid, and connected to an antenna tuner, the latter driven by a TR7 transceiver at approximately fifty watts.  The car itself served as a counterpoise.


A ten-turn secondary winding on the toroid drove a small diode and capacitor.  RF current in the antenna developed a DC voltage across the capacitor that I measured with a handheld DC voltmeter.  As the toroid slid along the wire, the voltage dropped and fell below 10 percent of the starting value a wavelength along the wire.


There was a small rise in voltage for a short bit farther along the wire, but at a full wavelength it fell below one percent, and never showed any further improvement. 

This occurred on 80 40, 20, 15, and 10 meters.


This measurement indicated that the current in the wire dropped almost exponentially along the wire, and beyond a wavelength was more than 20 dB down, so could produce little radiation.  The excess wire can simply be removed.  Thereafter my Grasswire deployments always used about one wavelength of wire at the lowest operating frequency.


Continuing the measurement at a later date, an assistant and I laid a center-fed wire dipole on a grassy field, 396 feet of insulated, 12 gauge wire - all that we happened to have handy.  Again a small toroid RF transformer and diode/capacitor, similar to the earlier one, had one side of the dipole threaded through it.  A fiberglass surveyor’s tape stretched from the center along the dipole to one of its ends.  The DC voltage, measured as a function of distance along the wire is a measure of the RF current.  Figure 5 and Figure 6 show the falloff of current along the wire at 7 and 29 MHz amateur Bands and its attenuation by at least 20 dB at one wavelength (seventy, and sixteen and a half feet, respectively.)

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Last Updated:

February 25, 2018 22:26