[TowerTalk] Voltage at ends of yogi parasitic elements?

[Steve, W3AHL]

Rob,

Since I needed to retune a portable 75M dipole, I took advantage of the opportunity to measure the voltage at the end.  I also modeled the dipole in EZNEC, duplicating the as-installed configuration, to hopefully verify the model’s accuracy.  The dipole was installed on my 60ft mobile tower trailer with the center at 32ft, the far end at 30ft and the test end at 11ft.  Testing was done at the new resonant frequency (3.64 MHz) that resulted from the lower overall height and one end being at 11ft.  My K3 was placed near the test end, connected to the dipole with 60ft of RG-8X and the TX power set to 10 W.

A Tektronix THS720 portable scope was placed on a 10 ft step ladder near the test end, with a 100 MHz Tek 10 megohm probe.  The probe is only rated for 300 vPeak at 3.5 MHz, but I exceeded that a little to get up to 10W TX power.  A ground reference wire was run to the top of the ladder and connected to a ground rod and the K3 chassis with about 16 ft of wire.  

EZNEC predicted the voltage at the end would be 276 vRMS at 10 watts at the feed point, using the zero amp current source object volt meter object referenced in previous posts.

The actual measured voltage was 265 vRMS with the probe’s ground lead unconnected or connected to a 3 ft floating wire (to simulate what would be connected to the other side of the relay in your Yagi).  With the probe’s ground lead connected to the reference ground wire the voltage dropped to 230 vRMS, presumably due to the additional resistive load from the probe’s internal resistor.   The voltage reading is very sensitive to any movement of the wire or my body, due to the very high impedance of the wire element’s end.

It is always comforting to confirm actual measurements agree closely to the EZNEC calculated values!

So back to your 40M wire Yagi with relays at the ends of the REF and DIR to reverse direction.  Based upon a model I have (used by our club for FD) at 1500 watts the REF end voltage is 2178 vPeak at 7.2 MHz resonance, rising to 3156 vPeak at 7 MHz.  The DIR is 1973 vPeak and the DE is 4474 vPeak, both at resonance.  Your actual antenna’s design should be modeled, since it may differ.  And of course a considerable safety margin would be nice in the relay’s voltage rating.

Steve, W3AHL