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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|