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Tom Donaly wrote:
I know you're champing at the bit to claim my test setup was all wrong and that I used standing wave current when I should have been using traveling wave current, and all that, but you'll just have to wait. That's not true at all, Tom. It's a trivial procedure to use standing waves to determine the approximate 1/4WL self-resonant frequency. But as with 1/4WL monopoles, there is *NO 1/4WL self-resonant frequency* without a ground plane or counterpoise and the quality of the ground-plane/counterpoise has an effect on the self- resonant frequency. Sorry, I don't have anything approaching a perfect ground plane for 4 MHz. I suppose one could use two identical coils and turn it into a dipole but I don't have another 75m Texas Bugcatcher loading coil. Let me say once again: A 1/4WL monopole, all by itself in free space is *NOT resonant*. After all, making guy wire segments 1/4WL long is one way of breaking up their resonance. We are not looking for super high accuracy/precision/resolution here. Almost everything is an approximation because we don't share exactly the same test environments. All I am after is the technical truth - there's nothing personal involved. Neither is a 1/4WL coil self-resonant all by itself in free space. I don't know what Reg was thinking if he advised hanging the coil from the ceiling without a counterpoise. A traveling wave can be used to determine 1/4WL self-resonance but it is a little more complicated than using an MFJ-259B with standing waves. A load resistor minimizes reflections while current probes are used to measure the phase shift through the coil. When the phase shift is 90 degrees, that's the 1/4WL self-resonant frequency. I'll tell you what, Cecil. If you'll test your coil, and describe how you did it, I'll tell you what I did. Fair? 1. I connected my 75m Texas Bugcatcher coil through about a foot of wire to the bumper mount on my GMC pickup and connected my MFJ-259B to the coax connector under the bumper. I tuned for lowest impedance above 4 MHz. The 1/4WL self-resonant frequency was ~6.6 MHz. These measurements were done and reported in March, 2006. 2. I used a tri-mag mount sitting on the hood of my GMC pickup fed through ~6' of coax. The 1/4WL self-resonant frequency was ~6.9 MHz. 3. I used a traveling wave on top of a wooden bench and found the frequency at which the phase shift through the coil was ~90 degrees. That frequency was ~7.2 MHz. 4. I modeled the coil with EZNEC and got some segmentation length warnings. EZNEC reported the 1/4WL self-resonant frequency to be 7.96 MHz. The spread in the above frequency figures is about +/-8%. Again Tom, the only thing I am after is the technical truth. If that is also what you are after, we should have no personal conflicts. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
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