Jim,
Great explanation!
steve ve6wz
> The Beverage is a "traveling wave" antenna so in the "ultimate" case it would
> be infinitely long, and the power gradually radiates from the wire (and is
> absorbed in the ground under the wire). The angle of maximum radiation,
> relative to the wire, is related to the propagation speed along the wire
> (which in turn is related to the height and soil properties). The reason
> rhombics work well is that you have two wires at just the right angle, so the
> forward lobes reinforce.
>
>
> The idea of the termination resistor is that if your wave hasn't attenuated
> (or radiated) much by the time it gets to the far end, you don't want it
> reflecting and travelling back, creating radiation in the opposite direction.
> If you've already radiated most of the power, then the reflection doesn't
> contribute much to "backwards" radiation. For instance, if you have radiated
> 90% of the power by the time you get to the end of the wire, the 10% that
> reflects back from a mis-termination isn't going to be much of a contribution
> to a back lobe.
>
>
> One way to examine it in a model would be to look at the power dissipated in
> the load resistor - you can get the segment current, and you know the
> resistance, so you know the power from I^2R.
>
> There is a report online out there (I'll see if I can find it again) that has
> extensive measurements on traveling wave antennas (from the 30s or 40s) with
> different terminations, etc.
>
> They were hot stuff before curtain arrays for broadcast became popular. All
> those HF TTY links, etc. were done with rhombics. They're cheap and fast to
> install - telephone poles and wire, so they were popular for commercial point
> to point links.
>
> And W6AM was famous for his field of rhombics on the Palos Verdes Peninsula
> in the Los Angeles area.
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