[TowerTalk] Beverage Termination video

[jimlux]

On 5/28/20 7:07 AM, VE6WZ_Steve wrote:
> How important is it to get that Beverage termination resistor just right?
> Using 4nec2 I do some modelling to show that the RDF and far field pattern of a Beverage is not greatly effected by the termination resistor value.  


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.