Topband: Loaded Beverages

[Tom Rauch]

I modeled a Beverage 250 ft long and six feet high. Using four loads 
spaced 20,40,60 and 80 percent, optimum pattern occurred with 
with 400 ohm reactance series connected inductors and 900 ohms 
termination.

If you double the loads the reactance for each will be 200 ohms.

It may be optimized further from this value, but the phase looked 
right (180 degrees shift in every 1/4 wl of length) so it is probably 
close to optimal. 

Back in the slinky threads, I tried to point out there is one optimum 
inductance per unit length for a slinky. It has nothing to do with the 
amount of wire in the coils except as that might change reactance 
per unit length. If reactance is too high, the antenna tries to fire in 
the reverse direction (null at termination end and response towards 
feedpoint). If reactance is too low, it doesn't have the best pattern 
you could obtain for a wire that spatial length. 

We sometimes have the misplaced notion that the length of wire in 
small helically loaded antennas is what sets the electrical length, 
but that isn't true. It is really the distributed inductance and 
capacitance. Results are the same no matter how you load the 
antenna as long as phase shift is about 180 degrees per 1/4 wl in a 
half wave antenna.

A string of beads, a string of lumped inductors, or a continuous 
helical (slinky) would all work the same. They are single band 
antennas when optimized. These will not be broadband antennas, 
because they depend on phase shift to "act longer" and the 
reactance will double with double frequency when you really need it 
to decrease dramatically as frequency increases!

73, Tom W8JI
w8ji@contesting.com


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