Sv: Topband: Question abt gain in bi- vs uni-directional Beverage

[Tom Rauch]

Hi Henrik,

> As a non professional I wonder how this in done in practice. How do I
> measure the right Vp value as you describe. Please forgive me for maybe
> asking a dumb question.

Measuring Vp is very difficult.

The easiest way to determine the optimum inductance is with a 
modelling program like Eznec that allows you to insert loads. 
Otherwise, the process is long. What you want to do is add 
enough series inductance to slow the wave in the wire down 
(remember an inductor causes a current lag) and distribute it at 
points less than 1/4 wl apart.

The "slinky Beverage" does this, but there is no reason to use a 
mechanically difficult slinky. It works just as well to use a lumped 
reactance at point closer than 1/8 wl or so.

Like the Slinky Beverage, there is ONE optimum inductance per 
length. Too much inductance or too little per unit length, and 
performance diminishes. By the way, as inductance is added the 
impedance of the antenna increases. It is not ~600 ohms, it 
becomes higher. That's the case in any antenna like this that adds 
series inductance.  

Antenna Engineering textbooks like those by John Kraus describe 
the pattern formation in mathematical detail as velocity is slowed in 
the wire, but you'd have to calculate the phase velocity through the 
wire using lumped components knowing the impedance at that 
point on the antenna. It would be a very complicated task!

MFJ sells a book (written by someone who taught me a lot in early 
years) called "Antennas and Transmission Lines" by John 
Kuecken. It has a one page blurb on this in the section on traveling 
wave antennas. It isn't a detailed Beverage book, but it is a good 
book about antennas in general and how things work.
  
73, Tom W8JI
w8ji@contesting.com


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