[Antennas] Impedance questions...

[Ronald KA4INM Youvan]

> Verticals with a ground plane bent downward around 45� has an 

> impedance of roughly 50 Ohms, 


   So true, this antenna is called "ground plane" and it was designed by
the communication division (mobile radios) of RCA, two radials are required,
all others are to make it look symmetrical to you.  The angle varies the
character impedance of the antenna.

 > ground planes 90� from the radiating element have an impedance
 > of roughly 30 Ohms...

  So true, this antenna is called a "quarter wave vertical" )previously
`Marconi') the more radials you have the better it is. (although the
changes become small after a while)

> Why do 99.9% of the mobile antennas come with 17' of co-ax?


  I have one with 33 feet (2 meter) and one with 15 feet (2 meter and 440 MHz).

> Is this a magic number for something?


   I don't think so.

> On a 1/4 wave antenna with  horizontal ground plane, can the impedance 

> be changed by altering the lengths of various elements without horribly 

> effecting the radiation pattern?

   I don't think you can effect the impedance much, other than reducing the
losses by improving the counterpoise, which would give you a slightly worse
match to 50 Ohm coax and better actual efficiency.

   In your E-gram you don't mention what band you are thinking about,
the 17' thing might be meaningful on one band, but I suppose it relates
to the distance from the rear bumper or trunk lid to the under dash
transceiver for mobile operation.
Base antennae seldom come with attached coax.

        73 (= Best Regards) de: Ron ka4inm@tampabay.rr.com
     100% LINUX, since July, 1997   SENT Time and Date are UTC
        Visit my HAM Web SITE at: http://www.qsl.net/ka4inm