[TowerTalk] 40 -2cd

[Frank Norton KB8XU]

At 03:24 PM 8/14/2001 +0200, you wrote:

>Hi folks.
>
>It's up again on the tower my 40-2cd.But i don't see any dip anymore.
>At 24m high it wass all ok.Just a dip on 7050.0 (swr 1.1)so very good.
>a moved it to the top at 28m.Now there is no dip anymore .at 7050 the lowest
>is 1.5 and going to 1.8- 2.0 at the lower or higher end.>SNIP<
>Shoud a take it down and tweeking the elements ?
>or is it just normal with this one ?
>
>Marc - on4ma
>73

Hi Marc,

It looks like u have a good dip, as well as full band coverage--very 
nice!  To perhaps shed some light on the 1.5 minimum SWR you might review 
characteristics of a center fed dipole in the "Antenna Book" from the 
ARRL.  The 40-2cd being a dipole with a director should demonstrate very 
similar behavior to the dipole alone.  The characteristic impedance, both 
resistive and reactive components, are essentially the same as the dipole 
alone.

Long story very short--1.45:1 is the best SWR one should see on a center 
fed dipole UNLESS u play a matching trick and place an inductance across 
the feed point of the dipole.  On 40 meters I would use 10 turns of #14 
solid copper with turn diameter of about 2.5 to 3 inches.  Surprisingly 
u'll need 8-9 feet of wire for this (I wind around a short section of PVC 
pipe but u can use any round form u can find, even a can since the form 
won't be staying in the coil.)  Once u make the coil I would make sure the 
solder lugs are solid to run full power and space the turns about 1 inch 
apart to begin with.  u may need to compress or separate the coils somewhat 
if u want a perfect match to 50 ohm cable but this will do quite well as 
is.  The only other point to remember is be sure and have the coil extend 
opposite the feedline balun.  (by the way, a 1:1 SWR may make u feel 
better, but all this work will only gain u about 5% improved power transfer).

Please note:  If u already have ur antenna up on the tower and if it is at 
all difficult to get at--I would leave it as is and enjoy....from ur 
description u have a well matched 40 meter beam.

73 es enjoy! de Frank kb8xu


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