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[TowerTalk] Shortened Dipole on 160M

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Subject: [TowerTalk] Shortened Dipole on 160M
From: (Tom Rauch)
Date: Wed, 19 Sep 2001 11:37:51 -0400
Hi All, 

This idea does not work for wide-band frequency changes as 

> >Mike: This is a very good report.  The L has been a very good antenna
> >even back in the 30's.  I started a trend of making it longer than a
> >1/4 wave and adding a variable BC variable in series to tune out the
> >XL from being too long.  It also raises the Rr closer to 50 ohms or
> >even above for 70 ohms coax. A COUPLE OF FOLKS RECOMMENDED THIS,
> >SOUNDS LIKE A GOOD POINT BUT== <SNIP>  The vacuum variable is nice
> >but is a problem to adjust from the shack.  The 1000 pf BC variable
> >does its thing in less than 180 degress across the whole 160 m band. 
> ><SNIP AGAIN> The idea is to get the best average SWR curve from 3.5-4
> >MHz or 1.8-2 MHz or the range of frequency you will use.  This
> >procedures should be obvious but there are still some that can't
> >grasp the idea. 

Using an inverted L for 80 meters, with the L cut for band center at 
160 ohms resistance, and fed through 17 meters of RG-71 (93 
ohm, 1/4 wl @ 3.7MHz) coax, and adjusting the variable for lowest 
SWR in the shack we get the following with 1500 watts:

3.5 MHz 

Antenna Z = 97.34 +j666 

Capacitor voltage = 3700 v peak (big receiving variable!!!) 

Shack impedance = 92.6 j0

SWR = 1.85

Cable loss = 58 watts (over cable rating)

4.0 MHz

Antenna Z = 457 j1611

Capacitor voltage = 4150 v peak 

Shack impedance =  19.3 j0

SWR =  2.5:1

Cable loss 65 watts

The SWR varies from 1:1 at band center to 2:1 at the low end and 
2.5:1 at the high end. It really isn't any better than a regular 
Inverted L with 50 ohm feeder. 

It is always a good idea to model or actually build things before 
deciding they work. This is the best case loss, with the reactance 
cancelling capacitor at the antenna. Things get much worse if the 
capacitor is moved inside the house!!!

73, Tom W8JI 

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