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Topband: Loading Coil

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Subject: Topband: Loading Coil
From: (
Date: Mon, 18 Oct 1999 21:32:33 EDT
Hello Tom,

I run a 68 foot half sloper (1/8 wave length on 160 meters) mounted on my 40 
foot tower, and really have a blast on 160 with it.  You can see my antenna 
in the March 1998 issue of QST.

Here are a few different lengths for your sloper, and a few different coil 
locations.  Efficiency is not as good with the coil at the feedpoint, but I 
found it very convenient to place it there for my situation.

I calculated the values below using 14 gauge wire as your sloping wire.  The 
formula I use is right out of the ARRL antenna handbook (Limited Space 
Section).  The handbook also has a chart that yields similar values.  I 
calculated the antenna for 1.8MHZ.  You can just shorten the end of the 
sloper to make it resonate up higher in the band.

For coil at feed point :
                                    Coil                    Coil
Half sloper length        XL (ohms)             L (uH)
80 feet                          376                       33.25
70 feet                          484                    42.80

For coil at mid point : 
                                   Coil                     Coil
Half sloper length        XL (ohms)             L (uH)
80 feet                          731                    64.6                    
70 feet                          927                    82.0

Let me know what kind of coil form you want to use (diameter, gauge wire, 
etc.), and I can give you some different winding combinations for the coil.

I had no problems getting my 160 meter half sloper to resonate, but the half 
sloper is dependent on many factors such as ground, top loading on the tower, 
etc, so be patient.

I am sure Tom (W8JI) and many of the other big guns on 160 meters laugh when 
they see such designs for antennas.  Nevertheless 160 meters sure is a fun 
band and I recommend putting up anything that will get you on the Top Band.

Don Kirk (WD8DSB) 

P.S. here are just a few of the countries I have worked in the last two 
seasons on 160 meters with my rinky dink 160 meter antenna.
VK, ZL, KH6, GW, G, TI, C6, HC8, HR, G, OZ, EI, HI, DF, IV                  

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