Topband: Capacitor for Inverted L

[Charles Moizeau]

> From: wesattaway at bellsouth.net
> To: dj7sw at kcag.de; topband at contesting.com
> Date: Thu, 13 Oct 2011 12:23:41 -0500
> Subject: Re: Topband: Capacitor for Inverted L
> 
> The question of "matching" an Inverted-L comes up every now and then, and
> under some circumstances it may be necessary to install some kind of
> matching network, particularly if you want to operate across the entire
> band.
> 
> However, for most situations I do not think a matching network is necessary
> because the SWR will likely be easily "controllable" by simply adjusting the
> length of the horizontal wire...<
 

It is of course possible to achieve a better match at an antenna's feedpoint by altering the antenna's total length ("simply adjusting the length of the horizontal wire"), but doing so will change the radiation pattern of the antenna, and that may very well be a bad idea.

To illustrate, my inverted L antenna is 85 feet vertical and 85 feet horizontal.  The horizontal length was intentionally set at the 85-foot dimension.  This was because I wanted the point of maximum antenna current to occur at the midpoint of the vertical section, in order to ensure that the greatest proportion of total radiation would be from the vertical section.  

The point of maximum current in any antenna longer than a quarter wavelength occurs a quarter wavelength back from an open end.  I reckoned a quarter wavelength to be 128 feet, and simple arithmetic shows that my selected horizontal length puts my antenna's point of maximum current at the midpoint of the vertical section, just where I wanted it.

Of course I did not have a good match to 50-Ohm coax at the antenna's base.  However, knowing that the antenna's total length was such that it presented an inductive reactance at its feedpoint, all I needed was an equal amount of capacitive reactance to achieve a match.  A series capacitor did the trick.  No lossy inductors needed.  

Recent comments suggest using a variable capacitor with as much as 1,000 pF maximum capacity to ensure a match.  The "bread slicer" variable capacitor I used probably has a maximum capacitance of 600 pF.  I adjusted it for a minimum SWR and was able to obtain a ratio of 1.5 to 1.  Initially, that was with 14 radials, having individual lengths averaging 0.2 wavelength.  Now, with 55 such radials, I have a SWR of 1.1 to 1.  I've measured the amount of capacitance being used and it is about 255 pF.

73,

Charles, W2SH