Topband: Inverted L Dimensions

[k8gg at voyager.net]

 Scott, KB0FHP wrote:

I have decided to take the plunge
and put up a simple antenna for 160M. 
> Several comments: 
> 
> It has to be simple and not excessively expensive; 
> I have limited room for antennas on my lot, and very limited room
for 
> radials. 
> I do have multiple tall trees that I
can use to hoist a wire 60 feet + in 
> the air. 
> My
soil, if you want to call it that, is poor and rocky (typical Eastern 
> PA). 
> My application is casual DXing and working
stateside. 
> 
> Based on this, it looks like an inverted
L is probably my best choice. I 
> want to feed it with 50 ohm
coax. Now the questions are pretty simple: 
> 
> What
dimensions are best - 5/16 or 1/4 wave? Both are indicated in the 
> literature. 
> What is simpler to match? 
> Am I
better off making a loaded vertical approximately 60 feet long? 
>

> I appreciate your help. 
> 
> Scott aka
KB0FHP 


OM Scott and fellow Topbanders,

 



 

My thoughts are as follows:

 



 

It doesn’t matter much whether one makes an inverted-L of 128
or 160 feet for 160 meters unless the vertical portion of the wire is 80+
feet high so that a series capacitor can tune out reactance without
further matching.

 



 

With a 160 ft long 160 meter Inverted-L antenna (5/16 wavelength)
about 60 feet high with 100 feet of horizontal wire, the feed point
impedance at resonance will be about:

 

Zant = 30 + Rg + 200j

 

where Rg is the ground loss, and  + 200j is the inductive reactance caused by making the
antenna longer than 1/4 wavelength (resonance).

 



 

Similarly for a ¼ wavelength 160 meter Inverted-L antenna
approximately 60 feet high and 68 feet horizontal, the feed point
impedance at resonance will be approximately:

 

Zant = 16 + Rg

 

Again, Rg is the ground loss, but since the antenna is a resonant
1/4 wavelength there is no reactance at the resonant frequency.

 



 

The easiest method to get 50 ohms is to make a 1/4 wl Inverted-L,
and ignore the SWR at the feedpoint, feed it directly with good quality 50
ohm coaxial cable into the shack and then use an L-network antenna tuner
to match the impedance at the shack end of the coaxial cable to 50
ohms.  TenTec model 229 or 238
antenna tuners, or some of the military surplus L-network antenna tuners
work very well for this application. 
There will be very little loss from SWR in the coaxial cable at 1.8
MHz.

 



 

If you want to have 50 ohms at the antenna end of the coax, there
are a couple of methods that work with 1/4 wl Inverted-L antennas.

 

A.  Make the antenna
wire approximately 7 feet longer than what is needed for resonance and
then put a shunt capacitor across the coax where it feeds the
antenna.  This makes an
L-network where the extra 7 feet of wire is the series inductor of
approximately 2 uHy.   If the impedance is 15 ohms, then
the approximate shunt capacitance  = 2500 pfd. 
If the impedance is 25 ohms, then the approximate shunt  = 1740 pdf, etc.

 



 

B.  Use multi-tapped
UnUn impedance transformers. 
Coil Winding Specialist, Inc.  sells one of these called “UN-MULTI”.  Their data sheet circuit diagram
is not quite correct but their multi-tapped UnUn will provide a good match
for various antenna impedances including 12.5, 16.3, 22.2, 28.1 and 36.8
ohms.  Their UnUn website page
is:  http://www.cwsbytemark.com/index.php?main_page=index&cPath=206_214

 

I have purchased and tested one of these and it seems to work
OK.  However, I have not had
one on the air for long periods of time.  Amidon made a similar model, but I no longer find it in
their on-line catalog.  If you
decide to use one of these commercially wound UnUn’s I would put it
in a weather resistant container.

 



 

Over eastern 
Pennsylvania
 rocky soils, make sure to use many radials.  Normally the minumum recommended
would be 30 radials, each 66 – 70 feet long.  Since the soils are known to be
poor, I would recommend installing another 1000 feet of radial wire as a
minimum, so that one has approximately 40 radials each 75 feet long or 36
radials each 80 feet long.

 



 

73 & Good
Luck     George  K8GG