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W8OFC    
Marana, Arizona


 

 

 

 

   ANTENNAS - Page 2 of 3
 
 
   W8OFC 20 METER "BIRDCAGE" ANTENNA
 
This is the latest of my antenna projects; and, as I'll explain in greater detail later on this page, I'm really quite surprised and pleased about how it performs.

As you can see, the antenna sits right on the ground, does not have any loading coils, and does not require any radials.  It is 7 1/2 feet high (that includes the 18 inch legs) and the sides are 4 foot wide.

The PVC supporting structure looks like a birdcage so that's how I've been referring to this antenna.  However, it should not be confused with the real Birdcage type antenna, which is an old design and is electrically different than this antenna.

 



Actually, this antenna is simply a variation of a standard full size center-fed half-wave dipole.  The feed point is in the center of a 6 foot copper-tube vertical section.  At the upper and lower ends of that 6 foot section, 2 foot copper-tube horizontal sections then attach to the upper and lower 4 foot square PVC loops.  At each junction of the copper-tube and the loop, a wire is attached to the copper section and goes about seven-eighths of the distance around the loop, the wire currently being held in place with tape.  The upper and lower wires were each trimmed an equal amount to achieve resonance in the 20 meter band.

The higher current portion of this antenna is in the vertical section; so this antenna is essentially a vertical dipole antenna and the pattern should, therefore, be omni-directional.  To prevent any rf from coming back on the shield of the coax and thereby distorting the pattern, I feed the antenna through a homebrew ferrite balun at the antenna feedpoint.



NOW THE IMPORTANT PART.  HOW DOES IT PERFORM?

Well, I've got to admit that I didn't initially expect a lot from this antenna.  This was one of those projects that I had aways wanted to try, but I built it as cheaply as I could so I wouldn't have too much money wasted if it didn't work.  Now I wish I would have built it out of something more substantial than PVC!  This sucker really works!  I guess a version 2 will have to be considered.

OK - now to some specifics.  First of all, the antenna I have been comparing this one to is not exactly a "world-beater" antenna, although it does seem to work moderately well.  It is an 83 foot long doublet fed with balanced feedline and tuned with a balanced L tuner.  I have used it weekly for the past 3 years as Net Control operator of a 20 meter net with check-ins from all over the country, so I would consider it as "satisfactory".

So how does the "Birdcage" antenna sitting on the ground compare?  Better(!), in almost all cases.  After a week of comparing literally hundreds of received signals from one antenna vs from the other, there was only one instance where the doublet was clearly better.  That station was local, about 10 miles away.

Except for that one case, around 50 percent of the stations received averaged 1 to 2 "S" units higher on the "Birdcage" and there were 2 instances where I could not even hear the stations using the doublet.  On the other 50 percent of the stations, I could discern no difference on the "S" meter, although MANY sounded louder because there was (is) much lower background noise when using the "Birdcage" antenna.

I realize that one could question whether the "Birdcage" is that good or is the doublet not very good.  It is probably a little of both!  I do know that the "Birdcage" is a definite improvement over what I have been using; so, after I "beef it up" a little, it will be my primary antenna for 20 meters until I come up with the next "better antenna".  That's the fun in building and experimenting with different antennas.

SOME ADDITIONAL NOTES:

The attachment point for each wire loop to its corresponding copper-pipe section is a nut-bolt arrangement, so that different length loop wires with terminal lugs on the ends can be easily substituted for use on 17, 15, or 10 meters.  I plan to try 15 meters next, and I'll update this web-page when I get some results to report.

I'm sure this antenna would work just as well if wire was used in the sections where I used copper tubing.  The use of the tubing is only to add some rigidity, particulary needed due to the weight of the ferrite balun attached to the center of the vertical section.

As far I as know, there is nothing sacred about the height and width of this antenna (unless I arrived at it just by luck).  Primarily, the height was determined by the copper tubing being available in only 10 foot lengths.  Also, I didn't want to make it any taller because I live in a CC and R restricted neighborhood and don't want to "press my luck".  The circumference of the PVC loops was dictated by the length of wire needed to make the antenna resonant on 20 meters.

The one thing that initially made me think that this antenna might not work well is the bottom loop's proximity to ground.  In fact, the reason I settled on 18 inches was not based on anything electronic; it was the longest that the PVC legs could be without making the whole thing very unstable.  In experimenting with that distance, it seemed to make very little difference in the resonant frequency, which is primarily a function of the overall length (changed by adjusting the length of the loop wires).  But, what effect does the proximity to ground have on efficiency, take-off angle, pattern, etc......?  Where's an antenna expert when I need one?

UPDATE: OCTOBER, 2010
CLICK HERE for the latest info on this great little antenna.


IF YOU ARE PLANNING TO BUILD THIS ANTENNA, HAVE BUILT A SIMILAR ANTENNA, OR HAVE SOME COMMENTS OR SUGGESTIONS, I WOULD ENJOY HEARING FROM YOU.  SEND ME AN EMAIL BY CLICKING ON THE "Email Me" BOX AT THE BOTTOM OF THIS PAGE.


 
 

 

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