I'm quoting from Jerry Sevick's (W2FMI) book "The Short Vertical Antenna and Ground Radial". A short antenna has been defined as one that is small compared to a wavelength.
He gives the following equation on p. 10:
Beta*h less than 0.5
where Beta equals 2*pi / wavelength and h is the height of a ground-mounted vertical.
BTW: the "less than" sign confused the HTML publishing process so had to remove it.
Solving for h gives us the following equation:
h less than 0.5 * wavelength / 6.283
For 40m antenna, the h is less than 3.2 m. If we use 130 feet for the wavelength, then h is less than 10.35 feet. The author uses 11 feet for his purposes.
He maintains that a 40m 1/4 wavelength ground-mounted antenna has a power gain of 1.62 when compared to the mythical isotropic radiator. This antenna is 33 feet high. The 11 foot vertical has a power gain of 1.513! Pretty impressive, don't you think? But HOW?
He further comments that "the very small value of its input resistance" is the important property that makes its capture cross-section nearly equivalent of a full 1/4 wave vertical. A ground mounted 1/4 wavelength vertical has a theoretical input resistance of 35 ohms and a 20 foot has an input resistance of about 8 ohms while the 10 foot antenna has about 2.5 ohms input resistance.
I'm not sure I understand all of this either, but it is very interesting (to me anyway). Well, I have some more to reading to do....AR
Showing posts with label W2FMI. Show all posts
Showing posts with label W2FMI. Show all posts
Wednesday, March 25, 2009
Monday, March 02, 2009
Short Vertical Antennas
"The Short Vertical Antenna and Ground Radial" book by Jerry Sevick, W2FMI, is loaded with thought-provoking information. I have been reviewing this book because I'm using a home-brew vertical produced by Tulsa Amateur Radio Club for $20. The 10 meter portion of this antenna just fits on my patio cover support post (4x4 wood post and I used a bunge cord to secure the antenna to it). I have been tuning this for 10 meters as a 1/4 wave-length and on 20 meters as a 1/8 wave-length. I have made contacts on both bands so I'm looking for ways to improve the efficiency of the antenna, i.e. more radials perhaps. So I've returned to Sevick's book for some insights.
In the preface, he says "...a very short antenna had about the same power gain and radiation pattern as a full size half-wave antenna. The main difference was that the resistive component of the input impedance, the radiation resistance, was very small in comparison to that of a 1/2-wavelength antenna and, depending on length, could be a matter of a few ohms. In turn, the short antenna has a very high capacitive reactance, which has to be canceled by various loading techniques. A short vertical has an even lower radiation resistance and, depending upon height, can be a matter of only 1 or 2 ohms."
A little background to put this in perspective might help. The antenna referred to is ground mounted and, as such, one of the main features of performance is signal loss to the earth. A half-wave that is ground mounted will have an impedance of about 35 ohms maximum when the ground losses are minimized. A short vertical will have a 1 or 2 ohm impedance with no radials.
One of the charts in the book shows that as radials are added, the antenna load impedance goes up (or gets better) because ground losses are lowered and eventually reaches a max of 35 ohms with 60 radials. Right now I'm using only 4 so I have lots of room for improvement.
His book spends a lot of time developing a short vertical for 40 meters, so my 20 meter antenna should be a little easier(?) ... AR
In the preface, he says "...a very short antenna had about the same power gain and radiation pattern as a full size half-wave antenna. The main difference was that the resistive component of the input impedance, the radiation resistance, was very small in comparison to that of a 1/2-wavelength antenna and, depending on length, could be a matter of a few ohms. In turn, the short antenna has a very high capacitive reactance, which has to be canceled by various loading techniques. A short vertical has an even lower radiation resistance and, depending upon height, can be a matter of only 1 or 2 ohms."
A little background to put this in perspective might help. The antenna referred to is ground mounted and, as such, one of the main features of performance is signal loss to the earth. A half-wave that is ground mounted will have an impedance of about 35 ohms maximum when the ground losses are minimized. A short vertical will have a 1 or 2 ohm impedance with no radials.
One of the charts in the book shows that as radials are added, the antenna load impedance goes up (or gets better) because ground losses are lowered and eventually reaches a max of 35 ohms with 60 radials. Right now I'm using only 4 so I have lots of room for improvement.
His book spends a lot of time developing a short vertical for 40 meters, so my 20 meter antenna should be a little easier(?) ... AR
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