"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
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Shouldn't the 20m antenna be usable on 10m as well ? I am still digesting some of the terminology from your last post. It's a lot to keep straight. But obviously, it's a worthwhile experiment.
ReplyDeleteThe antenna is 1/4 wave on 10 and 1/8 wave on 20. So makes a "short" vertical on 20. By the way, this antenna is sitting on the ground.
ReplyDeleteA vertical should have about 35 ohms impedance with sufficient radials to offset grounding effect.
Need loading coil to offset high capacitive resistance and radials to increase impedance. At least that is my take on the above quote.