KG6ABF
Member
Please take note the link below goes into great detail on this subject. Please see the link for charts, graphs and data to fully explain this area. I offer this in an effort to help readers and Amateurs better understand this area when building, erecting and using dipole antennas.
It covers frequencies from Amateur Bands from 160 meters through 50 MHZ and that is well beyond a simple "low band antenna" and illustrates how the impedance is effected by both frequency and height over ground. It also doesn't discuss other factors such as the ground's reflectivity, conductivity and other important considerations.
Putting up a 160 meter dipole, an 80 meter dipole or even a 40 meter may be a problem for some exceeding a 1/4 wave distance from ground. At 80 meters a band available to Technicians, General and Extra Class operators is for example approximately 66 feet, for a 160 meters is double that (132 feet) while 40 is a little better in that 33-35 feet off the ground may be sufficient. Coupling with the ground can occur however and depending upon the soil composition effects the antenna in a number of ways. You can find soil composition on the USGS website and determine what reflectivity and refraction, absorption your particular soil has and then compute the length and height needed to maximize your dipole antenna.
This brings us back to impedance and how frequency/height/soil factors determine what is needed to obtain a match and if it is possible considering the factors necessary in erecting an antenna for use on frequencies from 1.8 Mhz through 50 Mhz or beyond. Since impedance vary not only from band to band but ALSO within the band itself, a dipole for example can be built for specific (limited or Narrow Banded) bandwidth or made to cover a wider range of frequency (Broad Banded).
Couple the antenna height with other factors such as location to trees, lamp post, telephone/electrical wires and poles that have to be avoided in placement of a simple dipole. These other objects also effect not just radiation pattern, but can effect on the impedance due to the shape one must use to put it up. Not everyone in the city, apt, residential areas can run 132 feet or more for a flat top dipole. An inverted Vee antenna covering 80 -10 meters and the peak at the say minimum height for 1/4 wavelength off the ground for 80 meters (Lowest band of operation) at 66 feet and two legs running down at 45 degrees still involves some serious amount of real estate. Now add in ordinances from the County or City limiting height or antennas and now you might have an 80 meter dipole at 30 feet, or a 40 at 25 feet. Look at the charts and see what your impedance is. How about the scenario where you can't run a dipole straight down from the center and it has to be snaked around a few trees or other objects?
I think my point had been made and anyone reading this can think or examples where less than optimum conditions for an antenna are present.
Usually dipoles of this nature are used on several bands and as K7MEM reminded us it is important to consider a Balun (UnUn) or even a coupler or matching device such as a tuner when buying or building one or more of these antennas.
If you have a few minutes to look over the info below, I cut and pasted some of the material from the link for review. As you can clearly see it shows the changes of antenna impedance comparing height, frequency from 160 meters through 6 meters or the 50-54 mhz, basically the HF and low VHF band allocation for US Amateur stations. The page has charts, graphs and lots of other info you can review if you like beyond the dipole discussion.
From : Palomar Engineers
Dipole Antennas
Folded dipole antenna basics
[Edit: copyrighted material deleted]
It covers frequencies from Amateur Bands from 160 meters through 50 MHZ and that is well beyond a simple "low band antenna" and illustrates how the impedance is effected by both frequency and height over ground. It also doesn't discuss other factors such as the ground's reflectivity, conductivity and other important considerations.
Putting up a 160 meter dipole, an 80 meter dipole or even a 40 meter may be a problem for some exceeding a 1/4 wave distance from ground. At 80 meters a band available to Technicians, General and Extra Class operators is for example approximately 66 feet, for a 160 meters is double that (132 feet) while 40 is a little better in that 33-35 feet off the ground may be sufficient. Coupling with the ground can occur however and depending upon the soil composition effects the antenna in a number of ways. You can find soil composition on the USGS website and determine what reflectivity and refraction, absorption your particular soil has and then compute the length and height needed to maximize your dipole antenna.
This brings us back to impedance and how frequency/height/soil factors determine what is needed to obtain a match and if it is possible considering the factors necessary in erecting an antenna for use on frequencies from 1.8 Mhz through 50 Mhz or beyond. Since impedance vary not only from band to band but ALSO within the band itself, a dipole for example can be built for specific (limited or Narrow Banded) bandwidth or made to cover a wider range of frequency (Broad Banded).
Couple the antenna height with other factors such as location to trees, lamp post, telephone/electrical wires and poles that have to be avoided in placement of a simple dipole. These other objects also effect not just radiation pattern, but can effect on the impedance due to the shape one must use to put it up. Not everyone in the city, apt, residential areas can run 132 feet or more for a flat top dipole. An inverted Vee antenna covering 80 -10 meters and the peak at the say minimum height for 1/4 wavelength off the ground for 80 meters (Lowest band of operation) at 66 feet and two legs running down at 45 degrees still involves some serious amount of real estate. Now add in ordinances from the County or City limiting height or antennas and now you might have an 80 meter dipole at 30 feet, or a 40 at 25 feet. Look at the charts and see what your impedance is. How about the scenario where you can't run a dipole straight down from the center and it has to be snaked around a few trees or other objects?
I think my point had been made and anyone reading this can think or examples where less than optimum conditions for an antenna are present.
Usually dipoles of this nature are used on several bands and as K7MEM reminded us it is important to consider a Balun (UnUn) or even a coupler or matching device such as a tuner when buying or building one or more of these antennas.
If you have a few minutes to look over the info below, I cut and pasted some of the material from the link for review. As you can clearly see it shows the changes of antenna impedance comparing height, frequency from 160 meters through 6 meters or the 50-54 mhz, basically the HF and low VHF band allocation for US Amateur stations. The page has charts, graphs and lots of other info you can review if you like beyond the dipole discussion.
From : Palomar Engineers
Dipole Antennas
Folded dipole antenna basics
[Edit: copyrighted material deleted]
Last edited by a moderator:
