Saturday, June 27, 2020

A Ham Radio Series 6 – Choosing an Antenna

So far in this series, I’ve mostly talked about HF and low VHF propagation and some antenna overview topics.  My slightly snarky three laws of antennas have truth embedded in them.  If you don't have an antenna anything you put up will get you contacts.  There's a group of hams who try to operate with the lowest power or most meager antennas they can and sometimes with both low power and "compromise" antennas at the same time!  They measure miles per watt for bragging rights.  1000 miles per watt will get you a certificate suitable for framing, and the records are many times that.
The current QRP miles per watt record is 1,650 miles from Oregon to Alaska on the 10-meter band using 1 microwatt! That’s the equivalent of 1.6 billion miles per watt.
The second law was that nothing is best at everything, and it's pretty common to want to upgrade your antennas.  A saying that I agree with more almost every year is that if you suddenly got a windfall chunk of change and really wanted to upgrade your station, look at your antennas first.  

For the past several months, I’ve been looking at antennas I can put on my suburban lot that would give me better coverage on two different bands, and I thought some of you might be interested in what I’ve been doing to evaluate options.  For this post, I’m going to talk about the amateur allocation on 30 meters; or 10.100 to 10.150 MHz.

For antenna work, if you go with a resonant, full-sized antenna, there are a couple of formulas you need to know.  For a half-wave dipole, the length in feet for a half-wave at a frequency (f) in MHz is given by:
L = 468 / f
Since a monopole is one half of a dipole (one pole is half of two poles), it’s length is half of the dipole's or L = 234 / f. 
For the middle of the band, that tells me a half-wave dipole is 468/10.125 or 46.22 feet (that’s 46 ft 2.6 inches).  A monopole is 23 ft 1.3 inches.

Which one do I use?  As always, the answer is “it depends;” in this case it depends on what I want it for.  My preference for this antenna, as with all of my antennas, is for distant contacts.  That means I want a low angle of radiation.  Which antenna should I try to put up?

Let me stop here for a moment.  Why is there a choice between one or two “poles” and what does that even mean?  A half-wave dipole antenna has a length that closely matches the physical length of the radio waves.  As mentioned in the Antennas 101 Part 1, the voltage and current physically fit on the dipole like this.  Current is zero at the ends because there’s no where for the moving electrons to go.  Voltage is high were current is low because the power (voltage times current) is constant.


To show that conceptually for a monopole, you cut it in half at the middle, and rotate it so that it’s vertical.   Here, I cut off the right half and the left half is now vertical.


This antenna works because there’s a virtual second monopole; a reflection of the top monopole in the ground making it a virtual dipole.  In reality, if you’re mounting a monopole vertically like this, you need a better ground than most people get out of their actual ground.  AM broadcasters do this by burying a radial array of wires centered under the vertical.  In practice, four quarter wave long wires arranged radially just below the grass is pretty common.  Note that this sometimes called a ground plane antenna although usually just called a vertical.  If you live on a saltwater marsh (really, any body of water) you're good to go without putting down radial wires.

I’ve simulated several antennas in EZNEC 6 (freebie equivalent), and have some comparison plots.  First off, here’s a 30m dipole situated 20 feet above ground, so just under ¼ wave up.  This is an Elevation plot, that shows which direction the signal is strongest, marked from horizontal to vertical.  The farther out the red trace is, the stronger the signal is.  The antenna is conceptually at the bottom where everything converges, and you’re looking at one end of the wire sticking out of the screen at you. 


Notice that the strongest signal is going straight up.  The performance at 45 degrees from the ground isn’t that bad, it’s only down a little over 1 dB, but I can move a marker on the plot and find that the half power point, -3dB, is at 32 degrees elevation.  This dipole will be better for local contacts than those distant stations.  Very little power radiates at low takeoff angles.

Given that, let’s see how the vertical compares.


Because of the way EZNEC presents this data, I drew in a green line to represent the ground.  The radiation is strongest right along the ground.  This is close to ideal for those distant, DX stations I want to work.

There’s a gotcha.  Notice the gain figure on the right in both plots.  The dipole has more gain than the vertical but a worse pattern.  Even with the lower gain, the vertical puts more power into take off angles below 20 degrees than the dipole does.

To get a lower radiation angle from the dipole you need to put it up higher.  A half wave is better than a quarter wave. This is the same antenna when it’s 46’ up instead of 20.  Not many people have the real estate to support both ends of a 46' long dipole held 46' up in the air by two towers. 


I should point out something important.  All antenna modeling programs present impressive results that won’t match reality exactly because the world is much more complicated than the models.  The only thing in the universe of these models is what you model.  There are no buildings, no metal roofs, no towers, no cars parked across the street, nothing.  What that means is that this is an indication of performance, not an absolute model of what you’ll get.  The world around the antennas makes the patterns lumpy and asymmetrical.  The patterns are distorted, but reality resembles the model. 

You might ask if an electrically shortened antenna using loading coils or traps (parallel resonant circuits) effects gain.  It affects signal radiated but not the pattern so much and not by large amounts.  If you use an antenna tuner and a random length of wire, as many people do, you will lose power and received signal in the tuner.  If it gets you on the air and gets you contacts, you can worry over every milliwatt you might lose, but remember the ionosphere isn't constant and power levels flicker all the time, too.  My personal take on it is not to worry about loss in the tuner if the ionosphere takes out six or seven dB in a flicker.   If the radio's happy, I'm happy. 

Which way am I going for my 30m antenna?  I'm not really done with trying options, which is more about looking at my lot than running simulations.  My current antenna is a commercially made electrically short (trapped) vertical for 80 and 40 meters.  It's electrically long on 30m, and tuned with the radio's antenna tuner.  I've been using it on 30m since I got it in 2008, so it works, I just would like better.  Without dropping large sums of money on it. 



3 comments:

  1. A vertical with just two "Elevated Radials" works very well. My antenna back in Long Beach was a 33' vertical about 15' above ground, with 4 radials that were also about 15' above ground. I could easily work VK/ZL on 40 Meters, and could hear European station a couple of S-units above the noise. The SGC autocoupler at the base gave me "all band" capability, and it also worked extremely well on 20 Meters.

    Does your modeling program give you the capability of using elevated radials? I'm surprised that this configuration hasn't gotten more attention.

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    Replies
    1. Sure, it can do elevated radials. You enter (X, Y, Z) coordinates for the ends of the wire, so you just enter Z greater than zero. Describe the geometry.

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  2. Well, a 1/4 wave verticals with four 1/4 wave radials, elevated anywhere from a few feet to a 1/4 wave.

    I had a version of NEC on this Linux box, was never spent much time with it.

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