Antenna, Radials and Counterpoise

 Portable antennas - radials vs. counterpoise  

KATEKEBO

Edited by Kelvin Reynolds

Portable antennas - radials vs. counterpoise

« on: August 12, 2010,

Hi,

 

Just wanted to share some lessons I have learned over the past few days while “playing” with antennas.  I think some newbie hams (like me) might find these learnings useful when setting up a portable antenna system for the first time.

My project was to set up a portable antenna system for an FT-817 transceiver.  My goal was to have something highly portable, easily deployable, with acceptable performance, really easy and fast to tune, and something with sufficiently low SWR (<2:1) to be used without an antenna tuner.  The specific reason I wanted to avoid an antenna tuner is because, with only 5 W PEP, even a couple dB of losses can make a big difference between being heard or not.

 

I started with a Superantennas MP-1 portable vertical whip.  This is a popular portable antenna, and I like the built quality and the (relative) ease of tuning with a sliding coil.  I found that the antenna tuned perfectly (SWR <2:1) on 20 m band and higher frequencies, but for some reason, I could not get it to tune properly on 40 m.  I could find the point with the lowest SWR, but I could never get the SWR below approx. 4:1.  Clearly, something wasn’t working as it should.  I tried a different coax, placing the radials provided with the antenna in different patterns, moving the antenna to a different location, but nothing worked.  However, while doing all these experiments I noticed two things:

- The SWR improved (a little bit) with a longer coaxial cable between the radio and the antenna

- I could actually get better SWR on 20 and 10 m bands by retracting the radials a little bit, instead of using them fully extended.

 

These two observations put me on the right track to find the solution to my problem.  But before jumping into conclusions I did a bit of research and theoretical analysis.

 

THE CONCEPT OF COUNTERPOSE:

Let’s start with a little bit of antenna theory.  Any vertical 1/4-wave radiator is essentially only half of the antenna.  In order to make it work, there must be a second half that will “balance” the 1/4-wave radiator.  Here is a simple “kitchen logic” explanation.  As the current bounces back and force along the radiator, the tip of the antenna becomes alternately positively and negatively charged.   In order to keep the feedpoint electrically neutral, something else must develop an equivalent opposite electric potential to the radiator tip.  Without a “counterbalance”, it will be the other extreme of the coax cable, the one connected to the radio, that will develop the electric charge and, yes, it will “bite back” when you touch the radio.  Hence, in order to work properly, a vertical 1/4-wave radiator needs a counterpoise.

 

GROUND RADIALS:

In a classic, ground mounted 1/4-wave vertical antenna this counterpoise is the ground below the antenna.  The good Mother Earth acts as an infinite size reservoir that provides the necessary electrical charge to the radiator to keep the feedpoint electrically neutral.  However, it is not very practical (and in some cases impossible) to connect the outer shield of the coax cable directly to the ground at the antenna feedpoint.  This is because common soil is a relatively poor electrical conductor, and such direct connection via a small grounding rod will have a too high impedance.  An exception would be to stick the grounding rod into seawater or maybe a salt marsh or alternatively use a really huge grounding system with many rods.  Most of us live in places where the soil is dry and does not have enough conductivity to act as effective electromagnetic ground with only a single short grounding rod, and “digging for oil” to install a huge grounding system is not a cost-effective solution.

 

Here is where ground radials come in as a handy solution.  The function of the radials is to capacitively couple the antenna system with the soil below.  In order to use the soil as an effective counterpoise, we need to create a low impedance connection.  This is done by creating a huge capacitor between the antenna and Mother Earth, where the radials act as one of the capacitor plates, and the soil below becomes the other.  Logically, in order to achieve low impedance, we need high capacitance, and capacitance is directly proportional to the area of the capacitor’s plates.  This is why the number and length of the radials is important because each radial interacts with only a small area of the soil below.  Based on my experiments, 8 radials is probably the minimum you need, and some antennas may have as many as 100 radials.  There is plenty of information about radials on the internet.  A WORD OF CAUTION ABOUT RADIALS’ LENGTH:  The length of the radials is not very important as long as they are significantly shorter than 1/4 wavelength.  This is because at 1/4 wavelength the radials will become acting as radiators themselves and this will affect overall antenna behavior – this is not necessarily a bad thing, just something you need to keep in mind as explained below.

 

RESONANT COUNTERPOISE:

Ground radials are a very effective solution providing you can actually install them.  The main advantage is that a single system of radials can be used over a wide range of frequencies.  However, many times installing a good system of radials is not possible.  You need a lot of space around the antenna base, and in order to work properly, radials must be placed in close contact with the soil (lie directly on the ground or be buried a couple of centimetres).  Ground radials are also not a practical solution for portable operations.

 

A simple and portable alternative to this problem is a RESONANT COUNTERPOISE.  As implied above, you need a low impedance ground connection for the outer shield of the coax right at the feedpoint of the antenna.  In a radial system, this low impedance connection is provided by capacitively coupling the antenna with the Earth itself.  But when such capacitive coupling with Earth is not possible, learning a little about impedance in transmission lines will help to find an alternative solution.

 

A good explanation of the impedance in transmission lines can be found under this link:

http://www.allaboutcircuits.com/vol_2/chpt_14/7.html.  Without going into details about the theory, the important thing to remember is that a 1/4-wavelength, open-ended transmission line looks like a low-impedance “short-circuit” at the source end.  This means that a 1/4-wavelength piece of wire connected to the ground post at the antenna feedpoint will appear to the antenna (almost) identical to a low impedance ground connection.  This 1/4-wavelength piece of wire becomes the resonant counterpoise for the radiating element of the antenna.

 

PRACTICAL CONSIDERATIONS:

Building a resonant counterpoise for a portable vertical 1/4-wavelength antenna is really simple.  However, there are some important practical considerations.

 

While in theory, a resonant counterpoise should be exactly 1/4-wavelength long, in practice it will be a little shorter than that.  This is because electromagnetic waves travel slower through a wire than in empty space and because the counterpoise wire inevitably interacts with other objects around it.  Probably somebody can come up with a good theoretical model on how to accurately calculate the length of a resonant counterpoise, but a practical trial-and-error approach is much faster and simpler.  Just follow these simple steps.

 

1)   Start with a fully extended counterpoise that is 1/4-wavelength long (or maybe a little bit longer).  Then tune your portable antenna to the lowest SWR you can achieve (depending on the antenna this is done by using different taps on the loading coil, sliding a sleeve, or adjusting the length of the antenna whip).  It doesn’t matter what the value of the lowest SWR is, as long as you find the “sweet spot” where the SWR reaches the minimum.  In my case, the lowest SWR I could get at this stage was around 4:1.

2)   Coil back the free end of the counterpoise wire a little bit and check the SWR – you should see a small improvement.  Keep coiling the counterpoise back and checking the SWR until you find the length that gives the lowest SWR.  Assuming that you are using a decent quality antenna, and your coax is in good shape, you should be able to achieve a really good SWR – in my case, I can get to as low as 1.2:1.

3)   Prune the counterpoise to remove the excess wire and voila, you’re done.  You have built a resonant counterpoise for the specific frequency band.

 

So what about other bands?  You have several options.  You can build a separate resonant counterpoise for each band.  Or you can connect all the different resonant counterpoises together with a single connector to the antenna base, and extend the one appropriate for the band, while the other ones are coiled back at the antenna base (counterpoises that are coiled back at the antenna base are “invisible” to the antenna).  Or you can use a single counterpoise long enough for the lowest frequency band and coil it back to the appropriate length for higher frequencies.

 

Here are the counterpoise lengths that work well with my antenna set-up:

-   9 m (29.5 ft) counterpoise for 40 m band

-   4 m (13 ft) counterpoise for 20 m band

-   2.5 m (98 in) counterpoise for 10 m band.

I found that the length of the counterpoise isn’t really that critical and good SWR can be achieved as long as counterpoise is within +/- 5% of the length indicated about.  Lower frequency bands seem to be more sensitive to the length of the counterpoise wire, while on 10 m band, almost any piece of wire will do.  Finally, the counterpoise does not have to be perfectly straight.

 

FEW WORDS OF CAUTION:

The above information is provided for informative purpose only.  There are no guarantees implied.  Your results may vary depending on your particular antenna, feedline and other considerations.

 

One of the consequences of a resonant counterpoise is that the antenna radiation pattern will change depending on the location of the counterpoise.  If you elevate your antenna and leave the counterpoise hanging down in a straight line, you will effective create a vertical 1/2-wavelength dipole.  If the antenna is placed low, close to the ground, and the counterpoise is extended to one side, your antenna will display a fairly directional pattern – you can experiment to find what works best for you.

 

Finally, it is important to remember that radials and counterpoise are two different things.

- Radials are not a counterpoise - they are a capacitive coupling between the antenna and the Earth.  The counterpoise to the antenna is the Earth itself.

- A resonant counterpoise provides the necessary balance to a 1/4-wavelength vertical radiator without the need for a physical ground connection.

 

MY FINAL SET-UP:

I started my experiments with counterpoises in order to improve the performance of my MP-1 portable antenna (and to make it usable on 40 m band).  But the final set-up I ended up with is slightly different.  I use three different radiators for 40, 20 and 10 m bands.  For 40 and 20 m, I use MFJ-1640T and MFJ-1620T respectively, each one with its corresponding resonant counterpoise.  For the10 m ba,nds I use a high quality 2.7 m (9 ft) long telescopic whip, adjusted to approximately 2.5 (98 in) m, and a 2.5 m (98 in) wire counterpoise.  I like this set-up because it is a “tune once and forget” system.  I have carefully tuned the antennas for lowest SWR at the center of each band, and I don’t have to re-tune the system when I change bands.  All I have to do is to screw the appropriate radiator into the base, connect the corresponding counterpoise and I’m ready to go.  I quickly check the SWR, but I never found the need to re-tune the system.  On 10 and 20 m bands my antennas deliver <2:1 SWR across the entire band, while on 40 m I can stay below 2:1 SWR across approximately half of the band, centered right on the middle of the band.  If I want to transmit closer to the band edges, I just need to shorten (or lengthen) the antenna whip by 4 to 5 cm.

 

So how well does it work?  The same day I finished building my antenna system, I made contacts with Italy and Aruba, using 5 W PEP SSB with my Yaesu FT-817, transmitting from the deck in the back of our house.

73

 

S. Bucki  (KD8KQH)

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K4SAV

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RE: Portable antennas - radials vs. counterpoise

« Reply #1 on: August 13, 2010, 01:37:15 PM »

As much of a temptation as is to redefine the terms ground, radials, and counterpoise, I'm afraid we are stuck with what is currently being used.  Unfortunately, that is a mixed bag.  Probably the majority of people use the word counterpoise to refer to something similar to an elevated radial system (but not everyone does).  Many people (and textbooks) use the word counterpoise to refer to a radial system, whether those radials are elevated or not.  Other people use the terms interchangeably, so don't jump to the conclusion that you know what the system looks like when someone uses these terms.

 

The concept of thinking of a radial system as providing a capacitance to the earth doesn't work.  That would say that the higher that capacitance to the earth the better the system should work, and just the opposite is true.  If you take an elevated radial system and lower it closer to the ground, the capacitance to the ground increases, but the antenna gain goes down.  That's because more current now flows in the dirt, which is lossy.  The purpose of the radial system is to intercept as much of the field as possible near the antenna and return those currents back to the feedpoint so that the currents don't have to flow in the dirt.

 

The reason that a radial system may appear to resonate low in frequency is because of the proximity to the earth.  You can think of the effect two ways, as either as the capacitance to the earth causing this, or the dielectric constant in which that field is flowing has increased, which decreases the velocity factor of the wire.  That dielectric is made partly from the air and partly from dirt.

Shortening an elevated radial system does two things, it raises the resonant frequency of the system, and it makes the highest current (or lowest impedance) point on the antenna move up the vertical wire away from the feed point.  This increases the feedpoint impedance.  It may also cause more common mode currents to flow on the feedline.  (This is somewhat similar to what happens with and off-center-fed antenna.)  

 

If you have a need to create a resonant elevated radial system (or can't estimate or calculate the resonant frequency closely enough), the best way is to install two radials and feed them like a dipole and measure the resonant frequency.  Then build all the other radials like those two.

 

Jerry, K4SAV