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Ground Radials: Another Construction Approach?

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Ground Radials: Another Construction Approach?

I was surfing through some vertical antenna site for ham radio and I stumbled on some info that suggested that short vertical antennas (ground mounted) do not need radials 1/4 extending from the base, but rather only needed to be in the far field to be effective. An example might be a 18' Hygain that's ground mounted and used on 40 meters. Convention would suggest slighly longer that 34 feet in several directions. However, several radials about 19 feet will work as well.

Now applying that logic, one may conclude that several wires about 10 feet in length should do the job or as long as the antenna is high. So, that being said, I formed a 18" circle using 1/4 inch copper tubing, and soldered 300 ohm twinlead about every inch or so. I twisted and tinned the copper tubing end and then left the far end of the twinlead open, effectivly creating two radials per solder job. So 20 pieces of twinlead is 40 radials. Cheap and easy and it lays flat of the roof with a little help from silicone rubber. The metal ground stakes for camping from Walmart work well if your antenna is mounted on the ground. Just take a paper punch and poke a hole through the center being caeful not to damage the condductor and palnt it in the ground.

The copper circle is connected to the transmitter ground. Your situation may vary slightly.

Performance

So, did this new ground system improve your performace over what you had before?

Clarification please

Was that an 18' circle or an 18" circle? I'm thinking that you made an 18" circle and used that as a tie point at the transmitter to tie all of the radial leads to.

A lot of simulations have been done on ground radial systems that seem to generally indicate what you suggested-- that with a very short antenna, the radials don't need to be a full 1/4 wavelength. Even with a full sized antenna, the close-in region near the base of the antenna is the point where most of the current is flowing, so it is important to beef that up.

I am trying to think of a simple way to put a fairly substantial ground screen out to about a 10' radius at the base of my antenna once I get it installed.

WEAK-AM
Classical Music and More!

Answers

First, Yes Substantial Improvement

Second: I fixed the TYPO it was 18" (inches) Oops

WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager

Dade City Radio AM1610 & FM107.9, Part 15 John - WA4JM Owner-Operator-Chief Engineer-Program Manager http://wdcx.webs.com

Buried Radials

WEAK-AM: A lot of simulations have been done on ground radial systems that seem to generally indicate what you suggested-- that with a very short antenna, the radials don't need to be a full 1/4 wavelength. Even with a full sized antenna, the close-in region near the base of the antenna is the point where most of the current is flowing, so it is important to beef that up.

For some useful history on this topic...

In the mid-1930s, Dr George H. Brown (et al) of RCA Laboratories made some benchmark, real-world measurements concerning the number & length of buried radials with respect to the groundwave field strength generated by a vertical monopole over a given path, using a given tx power. The results were printed in the June, 1937 edition of the Proceedings of the Institute of Radio Engineers.

This experimental work shows in Figures 33 & 34 that 15 or more buried radials each 49 degrees long, used with a base-driven, earth mounted vertical monopole about 6 electrical degrees tall (~ 3 meters at 1700 kHz) produced a radiated field at 1 mile that was only about 31% of the field that was produced for that same radiator height/path/power when used with 113 radials each 148 degrees long (0.41-wavelength).

The paper states about the system with the 49-degree radials, "Evidentally, most of the earth loss occurred in regions beyond the the periphery of the ground system."

These data show that even electrically short antennas will benefit from using a large number of long, buried radials, and even greater than 1/4-wave long, each.

Rich http://rfry.org

Great idea

John,

The use of twin lead is as you described is a great idea. I have no clue about the electrical performance and I think Rich gave some input about that, but physically it sounds great. I made radials from some scraps of #14 wire and have a real three stooges time getting them to stay put on the ground. They are not buried yet since I am still experimenting but the antenna is definitely having a bad hair day with this wire. The twin lead seems much easier to manage.

Neil

Gutter cover

I have used lengths of gutter cover in the past with my ham radio antenna. It worked OK, although it sure looked like heck, especially after it had been out in the elements for awhile! The twin lead idea sounds like a more reasonable approach.

On the number and length of ground radials, a lot of excellent modeling has been done on that subject and therefore I will not go over it here since the information is readily available in the literature. It is well accepted that 120 quarter wave radials will give you a very effective ground system, but few private individuals have space for that. So it comes down to using a reasonable number of radials that will fit within your available real estate and accepting the efficiency that can be achieved. For example, I use a set of 16 0.1 wavelength radials under my 40 meter ham vertical, which if memory serves me correctly results in approximately 50% efficiency. That is not a bad tradeoff in my mind, considering the space I had available for that system and the effort required to install it.

WEAK-AM
Classical Music and More!

Buried Radials Cont.

Hi Rich,

YouSaid:
"These data show that even electrically short antennas will benefit from using a large number of long, buried radials, and even greater than 1/4-wave long, each."

Isn't there an argument to be made that burying radials in lossy soil as we have here in Florida is counterproductive. Many radio stations including the one we have here in Dade City, opted when replacement time came to cut the grass very short and lay the radials on the surface of the soil. In other words, working a radial system through crappy soil conditions reduces the efficiency of the system.

John

WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager

Dade City Radio AM1610 & FM107.9, Part 15 John - WA4JM Owner-Operator-Chief Engineer-Program Manager http://wdcx.webs.com

Burying Radials

John (WDCX) wrote: Isn't there an argument to be made that burying radials in lossy soil as we have here in Florida is counterproductive.

The radials can be placed directly on the surface of the earth, but that gives them no electrical advantage over being buried a foot or so, and they will be more subject to damage.

The George Brown experiments I referred to were done in the sandy soil of New Jersey where ground conductivity is poor, similar to some parts of Florida. When using a buried system of 113 radials each 0.41-wave long, his field strength measurements 3/10 of a mile from the antenna were within a few percent of the theoretical maximum possible over a perfectly conducting earth. This shows that if such a ground system is installed then the conductivity of the earth at the installation site is not very important.

Of course, once the groundwave has traveled over a fairly long path of low earth conductivity, that conductivity becomes very important. But there is little one can do about that, other than to move the tx site to a location giving better path conditions.
//

Radials

Easiest way to do is to lay the wires flat on the ground, then secure them every meter with those long staple-looking things that landscapers use to nail down sod. Best to do this in the early Spring. By Summer, the wires are totally hidden and lawnmower proof.

1610AM The Copperhead
It's All Rock n' Roll
Lucama NC

Radials Buried Cont'

TNX, Rich :-)

WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager

Dade City Radio AM1610 & FM107.9, Part 15 John - WA4JM Owner-Operator-Chief Engineer-Program Manager http://wdcx.webs.com

Ground planes for electrically short antennas

Many people have noticed that electrically short antennas are "hot." This is not apparent with 100mW transmitters, but applying only a few watts to a short antenna can cause RF burns. This is because the field strength near short antennas is higher than for full-sized antennas generating the same amount of radiated power. This higher field strength makes the ground plane requirements for short antennas even more demanding than for large antennas.

Licensed AM broadcast stations using electrically short antennas have been found to be more efficient if they use metal sheets near the antenna rather than radials. The diverging radial wires have too much ground resistance in the high near field region. It's beneficial to make the ground plane as big as possible.

A large, low resistance, ground plane is difficult for Part 15 AM operators to obtain, and so they usually get a lot less efficiency than even the poor theoretical efficiency of 3 meter antennas.

More Data

Some Rules of Thumb for Ground Radials

Here are rules that seem to be reasonable for ground radials (not to be confused with elevated resonant ground plane systems):

Radials can be rather small diameter wire since so many of them exist to share the return currents and they are in parallel with the ground currents in the earth as well. Each radial is going to carry very little RF current.

Ground radials need not be resonant. This is a misconception based on elevated or ground plane type elements. True ground laid radials designed to supplement ground return currents in the earth need not be resonant. They are different from the elevated ground plane radials in this regard since ground radials supplement ground currents and do not try to replace them entirely. Elevated ground plane radials, especially if few in number, need to be bit longer than 1/4 wave at the operating frequency.

Ground radials seldom need to be longer than .2 (two tenths) wavelength regardless of the height of the antenna, even a halfwave vertical radiator. A maximum of .28 wavelength seems to certainly be an upper limit for ground radial length. Due to detuning of the ground, insulated wires laid on the ground tend to be electrically 1/4 wavelength when the physical length is close to .28 wavelength.

Ground radials do not need to be much longer than the antenna is tall. A shortened antenna with loading coils will have a more compact "near field" where the majority of the antenna field is. The ground needs only reach out as far as the near field extends. Field intensity drops off with the square of the distance from the base of the antenna.

Minimum number of ground radials is probably 8, closer to 16, well you would do better with 32... You get the idea, the more the merrier. Four ground radials is going to be a horrible system. More than 32 radials gets you into the area of diminishing returns.

The ground around a vertical monopole type antenna can be viewed as strings of series connected resistors fanning out from the base. The purpose of the radials can be viewed as attempting to short circuit as many of these resistors near the base as possible. This is especially critical very close to the base where RF field density is highest, and its importance drops off quickly beyond 1/8th wavelength from the base of any vertical antenna, where the RF field density per unit area goes down sharply.

It is important not to confuse this application with elevated ground planes. We are talking about radials that supplement the return of ground currents to the base of the antenna, especially in the near field. They work "in parallel" with the existing earth ground surface to supplement it. Elevated radials are a resonant element and serve a decoupling function and establish a completely artificial ground. They should be resonant, quarter wave wires, but still in fair numbers, probably more than the four usually seen, for best results.

WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager

Dade City Radio AM1610 & FM107.9, Part 15 John - WA4JM Owner-Operator-Chief Engineer-Program Manager http://wdcx.webs.com

Number & Length of Buried Radials

John (WDCX) wrote: Ground radials seldom need to be longer than .2 (two tenths) wavelength regardless of the height of the antenna, even a halfwave vertical radiator. A maximum of .28 wavelength seems to certainly be an upper limit for ground radial length. Ground radials do not need to be much longer than the antenna is tall.

John, I don't know where your information originated, but it is not supported by the 1937 measured data of George Brown, et al that is considered to be authoritative on this subject. In fact it was Brown's work that was used by the FCC to define the minimum acceptable fields for AM broadcasting that are in use to this day.

This * link * leads to two plots showing the MW groundwave field strengths generated by 1 kW at 1 mile for various radiator heights and length/numbers of radials.

It is easy there to see the importance of using a large number of radials each longer than 0.274- wavelength, and especially so for the electrically short antennas used for Part 15 AM.

Field intensity drops off with the square of the distance from the base of the antenna.

This is incorrect, sorry. Power density is inversely related to the square of the distance. Field intensity is related to the inverse distance, only (not that distance, squared).
//

Hi Rich: The Referenced Source

I actually just through this out for consumption. I should have referenced the link.

http://www.chem.hawaii.edu/uham/radials.html

WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager

Dade City Radio AM1610 & FM107.9, Part 15 John - WA4JM Owner-Operator-Chief Engineer-Program Manager http://wdcx.webs.com

Thickness/insulation of ground radials

What is the optimum way of doing ground radials? Should they be thick bare copper wire like Antenna Guy uses (#12 AWG) or would insulated feed line with #20AWG as John suggested be just as good (a very neat idea). Would the the 2 wires in the feedline be better than one #12 bare wire? Also wouldn't feed line be hard to strip the unsulation? It's OK if you have to do the odd one, but to do larger quantities is a pain. With bare wire, you just wrap and solder.

Has anyone had range experience with Antenna Guys ground radial schemes compared to say a single grounding rod? He currently offers these:

qty 10 x 10' long
qty 20 x 10' long
qty 25 x 10' long
qty 40 x 20' long

It would be nice to hear if anyone has any data.

Gerry

Gerry
www.chezradio.com
Airwave Freedom

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