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Following real-world experience I am taking the position that only ground radials are capable of pushing a part 15 AM station’s range to its maximum legal outreach.
So many of us have tended to believe that a ground rod can accomplish something of the same thing but I find otherwise.
Think of it as a 3-choice process…
A part 15 transmitter with only a 3-meter vertical antenna and no serious ground will not provide impressive reach. The relationship between antenna and ground is at best a very high impedance affair with little useful result.
A part 15 transmitter attached to a ground rod will tune-up beautifully on the output meter because the impedance between transmitter and ground is sufficiently reduced to provide a ground path to complete the circuit… except, that the highly resonant well tuned signal will be confined to an area near the ground rod and will not extend to the far field. In fact a ground rod is merely a negative dipole element buried in the ground where it loses potency.
Only ground radials will serve the radiated signal out to the distance by “claiming” the ground-plane and coupling it with the antenna system.
The final frontier amounts to trying different numbers of and lengths of radials.
Total posts : 45366Yeah.. The advatages appear appear clear, though I’ve never tried it… Would have, but the bigger problem is actually having the patch of land to do it.
Thinking about building a big planter on the roof and installing the transmitter inside that, and running the ground wire thruogh an attached dirt filled pvc pipe leading down to the bottom.
Technically legal
No, I’m not serious.
Total posts : 45366Carl, have you been experimenting with an outdoor install?
Total posts : 45366Rich Powers wonders: “Has Carl been experimenting with an outdoor install?”
Only partly.
In the back of the house my Wintenna, a metal window plus added wire to achieve 3-meters, has a below the floor set of two radials, one going north through the basement, the other going south up into the outdoor yard. Very excellent range toward the south.
In the front of the building is another Wintenna with the ground clamped to the I-beam which connects to the water pipe ground and electrical ground, the field strength strong only local to the ground, Very little range.
The front system totally penetrates the whole building and the outside walls, but is very weak in the distance.
Total posts : 45366This has certainly been my experience. Right now I am at (20) 30 foot radials and it made a heck of a difference.
Total posts : 45366The principles that I have claimed in this thread did not arise in my mind right away because I would have expected them to be more clearly spelled out here on the part 15 forums, but I don’t recall that they have.
Both of my systems serve their own purposes…
The direct grounded transmitter completely covers the interior building and the outer walls of the building out to about 20-feet.
The system with ground radials penetrates the immediate neighborhood and is fully present anywhere in the yard which is very convenient.
What we wonder next is what would happen if both transmitters were
A.) Equipped with a switch so as to have EITHER direct ground OR radials;
B.) Be COMBINED to give both transmitters BOTH systems… would it IMPROVE the overall result?
We are guessing that any of those switches would require re-tuning everytime the ground system was changed.
Oh good, just what we needed… another project on the list.
Total posts : 45366Carl,
Regarding ground radials, is there anyone of the opinion that more is not always better? I realize at some number you reach the point of diminishing returns. In my very limited experience I just know I saw a significan increase in range when I went from (10) to (20) 30 foot radials. I was considering adding another 10 once I get the xmtr setup finalized in the outdoor enclosure.
Total posts : 45366There was an interesting discussion about this by several of the more knowlegable here a few years ago. You should find your answers in this thread:
Understanding Vertical Antenna Ground Radials
http://www.part15.us/forum/part15-forums/antennas/understanding-vertical-antenna-ground-radials
Total posts : 45366Jim Henry is in the lead: “Regarding ground radials, is there anyone of the opinion that more is not always better? I realize at some number you reach the point of diminishing returns. In my very limited experience I just know I saw a significan increase in range when I went from (10) to (20) 30 foot radials. I was considering adding another 10 once I get the xmtr setup finalized in the outdoor enclosure.”
As of now my 2-radials are the sum of my experience in the area.
And yes, Rich Powers, thanks for that link! I want to start reading and re-reading all the postings about ground radials, some of which (as I recall) are about part 15 installations and some are about amateur or full-power stations.
Jim, your findings contribute to the knowledge data base…
… and I am compelled to add a hunch:
Again, this is a hunch… I SUSPECT that the ideal number of radials might vary from one installation to another, which is probably obvious, but I also suspect that a given installation will vary because of ground conditions and climate changing over time!
Of course you can’t instruct your radials to grow or shrink based on ground conditions, so at some point we must settle for a fixed solution.
Total posts : 45366There is a very interesting report of a 1980 study concerning grounding and ground radials of outdoor transmitters (portable whip and mast type) on pages 57 thru 63..
Highway advisory radio operational site survey and broadcast equipment guide (Report – Federal Highway Administration ; no. FHWA–RD–79-87)
https://archive.org/stream/highwayadvisoryr00turn#page/56/
There’s some curious things pointed out in this document.
Total posts : 45366antenna experiments were good because some Part 15 AM experimenters will never be able
to do an outside set-up in the
“middle” of the yard.
At the other house,
I was able to go outside with
the set-up. I started with 4 radials
and then 8 and then 16. It was
interesting doing that and seeing
the range grow. Some of my radials
were 10 feet and some 20 – – there were
lots of big rocks and tree roots in that
yard. Some of the radials couldn’t go
very far. The AMT-3000 was inside a
children’s playhouse. The 9.5 foot stick
was attached to the outside of it.
Brooce Part 15 Hartford
Total posts : 45366Ideally a radial would be 1/4 wavelength. I noticed as my radials degraded, so did my signal. It’ll jump up to its normal range when it rains but as soon as the soil drys the coverage will drop dramatically. When the radials were good there was no difference between wet soil and dry soil as far as my signal was concerned.
Total posts : 45366For what it’s worth, the study conducted by the Atlantic Research Corporation for the highway department concluded the following concerning legth and number of radials (from the above linked document):
“… It may be observed that the resistance (which is primarily ground resistance) does not change appreciably with radial length, but changes significantly when the number of radials is changed. In fact, a single 1.8 m ggground rod at the base of the monopole gave lower resistance than did the use of only two ground radials.
It is probable that the multiple grounding points resulting from pinning the ends of the radials to damp earth had greater effect than the lengths of the radials.
Antenna capacitance may be seen from Figure 20 to increase with either length or number of radials…..Model results show that with eight radials, capacitance does not increase with radial lengths greater than the monopole height. Radiation resistance increases rapidly up to radial lengths
equal to monopole height, and very slowly as radial lengths are made longer.Data on numbers of relatively long radials (3.4 x monopole height) show a 21% increase in capacitance going from two to eight radials, a 4% increase going from eight to 16 radials, and only a 2% increase going from 16 to 32 radials.k
Radiation resistance varies very little above eight radials.Based on the preceding measured and model data, the following recommendations can be made with respect to the design of a radial ground system for HAR monopoles.
1. Length: At least twice the monopole height.
2. Number: At least eight.
3. Material: No. 10 or 12 AWG bare solid copper wire has proved effective.
4. Configuration: Symmetrically arranged in a circle if practical. However, the pattern may be altered to fit space available without appreciable loss of coverage. For example, a “bow-tie” configuration might be used to fit a restricted highway right-of-way.
5. Burial: Burial to a depth of .5 m to .75 m is recommended both to protect from damage and to provide good earth contact. An alternative when concrete or asphalt pavement makes burial
difficult is to drop the radials into shallow slots cut into the surface, and to connect the ends of the cable to ground rods driven into the earth.///So this appears to conclude that longer the radials, the better.
Total posts : 45366Rich,
Good info, thanks. I have not yet read that study yet. When I went from 20 to 30 thirty foot radials I was able to reach out 9,000 feet on the south side of my antenna. One thing is puzzling to me is that 9000 foot was on the other side of the hill I am on. My home is about 875′ above sea level and about 160′ above street level. The antenna is mounted in my back lot and that is at the level with the 2nd floor of my home, BUT, it is still about 100′ below the very top of the hill. The audience I want to reach is not in that direction but rather to the north, NE, and NW of me. I was getting about a mile in those directions. BTW these results were when testing and running through about 150′ of coax from xmtr to antenna. I know that’s not in compliance, was just for initial testing, and the xmtr is shut down now while I get it ready (almost done) to eliminate that coax run with the xmtr mounted on the antenna. My soil is very rocky with just about 6″ of topsoil above what seems to be bedrock granite in most places, so I expected to have ground challenges. I expected to have to use bare wire for the radials but was told that STP would work fine. That’s what I used and it does seem to be fine.
Total posts : 45366“..I was able to reach out 9,000 feet on the south side of my antenna. One thing is puzzling to me is that 9000 foot was on the other side of the hill I am on.. …antenna is mounted in my back lot and that is at the level with the 2nd floor of my home, BUT, it is still about 100′ below the very top of the hill.. “
This is just an assumed guess, but maybe the improved range on the hill side has something to do with a higher collection of moisture content collecting at the bottom of hills, and since your antenna is below the hill, there is thus better ground wave propagation there.. I don’t know how scientific or accurate that assumption might be, but it’s what first came to my mind.
I get the impression.. based only upon years of periodically reading about ground radials — so I can’t genuinely give personal experience know-how advice about it, cause I’ve never done it… But my strongest impressions causes me to conclude that biggest catalyst for making a ground radial system most effective, is not only having an equal multiple of radials, but also perhaps more importantly having ground rods at the ends of several of those radials.
This was mentioned in the above: ““It is probable that the multiple grounding points resulting from pinning the ends of the radials to damp earth had greater effect than the lengths of the radials”
And of the top of my head, I recall similiar emphasis about using multiple ground rods is found in some of the Hamilton Rangemaster documents, for example:
“What you are trying to do is make an electrical connection to the earth over a broad area. What that means is if you have multiple rods keep them at least 6 feet apart, don’t concentrate on just a small area of dirt. For example don’t place 10 rods in a 2 foot circle. The more yard area you can cover with your system the better. A 20 foot diameter circle would keep the rods about 6 feet apart. Keeping the system spread out allows the currents to flow more efficiently.” http://www.am1000rangemaster.com/amradiogrounding.pdf
I suppose your bedrock lawn isn’t helping anything, and driving additional rods may not be feasible in your situation, but it does illustrate the point that the key objective is to bond with as much ground area as possible
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