This project started innocently enough as a test rig to examine the range obtainable with a 3 meter monopole over ground radials. It worked so well that a permanent installation was pursued. What's reported here is not a step by step how to but rather is presented to give you ideas. Here's how the sausage was made:
It begins with an acrylic coil form 3" D by 6" L. End caps were turned from 1/4" acrylic sheet on a lathe to support the form and close the ends. These caps have a shoulder which provides for a press fit into the cylinder and press fit on the mast.Photo here: http://postimage.org/image/r4vezlsnx/
A view of the form installed on the mast: http://postimage.org/image/4cgzxiv6l/
The radiator was found in the attic junk pile and is the center element of a CB roof top antenna. The telescoping sections are easily adjusted to a total height above ground of 3 meters. To support the radiator a mast was made from 1" and 3/4" PVC sch. 40 pipe. The parts were sized on the lathe so they press fit together and adapt the radiator to a 1" pipe which is sunk into the soil. The mast and antenna assembly are removable from the portion in the soil for maintenance and changes.
Photo of the mast assembly: http://postimage.org/image/4b7243tct/
Connection from the transmitter output lead to the aluminum tubing was made as shown with the aluminum cleaned and coated with NoAlox to resist corrosion and held in contact by a PVC bushing in the mast. The lead wire was brought out into the coil form through a hole drilled in the side of the mast inside the coil form. Photo: http://postimage.org/image/p6tcfcpjx/
The coil was wound to resonance and includes a movable three turn coil in series which is used to fine tune the resonance. The assembled coil and mast are in this photo:
http://postimage.org/image/eu2gfj6n1/
Twelve ten foot radials were arranged around the base of the antenna and are buried beneath the grass. This was done by using a flat gardener's shovel to slit the sod and the wires were pushed into the slits by hand. There is minimal damage to the turf. The mast was sunk into the soil by using a drill powered 1" auger to a depth of 18". The completed installation is shown here: http://postimage.org/image/6v2oy7gwt/
The transmitter used is a Ramsey AM-25 modified for high efficiency and is contained in a 6 inch square PVC electrical enclosure. It is placed at the base of the monopole and connected to the monopole and the radial ground. Photo: http://postimage.org/image/hmvnzk6zh/
Just out of curiosity an experiment was conducted using a large metallic "ground plane" as shown here: http://postimage.org/image/dch03z1wd/
This ground plane had no effect on the radiated signal strength at all and is only here to show off a tractor that I reconditioned.
The project was actually quite fun to do and it works well. Perhaps this will inspire you with ideas to build your own.
Neil
Enjoyed the antenna tour.
I'm gettin' me a tractor!
Good looking install. Nice shiny tractor too!
What I wonder is your field intensity being absorbed by all the nearby trees and could be better if the setup was in the clear.
Have you considered enclosing the coil for weather protection?
Nice all around!
RFB
Yes, there is a bunch of green near the antenna. When the wind blows and moves the honeysuckle a bit I can see the FS change on the S meter. The leaves will drop in late November and I'll check if it makes any difference.
There was no in the clear location here that would satisfy the wife so I compromised.
I am considering getting a piece of 4 inch PVC pipe and an end cap to slip over the coil but held off since the price is pretty high and I wanted to make sure I was happy with the antenna before going ahead. Good suggestion though.
Neil
Great presentation! I enjoy seeing projects come to life. I wish I would have taken more pics of me playin' around when I was. You woulda laughed at me, installin' chicken fence onna platform 20' off the ground.. Solderin' 6 ga. wire to ground rods with a torch.. I spent an entire summer "experimenting".. Had a blowtorch FM station goin' for a while.. (.7watts) I know, I know..
Haven't played around for a good while but I'm gittin' that itch again. Most of my copper ground plane is still out there.. 😉
I want that remotely tuned antenna. Sounds like a good project to me. Wish I could get some brainstormin' goin' on..
I got the same problem with all the trees eatin' up my power. Big difference when I moved things out into the open field..
I'm really really glad you did this, Neil!
So - when you had the antenna mounted on
the tractor, could this have potentially
been a mobile AM Part 15 set-up - if everything
was on the tractor - I suppose so...?
Anyhow, what you did is great! What a nice
installation!!
Bruce, DRS2
How far is the run for the audio line from console to transmitter, and how good is the noise rejection and immunity? I've found long-run audio lines - unless they are balanced and nicely terminated - pick up hum like cicadas in Georgia.
You've put some real care into that antenna, so you must be putting some care into maintaining audio integrity. What's your method?
-a.
Audio lines longer than around 8-feet really need to be balanced asnd, in certain designs, terminated.
Very long unbalanced audio lines tend to suffer from high frequency roll off and noise pickup.
But there is a trick that (I think( can give perhaps up to 20-feet with unbalanced lines, and that is to use a kind of "fake balance"....
Use a three conductor shielded audio cable, ground the shield at both ends to circuit ground points, not audio ground points. The shield will be used to "shield" the inner two wires, NOT carry audio ground.
Run audio ground down one inner wire, maybe the white wire....
Run the positive audio down the remaining (maybe black) wire.
This has worked in some installations, and I read about it in an engineering magazine.
Some references recommend grounding only one end of the cable shield when using shielded 3 wire cable.
This is because if the equipment at opposite ends of the cable have different ground potentials it will cause current to flow through the ground wire (ground loop current) which can induce noise.
The audio/power feed runs from the "console" for 30 feet through the basement and this line is 4 conductor twisted pair (two pairs) with one pair in a foil shield. The shield is connected to audio ground at the console end. At the house outside wall the line is connected to an abandoned underground 12-2 With Ground line which runs an additional 30 feet to the transmitter. The power is fed on one conductor, the audio on the other, and both share the common ground conductor. Since the DC power is well filtered the voltage is time invariant and does not cross couple with the audio nor interfere with the shared ground signal.
I chose to use a RF transformer at the output of the transmitter to serve as an impedance matching device and also to provide ground isolation between the antenna ground and the power/signal ground. This breaks the ground loop with the antenna ground separate and isolated from the transmitter ground which is connected to the inside ground for both protection (connected to the house wiring ground lead at the point of entry) and signal grounding. Despite the outside run being unshielded and a two point ground inside the house there is no noise pickup and no audible hum. This may be due to the line being buried down about a foot. This was not planned but it was a "try it and see" experiment which yielded good results. The audio source impedance is low enough that there is no roll-off for this run.
If I were beginning again from scratch, or if this installation failed, I would use an audio transformer coupled twisted pair with a transformer (or differential amplifier) on each end for the signal and a separate pair for the DC power feed. If the source and load impedances are kept low and match the line impedance (~100 ohms balanced to ground for twisted pair) then frequency roll off is not an issue for audio applications for runs up to about 1000 feet.
Neil
Edit to add: Check this LINK for an update on the cover. End edit.
Since declaring that this installation is going to be permanent it became necessary to provide a bit of protection from the weather and to make the coil less visible. To do so a cover was made using a length of 4 inch PVC pipe and an end cap. The cap was drilled to fit over the mast above the coil and it was painted to blend into the surrounds as shown in this photo. Here's another view.
The transmitter enclosure is temporary and when the final one is installed it will be painted also.
"Duct sealing compound" was used around the joints with the mast to keep out rain and provides for easy disassembly for tuning or changes. The PVC cover pulled the tuning a bit requiring removal of one turn from the coil to maintain resonance but it did not reduce the field strength at all per a remote field strength measurement.
An open space gap was left at the bottom of the cover so that air can circulate around the coil in case moisture gets in. The problem with this is that, from experience, spiders and mice like to crawl into such spaces and do their nesting things. This is a good reason to make the cover removable.
Neil
The coil enclosure is environmental for its blendedness.
Truly fine work.
The inspiration of it has me out in the back engineering the land clearing prior to installing ground radials back there.
The heat wave provided the best excuse for no activity, but temperatures have dropped, and land clearance has begun.
One thing very curious is the surround of foliage near to the antenna (not here, there!).
May I suggest that you are broadcasting from "Attenuation Forest".
"An open space gap was left at the bottom of the cover so that air can circulate around the coil in case moisture gets in."
Another way to keep moisture out is to use those dry packs put into product packaging to keep items from getting moisture damage during shipping, storage etc.
Use those and you can enclose that coil cover completely and no worries about moisture or critters turning it into a nursery.
RFB
RFB,
Good idea and worth considering.
Carl,
I have baseline antenna impedance, relative field strength readings, and range observations with the foliage in place and will be watching for changes in these when the leaves drop. Another effect of foliage can be that the trees and bushes suck water out of the ground which raises the ground R. Living in a woods as do I gives little leeway for antenna placement and add to this that the raccoons are digging up my radials as they forage for grubs and this is not an ideal location. Another hazard is that deer run through this area and they will probably knock the antenna down. Arrrrrgggh!
Neil
When it comes to part 15 being installed in the natural environment I am a stickler for agreeable integration.
I think every 3-meter pole should hold a birdhouse on top.
That's as far as I've gotten.