Needing help in antenna design

Is there any way for you to add more lengths of wire from the ground rods around the base? Believe me, you can't get enough ground radials for that antenna. The more I added, (around 75' in length) the better results I got with the tuning.
I also had an issue with the audio cables. Unhook them from the xmtr and check your test point voltage again. If it is closer to the desired 15 volts, you DO need more ground radial area around the base.

Sparky & the rest,

Simple truth: 12vman's got it right. I will add that there are differences of opinion about the maximum length of any radial wire and about the placement of such radials relative to the geological earth, but one way or the other, a handful of copper isn't going to make much difference.

Add to that the distance between the real antenna and the ground system you claim, well, you're lucky. Cheating the expense of not falling off the roof ain't savin' you a dB anywhere. Having lived and tried to play radio in third-story apartments taught me decades back that.

The only reason you're supposed to use copper pipe for the vertical element (as opposed to say, a steel CB whip) is to lower any ohmic resistances (and thus, losses) in the EMF field you're trying to create around the antenna. Yeah, it sounds like StarTrek, but that's the way it works. Me? I'm gonna go for that pile of silver plated teflon coated 00 that I got surplus years back. Great for 40A power supply wiring too.

And it's winter, ain't it? Perfect time to be putting up an antenna.

As for reference material, try the ARRL antenna book first. Older editions are a lot more "scientific." But a recent edition will have info you need to figure out all the schematic/design kinks.

I also highly recomment L.B. Cebik's web site ( http://www.cebik.com/) for source material, descriptions and straight talk about how antennas work & why you should put more copper on or in the ground. Among other things.

I just got my AMT3000 today. After my annual poke & prod tomorrow I'm gonna start on putting it together. Maybe I'll find that collection of Norwegian julemusikk to try it out on. Presuming I can get an antenna up off the snow-covered lawn with a minimum of cussin' and growlin'.

73

Nils
W8IJN

Hi all,

Interesting posts, but I have a question. If the ground is in place and connected why does connecting the audio cables lower the monitored voltage? Does tuning the antenna with the audio cables connected bring it back to spec? I also wonder, because I do not know, if the 12 volt target is the same for the antenna described or if it applies only to a ground level antenna.

For Nils:

Have you never heard of the Law of Maximal Inclemency which states "An antenna's performance is proportional to the inclemency of the weather at the time of construction."? Worked for me....put up a dipole from tree to tree in my woods in December 18 years ago and except for having to replace the buried feed line, it has worked great.

Neil

In my situation, I couldn't get the voltage reading I wanted at the test points. I removed the feed line and the voltage would increase a couple of volts. I figure the xmtr was using the feed line for a source of ground. I then STL'd with an FM xmtr, only having the power line ran to the AM xmtr, and a lot of the tuning issues went away. This even enhanced the quality of the audio. (Took the pillow off of it).

Every situation is different. I'm 20' above my ground area using a wooden structure to support everything.

Length/amount of ground leads is always questioned. In basic antenna theroy, there needs to be some resonance and the ground area is not excluded. The earth is a poor conductor and the RF needs something to work with. Even at such a low power, the basic laws still hold true. I know not all folks can install such a long wiring system but from my experience, the more/longer theroy does work.

"What are these rf's and how do they do that..?"

The earth is a poor conductor and the RF needs something to work with.

____________

Below is a paste of my comments on this topic that I posted on another board.
+ + +
The purpose and function of buried ground systems is to provide a low resistance path for the RF currents induced in the earth by radiation from the antenna. Those currents flow through whatever conductor is available, back to the ground terminal on the tx. They are a necessary part of an efficient MW antenna system. A high-resistance path for those currents reduces the radiation efficiency of the antenna system.

These induced earth currents are present out to several 1/10s of a wavelength from the radiator. So no matter how much metal you bury under the antenna, the currents it needs to collect from the earth will have to travel through several 1/10s of wavelengths of earth to get there -- and that means the total resistance in the ground system will be very high.

Likewise, even if you can connect to a buried water pipe that is miles long and has a huge surface area in contact with the earth, it still is a very poor MW ground because most of the radiated earth currents it needs to collect will have to travel through a lot of earth before arriving at the pipe.

This is the reason that radials are used. They need to be placed in enough numbers and in the locations around the antenna where they can collect the RF earth currents with only very short paths through the earth, and then conduct them back to the tx.

Here is some data relating the number of symmetrically-placed, buried radials each 0.1-wavelength long, to the current each one collects from a 0.1-wavelength MW vertical radiator, for the same tx power in each case.

No. of Radials > Current (amperes)
8 > 0.3
16 > 1
32 > 1.5
64 > 1.55

So in this example a system with 8 radials has 5 times more ground loss than a system with at least 32 radials. And as radial length increases, the differences in the currents in each one are multiplied. For example, if the radials are all 1/2-wave long, the current in 64 radials will be about 250X greater than the radial currents when only 8 are used.

These increases in current result from the reduced losses that the induced earth currents have as they travel back to the tx. And that higher current flowing in the antenna produces greater useful radiation (for the same tx power).

So that's why a localized "ground" under a MW antenna is not such a good idea. Pardon my rant.
//

I find your comments very useful but I'm in a little
different situation.. My xmiter (the sstran unit) is in my
attic for a number of reasons. I am presently using the
netural conductor of the AC mains as my RF ground with
another ground path possible through the audio cables.
My question is: would I be better with a counterpoise
of radials in the attic or run a separate wire to ground
system of radials in the actual earth. The distance from
the xmitter/antenna to earth is about 15 ft. I am also
using a small capacitance "hat" which seemed to increase
my coverage area by about a block further out. I am interested
in any technical information on how radials and the "hat"
affect the radiation angle. thanks - John Anderson

First... using the AC line neutral as a chassis ground is not permitted by the US electrical code, because it is possible for the neutral to have substantial AC voltage/current, which can damage whatever is connected to it that way, and pose a human safety hazard.

But regardless, the neutral becomes a part of the antenna system, and probably radiates at least as much as the 3-meter section above the tx (see paper 3 at http://rfry.org/Software%20&%20Misc%20Papers.htm ).

Even if you replace the neutral with an isolated wire leading from the tx chassis to a cold water pipe ground (or similar) -- it also becomes part of the radiating antenna.

Installing buried radials somewhere near the antenna, and connecting the tx chassis to it again results in a radiating "ground lead."

Using 3 or 4 symmetrically-arranged, horizontal radials laying across your attic floor would give you an RF ground plane for the 3-meter vertical to work against. Make them as (equally) long and straight as possible, solder them together where they converge below the tx, and connect the tx chassis to that common point. Remove any wire connecting the tx chassis to the AC neutral, or a separate wire to a water pipe ground.

This will change the feedpoint conditions at the antenna, so the loading coil settings will need to be checked and re-optimized.

The radials and any top hat used won't change the vertical pattern of the antenna as long as they all are in the horizontal plane.

How an FCC inspector might view this installation with respect to the "3-meter rule" is a judgement call. The technical reality is that the top hat and radials are not contributing to the radiation of the ~3-meter vertical, but assuming that an FCC inspector might know that probably has some risk.
//

Amigos,

Came across these few URLs with connections to shortened antenna systems such as MF Part 15'ers might find interesting:

Checking Claims of Antenna Designs - http://www.antennex.com/Stones/st0403/claims.htm

Antenna Coupler for short antennas - http://beacon.nautel.com/Resources/Docs/Whitepapers/Dev%20of%20Antenna%20Coupler.pdf

Canadian company with short antenna systems - http://valcom-guelph.com/Guelph/products/lowfreq/v147-cl2_photo.html

There was another one tied to Globalsecurity.org but I didn't track that URL so well. Most of it was stuff from a USN manual on antennas, if memory serves.

And as for my smug attitude that I'd have no problems with my own personal touch on connecting an antenna to an AMT3000, well, oops.

Tonight I get out the carpentry tools and set myself up a test jig to fiddle all the wires & taps and coupling with. Can't let my obsessive nature get the best of me. And I don't want to open the window to toss stuff out. Too dagnab cold for that.

Oh, also: "Practical Wireless Telegraphy," by Elmer E. Bucher. Wireless Press, NY 1918.

From back when radio was mechanical & electrical (but not quite electronic). Good info on matching antennas down in the MF & high LF spectrum. If you like sparks and arcs. Still good antenna info.

73

Nils
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La Estancia de los Guajolotes Sonrientes
W8IJN - http://w8ijn.tripod.com