Radials connected in the shield connection

[YvesRoy]

Radials connected in the shield connection

Hello,

Finally, i have installed 4 radials at the base of my 3-meter antenna with my loading coil, and are attached to the ring. These radials are 3-meter long. The results is very good. However, i have tried to installing another 4 radials for a total of 8 radials and the field strength is much less than if it would have 4 radials. I don’t know what is the problem. It supposed that the field strength would be much better if someone installed many radials.

Why do i have this problem ???

My radials and the ring are just a simply copper wire 20 GA non-enameled, installed 1 inch above the soil.

Regards
Yves

[Rich]

Adding radials may have changed the feedpoint impedance of the antenna system to the extent that the tx matching into it may need to be re-optimized.
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[YvesRoy]

Hello,

Just to let you know that i’ve installed successfully my 3-meter antenna this weekend including the loading coil made by myself (inductance of 345 uH for the frequency of 1470 kHz) and 4 radials of 3 meters long. The coverage zone is about 600 meters if i do “auto-scan” my car digital radio.

You can see 2 pictures of my AM base-loaded antenna at the following web page (sorry, it’s in french language… i live in Quebec):

http://www.geocities.com/yvesroy.geo/mini-station-am.html

However, i still have some reception problem when i’m adding some more radials (very strange). I’ll try to improve my antenna within a few weeks by adding some radials once i have resolved the feedpoint impedance of my antenna. My goal is that the coverage zone rayonates up to 1 or 2 kilometers.

From a happy canadian guy. 😀
Yves Roy

[Nils]

Yves-Roy et les autres,

I remember reading an article in the ARRL magazine “QST” back around 1972 or 1973 on shortened antennas. I remember ’cause I remember going to the Dayton Hamvention that year and sitting in the crowd as the author of the article gave a talk about how tuning up shortened antennas worked.

So somewhere in the QST archive for 1972 or 1973 there’s that article. I suspect that you’d find a lot of good info (and technical stuff) about how shortened antennas work &c.

I do know from experience that any change to any antenna feed point (adding radials, adding the neighbor’s dog’s leish &c) changes the feedpoint impedence & makes for an interesting dance back and forth between radio & antenna. And you get to say lots of bad words very loudly so as to scare your family out of the house while you go legally insane for a while.

Every time I get started on one of these radio projects, I keep hoping that someone will give some intervention. But no. I get to go crazy doing simple things.

Nils

[YvesRoy]

Hello Nils,

[quote:69e5bc2127]Yves-Roy et les autres, [/quote:69e5bc2127]

Ton fran

[PhilB]

I looked at your antenna pictures and I have a few questions that may relate to performance.

1. It’s hard to see in the pictures. I see a bunch of insulated wires laying on the ground around your antenna. Are they the radials? Radials must be bare copper wire buried at least a few inches underground. The longer the radials are, the better ground contact you will get. Also, one or more 8 ft. ground rods driven into the ground will help you get contact with moist soil deep down.

2. Does your coil have taps? Taps allow you to fine-tune the inductance to resonate with your antenna pipe capacitance.

3. I don’t see a way to adjust your antenna pipe height. This is an important feature for this type antenna. Tuning is very critical. A tapped coil will get you in the ballpark, but you must have a way to fine-tune the antenna capacitance (adjust the height) to get absolute resonant peak.

You may want to consider elevating the antenna as high as feasible for you to get better performance. Whatever you decide, remember you must be able to access the antenna for tuning in its final position.

[Rich]

[quote:6c9505cb7a=”PhilB”]Radials must be bare copper wire buried at least a few inches underground. [/quote:6c9505cb7a]
Quoting Edmund Laport’s [i:6c9505cb7a]Radio Antenna Engineering[/i:6c9505cb7a] (McGraw-Hill 1952) about installing radials for MW antennas:

[color=blue:6c9505cb7a]”The depth of the wires is immaterial, whether the soil be moist or dry. They may be placed on the surface except that they are then subject to injury and prevent the use of the land for other purposes.”[/color:6c9505cb7a]

When the radials are not buried, it doesn’t matter whether or not they are bare. The currents they carry are r-f currents, which enter the wires by radiation. The earth itself is a rather good insulator, and bare wires buried in the earth are therefore surrounded by “insulation,” also.

[quote:6c9505cb7a]The longer the radials are, the better ground contact you will get.[/quote:6c9505cb7a]
The improvement in the performance of the ground system with added radial number/length comes not from “better ground contact,” but from the fact that they then intercept more of the r-f current induced into the earth by the vertical radiator– most of which is contained in a circular area around the radiator having a radius of about 1/2 wavelength.

If better ground contact was the only issue, that could be achieved by burying short coils, zig-zags, or loops of wire of the same total length, or copper plates having the same surface area as the normal radial system layout. But it wouldn’t work well, because the r-f currents then have to travel through 1/2-wavelength or so of earth to reach those conductors, and in doing so they have encountered high r-f losses before they ever reach the buried ground conductors. Those losses are in series with the antenna current, and so the radiation efficiency of the antenna system is reduced.

//

[YvesRoy]

Hello PhilB,

[quote:17214a266c]I see a bunch of insulated wires laying on the ground around your antenna. Are they the radials? [/quote:17214a266c]

Yes, they are 4 radials of 10 ft suspended about 1 inch above the soil.

[quote:17214a266c]Does your coil have taps?[/quote:17214a266c]

No. The main reason is that i am not planning to change the frequency for broadcasting.

[quote:17214a266c]I don’t see a way to adjust your antenna pipe height[/quote:17214a266c]

My antenna is 3-meter fixed height. So i can’t “retract” antenna like a TV antenna.

[quote:17214a266c]A tapped coil will get you in the ballpark, but you must have a way to fine-tune the antenna capacitance (adjust the height) to get absolute resonant peak. [/quote:17214a266c]

It exists an application (DOS version) which calculates the loading coil inductance based on the frequency, the height and the diameter of vertical antenna which is called [b:17214a266c]BOTLOAD2[/b:17214a266c]. You can download this application at this following link:

http://www.btinternet.com/~g4fgq.regp/page3.html

The application will also gives the number of turns on coil based from the coil former diameter and length.

Regards.
Yves

[PhilB]

Rich,

I think your post regarding radials may be a little misleading with regard to YvesRoy’s situation. Would you care to comment on the effectiveness of four 10 ft radials suspended just above the ground vs. equivalent number/length buried bare wires and ground rods?

Part 15 broadcasters want to get best antenna performance with a PRACTICAL installation. Often property boundaries restrict radial length possibilities severely. A real broadcast class radial system is completely out of the question for 99.999% of part 15 broadcasters.

Assuming an area of say 50 ft by 50 ft, what is your recommendation for the best possible ground system?

[PhilB]

YvesRoy,

I’ll give Rich the opportunity to give more precise technical comments on your radials after my last post, but I will venture to say that four insulated 10 ft radials are, for practical purposes, totally ineffective as a ground system. Your real ground path is most likely through your audio and power feed wires back to your house wiring ground point. This might be good if the audio and power wires ran substantially vertically to an elevated antenna, but this is not your situation.

As I commented a while back regarding the Manteca Magnum antenna, it is very unlikely that a fixed-tuned base-loaded antenna will perform optimally. These antennas have a very high Q, which means they have a very sharp resonant peak. It isn’t realistic to expect a fixed tuned antenna constructed from calculated parameters to perform optimally.

From my experience, the coil is the biggest factor that can throw you out of tune relative to calculated values. There are just too many variables working against precisely achieving the calculated turns per inch and overall length of a coil. Variations in insulation thickness and minor zig-zags between turns add up to sigificant variation from the calculated parameters over the entire coil.

Here is something easy to do for an approximate check on the tuning of your antenna. Try setting your transmitter frequency 10 kHz on both sides of your operating frequency. Check the field strength or RF metering to see if it increases on either side. If it increases, the antenna is not tuned optimally to your desired frequency. If it doesn’t, you are in pretty good shape except that you may be off by a few kHz which could be corrected by having a few inches of antenna height adjustment range.

[Rich]

[quote:f7cd9dd5ba=”PhilB”]Would you care to comment on the effectiveness of four 10 ft radials suspended just above the ground vs. equivalent number/length buried bare wires and ground rods?[/quote:f7cd9dd5ba]
A 3-meter vertical working against four 10 ft radials insulated from, and suspended above ground really is in the form of a ground plane antenna, and as you probably know, they work well. Depending on r-f losses in the buried radial system, the “ground plane” version might even be more efficient (other things equal). I think you told us you have EZNEC. Why not model the system near ground, but with elevated radials? I think you’ll be surprised.

Earlier in this thread Yves-Roy has a link to a very good paper on ground systems for vertical antennas, written by Rudy Severns. Reading/understanding that paper and applying that information will be very useful to Part 15 AM-ers.
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[Ebacherville]

I basicly did the same as you did used a formula to calculate the exact antenna i needed, i quiclk found out that there are way to many avriables even the most complex formula cant take in to account..

My antenna was a fixed mast (non adjustable) and my coild has no taps… Well it worked ok but not near what it could be doing….

SO first thing i did was made the mast adjustable.. this helped quite a bit.. but i knowi can still do better, so now im making a tapped coild and new mast exactly as the SStran specs say.. this should be a big improvement…

there are way to many variables to consider to build a totaly fixed antenna system.. thers soild conductivity, efficience of the wiring to conduct the connections of wires to the mast etc. This is why the antennas need to be adjustable, were dealing with such constrained rules thatwe absolutly have to be able to tune the antennas to there surroundings.. I would also bet after time goes by we will need to retune due to currosoin on ground radials or other electrical joints in the antenna..

SO you probably in the same boat as i am… reconstructing the entire antenna… but look at it the way im looking at it.. the first one provicded a great trial run at construction methods.. i feel that my second one will be much more sepiorly build and will most likly look like a pro built antenna 🙂

Jason

[Nils]

Compa

[Author]

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