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ANTENNA DIAMETER

S

Surfside

Here's something I always wanted to know. Is antenna diameter covered in the rules? Would increased diameter affect the signal.

For instance, a radio shack whip is one size. A tree trunk is another.

I don't see it in the rules anywhere. Seems like you could have a 400 foot diameter antenna that, if it were less than 10 feet high would be legal.

I am sure I am missing something and my knowledge of antenna design is null- so who can chime in?
 
Surfside said:
Is antenna diameter covered in the rules? Would increased diameter affect the signal.
Click to expand...

As the diameter increases, the reactance decreases, and the change in reactance with a a change in frequency also decreases.

This means the that loading coil needed to resonate the antenna will be smaller, and that the r-f bandwidth of the antenna system will increase.

The reduced coil size might mean that the DC resistance of the coil can be less, which can improve the radiation efficiency of the antenna system by small amount (probably not enough to be noticeable, other things equal).
//
 
Surfside said:
Here's something I always wanted to know. Is antenna diameter covered in the rules? Would increased diameter affect the signal.

For instance, a radio shack whip is one size. A tree trunk is another.

I don't see it in the rules anywhere. Seems like you could have a 400 foot diameter antenna that, if it were less than 10 feet high would be legal.

I am sure I am missing something and my knowledge of antenna design is null- so who can chime in?
Click to expand...

Surfside

If you did use anthing large for an antenna say a 15 inch dia pipe, the loading coil can also be a better match as you can also use a 15 inch diameter loading coil and the number of turns can be reduced to 25-30 depending on frequency if you use .25 dia copper tubing (the type used for plumbing), it would also have less resistance and therefore the efficiency should be better.

Use this calculator to get the inductance you need for the loading coil...

http://www.crystalradio.net/cal/indcal2.shtml

Radiopilot
 
radiopilot said:
Use this calculator to get the inductance you need for the loading coil...
Click to expand...

But please note that this on-line calculator has no input condition to relate the OD of the radiator to the coil parameters needed to produce resonance of a 3-m Part 15 AM antenna system at its operating frequency when mounted with its base at earth level.

If you are not able to analyze this scenario accurately, you will not not be able to define and construct the coil that will be needed to achieve system resonance.
//
 
R. Fry said:
radiopilot said:
Use this calculator to get the inductance you need for the loading coil...
Click to expand...

But please note that this on-line calculator has no input condition to relate the OD of the radiator to the coil parameters needed to produce resonance of a 3-m Part 15 AM antenna system at its operating frequency when mounted with its base at earth level.

If you are not able to analyze this scenario accurately, you will not not be able to define and construct the coil that will be needed to achieve system resonance.
//
Click to expand...

Reg Edwards G4FGQ has written several free downloadable antenna analysis programs that allow the user to enter the antenna diameter as well as the other parameters. The programs ENDFEED (for end-fed antennas), TANT (for Marconi "Tee" antennas--just set the tophat dimensions to zero for a 3 meter Part 15 AM vertical antenna), and TOPHAT2 (a variation of TANT) can all be used to analyze Part 15 AM antennas.

The programs demonstrate how variations in antenna diameter (and length, of course) affect the required value of the loading coil (and the component values for L-Match and T-Match antenna matching networks). They can be downloaded from here: www.btinternet.com/~g4fgq.regp/#S102 . Also, you can run them on the web site without downloading them if you wish.


-- Black Shire
 
As stated previously, I found that increasing the width of the antenna did have an effect on my particular signal. I can not say for sure, depending on your area and installation as to if you will experience this.

Giving an example of a tree trunk of copper is a pretty good scenario. AFAIK, there is no rule saying you can not have such a large width antenna. but my thinking was to make it larger, but no larger that would make it totally noticable (like sticking out in a crowd with a weird hairdo).
As Mr. Fry pointed out, this reduced the width (and work involved) with building the coil, so I was for it, should it prove to be better than the original designs.

We have very little power; so any basic improvement within the rules is to our favor.

Believe me, the FCC knows we do try to stay within rules but will try things to not so much bend them, but to live within them while still attempting to improve our signals.
 
Thanks for the tips guys! It begs to ask the question- would a horizontal 200 foot antenna serve us better than a 10 foot verticle? Could it be argued that the antenna length is what is vertical?

Also- how about a 10 foot tall antenna with horizontal side elements sticking out 10 feet with vertical 10 foot elements atached.

IE An "H" design with numerous elements. As long a sthe over height is under 10 feet, are we still legal?

Seems like it may improve performace. What do you think?
 
Surfside said:
Could it be argued that the antenna length is what is vertical?
Click to expand...

It always could be argued, but maybe not to the satisfaction of an FCC field inspector.

Part 15.219 doesn't say what the physical configuration and orientation of the antenna+ground conductor must be, only that the total length of that system cannot exceed 3 meters.
//
 
Horizontal polarization stinks for MW Part 15. Stick with vertical.
 
Surfside said:
Thanks for the tips guys! It begs to ask the question- would a horizontal 200 foot antenna serve us better than a 10 foot verticle? Could it be argued that the antenna length is what is vertical?

Also- how about a 10 foot tall antenna with horizontal side elements sticking out 10 feet with vertical 10 foot elements atached.

IE An "H" design with numerous elements. As long a sthe over height is under 10 feet, are we still legal?

Seems like it may improve performace. What do you think?
Click to expand...

Surfside

200 foot would be better but not legal for Part15.. also you are better with 15 feet vertical than 200 foot horizontal, also not Part15.

Actually if I had to do my setup again I'd use 7 feet of vertical antenna followed be a 2 foot diameter top hat or some sort of disk top hat... been reading about these top hats, or 6 foot vertical with 3 foot diameter top hat, may have to experiment.

Quote:

"Capacitance loading of a short vertical antenna can improve radiating efficiency more than inductance loading. Best possible improvement for a 2/3-height loading coil relative to base loading is typically 3 dB. But a large top hat can improve performance by more than 6 dB.

Top-capacitance loading improves radiating efficiency in two ways:

A more uniform distribution of antenna current versus height results in an increase in radiation resistance for no loss in the top-hat. Whereas the high loss resistance of a raised coil can outweigh the benefit of increased radiation resistance.
A tuning coil is always necessary. But its inductance is less when a capacitance hat is fitted and if overall coil dimensions are unchanged then loss in the coil is much smaller."

So adding such a top hat is 6db improvement... that is huge for our small setups.

Radiopilot
 
I agree! Sounds like double. Maybe a hat is what I need. All my transmitters are rangemasters so doesn't the tuning inside the box serve as all the coil I need or should I consider an outboard coil?
 
Surfside said:
I agree! Sounds like double. Maybe a hat is what I need. All my transmitters are rangemasters so doesn't the tuning inside the box serve as all the coil I need or should I consider an outboard coil?
Click to expand...

Surfside

I don't think you can change or use another coil with the Rangemaster.. You might want to talk to Keith about that.

I was having a conversation with a someone I know and he is pretty heavy on dx ham stuff... anyway he built a Part15 LowFer transmitter 100mw power and fed it into a 3 meter 2" aluminum antenna (conduit) and used a satelite dish with metal screen for the top hat and a large loading coil 10 inches dia. using 18 gauge insulated wire and the frequency was 510 khz. The antenna is mounted to the roof gutter line 12 foot high.

He lives in North Carolina

Here are the distances he recieved contact as dxg:

1st Reception report for this beacon was made by M. P., V***T, London Ontario 537 Miles :D

Best DX Reception for this beacon is from L. K., K**R, Aitkin, Minnesota 1035 miles :eek:

27 Jan 2001 M. P. London, Ontario, Canada 537 Miles 1st Reception on 1st day
17 Nov 2001 P. S. Raleigh, North Carolina 104 Miles
08 Dec 2001 L. P. Pasadena, Maryland 339 Miles
09 Dec 2001 B. R. York, Pennsylvania 380 Miles
09 Dec 2001 B. F. Rockville, Maryland 323 Miles
09 Dec 2001 L. C. Marriottsville, Maryland 337 Miles
10 Dec 2001 L. K. Aitkin, Minnesota 1035 Miles Best DX
10 Dec 2001 J. S. Normal, Illinois 309 Miles
12 Dec 2003 L. C. Marriottsville, Maryland 337 Miles
15 Dec 2003 L. P. Pasadena, Maryland 339 Miles
16 Dec 2003 S. D. Elm, Pennsylvania 405 Miles
16 Dec 2003 J. R. Burlington, Connecticut 603 Miles
14 Jan 2004 R. T. Smithville, Mississippi 463 Miles
20 Jan 2004 M. P. London, Ontario, Canada 537 Miles


Now I asked him what about the local signal...

He got a clear signal out to 20 miles on a truck radio! I asked him if he tried the signal at a higher frequency... he said at 1700 Khz. the signal did much better. :eek:

I will definately be changing my setup soon! I will get more info and pictures about his setup.

Radiopilot
 
WOW!! That certainly beats the 1/2 mile I get from my Rangemaster whip installation!!

Time to build a cap hat!!

Thank you so much for your help- does he have a grounding system built for this. Looked at the pics and it's seemed pretty cool!

Dave
 
radiopilot said:
Black_Shire said:
Is this his MedFER beacon?

MedFER Beacon NC 510 kHz (old): http://www.w4dex.com/medfer.htm

MedFER Beacon NC 1700 kHz (new): http://www.w4dex.com/hi_medfer.htm


-- Black Shire
Click to expand...

You are correct! I didn't want to post the website without his permission. Anyway I stand correct on his assertions about what he described regarding the local signal going out 20 miles...

Radiopilot
Click to expand...

Oops...I hope I didn't "post out of school" there... But I had known about the MedFER beacon NC for quite some time from the Long Wave Club of America www.lwca.org web site. I'm curious about the bandwidth of his two antennas--would they pass a 10 kHz wide AM signal? (MedFER, LowFER, and HiFER beacons often transmit very narrow-bandwidth [1 kHz - 2 kHz or so] signals, ranging from slow CW [Morse Code] to various digital modes.)


-- Black Shire
 
Hey Radio Pilot!
I actually built several antennas similar to what you are posting about....and I'm sure we discussed it privately....but here is how I did it.
Using the main plan in building the sstran antenna, I cut the actual verticle radiator down three feet (yes.....three feet).
I then built a hard to configure three prong "top hat"...each prong being one foot long (actually about 11" long to accomodate the fittings), and placed it on top of the antenna.

What I was "going for" was providing as much copper as was legal....in order to be able to house it "inside" a building. I actually had, and still have requests for antennas that will fit inside a house, probably due to property owners' association rules. I also felt it would be a somewhat directional antenna (and it is, depending on how you configure the top), and I still don't know why that is. However, what I did notice was an increase in loudness (6db?.........not sure of that!).

I suspect that this "top hat", as part of the antenna, and...part of the 98" allowed on our antenna kind of works as a counterpoise. And I wished I could prove that to be true, because it would take away from the grounding somewhat if that were so.

Nevertheless I would rather have a verticle radiator than a horizontal one.
I used the same coil (the basic one, not the one we named after you!) in this particular installation.
 
If the top hat is symmetrical and the top of the antenna is connected to its center, the antenna should be omni-directional...but inside a house, metal objects like studs or conduits can affect the antenna's pattern.

The antenna's top hat does not function as a counterpoise (the radial wires and/or ground screen connected to the transmitter's or the ATU's ground terminal function as the counterpoise). Rather, the top hat and counterpoise act as plates in a capacitor, and the top hat forces more current to flow in the upper part of the antenna which causes the upper section of the antenna to radiate more signal.

One could use two such top-hatted vertical antennas mounted "base-to-base" (although it would exceed the 3 meter Part 15 antenna size limit) as a shortened, loaded dipole.


-- Black Shire
 
carlvenorden said:
Hey Radio Pilot!
I actually built several antennas similar to what you are posting about....and I'm sure we discussed it privately....but here is how I did it.
Using the main plan in building the sstran antenna, I cut the actual verticle radiator down three feet (yes.....three feet).
I then built a hard to configure three prong "top hat"...each prong being one foot long (actually about 11" long to accomodate the fittings), and placed it on top of the antenna.

What I was "going for" was providing as much copper as was legal....in order to be able to house it "inside" a building. I actually had, and still have requests for antennas that will fit inside a house, probably due to property owners' association rules. I also felt it would be a somewhat directional antenna (and it is, depending on how you configure the top), and I still don't know why that is. However, what I did notice was an increase in loudness (6db?.........not sure of that!).

I suspect that this "top hat", as part of the antenna, and...part of the 98" allowed on our antenna kind of works as a counterpoise. And I wished I could prove that to be true, because it would take away from the grounding somewhat if that were so.

Nevertheless I would rather have a verticle radiator than a horizontal one.
I used the same coil (the basic one, not the one we named after you!) in this particular installation.
Click to expand...

You're thinking of the top-fed AM towers where the top hat *was* indeed the counterpoise. A matching network was installed between the top of the tower and the counterpoise top hat. The coaxial cable from the transmitter ran up inside the tower (whose base was insulated from the Earth) and to the matching network (center conductor to tower, braid to top hat). There were no ground radial wires running out from the base of the tower.

These top-fed towers (really just upside-down verticals) had the high-current point which produced most of the radiation at the top, which was an advantage. However, they weren't too popular with broadcasters because the base of the tower was the high-voltage point, and feeding AC power to the tower aircraft beacon lights while avoiding RF flash-over discharges wasn't easy.

Also, the top-hat counterpoise was much smaller (and probably considerably less effective) than the buried radial wire counterpoise of a conventional AM tower.

At Part 15 AM power levels, RF flash-over discharges probably wouldn't be a problem for a top-fed vertical antenna, but whether a top-fed 3 meter vertical antenna would be worth the trouble would require testing to determine its performance gain (if any) over a conventionally-configured 3 meter vertical antenna.


-- Black Shire
 
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