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Aerials/Antennas

C

Chernobyl2

Member
Sep 2, 2009
18
0
11
Hi,

Regarding aerials/antennas for HF, does anyone have any information on the different types, etc..?

I'm not a mathematician, but can handle complex concepts of operation.

I'm just looking for the basics for now. I'm sure I'll have more questions later.

Thanks! :biggrin:

Chernobyl2.
 

Chernobyl2,
Reduced to it's very basic state, an antenna is a conductor of electricity of an alternating current form. The size and shape of that conductor has an affect on it's being an antenna, it's length in particular. That 'length' is frequency specific and deals with how far energy at a particular frequency travels during one complete 'cycle' of it's existence. From there it gets a little more complicated because of some other characteristics of alternating current at a given frequency. [That's about as far as I'm going with that. I'm too lazy to 're-think' what little I know about antennas, and I'd probably get something wrong anyway.]
Believe it or not, all antennas can be reduced to having the properties of a 1/2 wave length antenna. Some times half of that particular length doesn't appear to be there, but it is in some form or another. That 1/2 w can be made even shorter by 'loading' it (meaning some inductance is being added in some manner, a coil of some form most commonly). That's also where things get more complicated again because of something called inductive reactance. Reactance is how 'stuff' reacts with alternating current, and is a part of impedance, which is resistance in alternating current terms. Impedance is important because if there's enough of it, no signal (electrical energy) ever get's to an antenna to be radiated.
Which brings us to another property/characteristic of antennas which is their radiation patterns, where that antenna sends a signal when it's radiated. If a particular 'size/shape' antenna puts a signal where you want it to go, then it's a pretty good antenna, which is the whole point anyway, right? And from there it gets really complicated. All the juggling that can be done to end up with getting that signal where you want it.
If you have the required basic knowledge of electricity, all of this starts to make 'perfect' sense. The different qualities/characteristics/abilities/liabilities and who knows what can be manipulated to produce a 'good' antenna. Don't ask me why, but painting the @#$ thing pink seems to make it work better. (That last supplies your minimum daily requirement for B.S., by the way.)
That's about as simple as you can get. It isn't complete by any means, and hasn't even touched on any of the math involved. But it's the general idea of what antennas/aerials are all about.
Now for some 'magic' numbers!
1/4 wave length: 234/f in Mhz = Feet.
1/2 wave length: 468/f in Mhz = Feet.
1 full wave length: 984/f in Mhz = Feet.

Those 'magic' numbers, 234, 468, and 984 will get you very close to three of the 'natural' resonant lengths for whatever frequency. They are the 'real world' numbers which take radius to wave length ratios of conductors into consideration. Considering that everything around an antenna will affect it, they will get you 'close enough' in most cases, but are never exact.
Sorry you asked about this stuff yet? Oh well, if it were simple, no one would think anything about it. There's a huge amount of information available about the theory and practical thingys about antennas. If you really want to know all that, I'd suggest the ARRL's HandBook as a starter. Not that it's the 'end all' about antennas, it just has most of what information you may want, in one convenient book. It's probably 98% accurate/true, and is 'up dated' sort of regularly.
Knock yourself out! Have fun.
- 'Doc

None of this is strictly for HF, it applies to all frequency ranges till you get into the microwave ranges. Then it get's really weird.
 
Your so called magic numbers are a bit mixed up,in the first two you include .95 velocity factor (the standard vf for copper/aluminium in air) and in the last one you don't.Insulated wire will have a slightly different length depending on the type of insulation and its dielectric properties.

It should read:

1/4 wave (feet) = 233.7/Freq (Mhz)
1/2 wave (feet) = 467.4/Freq (Mhz)
1.0 wave (feet) = 934.8/Freq (Mhz)

Rounded up for trimming,234,468 or 935 divided by frequency in mhz.

To find free space wavelength:

1/4 wave (feet) = 246/Freq (Mhz)
1/2 wave (feet) = 492/Freq (Mhz)
1.0 wave (feet) = 984/Freq (Mhz)

To find wavelength in coax multiply free space wavelength times cable velocity factor (vf) usually around .66 for solid dielectric cables or .79/.80 for foam dielectric cables,or by whatever your particular cable is according to manufacturers specs.

To convert any of these lengths to Metres divide by 3.28.
 
jazzsinger,
Those 'magic' numbers are from common usage and experience. They are derived from the use of 'small' conductors for antennas and are a very general thing. Take a gander at their derivation in that Hand Book in the antenna section, it's there. Has nothing to do with copper, as such. As for rounding up/down, would you rather have it too long, or too short?
- 'Doc
 
HiDef,
I'd be willing to bet that almost every antenna you've used has been resonant. Or at least as resonant as you could make it. The definition of resonant is no reactances present. Has little to do with physical length if you load it to neutralize any reactances present.
- 'Doc
 
Chernobyl2 - you mentioned the types of antennas. Most of them can be referred to as either vertical or horizontal. Vertical antennas are a little more tricky to build. Horizontals can be easier; I'll get to that in a second. They are generally in increments of 1/4, 1/2, and 5/8 total wave length of the frequency to be used. No one antenna 'fits all'; as they must be tuned for the bandwidth used for transmission and receiving. An antenna for the 2 meter band (~144.000mhz) is considerably shorter than a 11 meter 'CB' antenna (~27.000mhz) - because any antenna must be resonant -or 'sympathetic'- to conduct the flow of transmitting energy out of that radio. So, you must be specific as to which antenna you intend to build in regards to the freq you intend to use it on.

Since you are asking about the particulars of antennas, I must assume that you are considering to build one. The horizontal dipole is a very easy to build antenna, and is quite common. Maybe not the best antenna to have - but it is very easy to make - provided you follow the guide lines. Materials needed are as simple as some wire and the means to measure and mount them to become effective.

This is a thread that goes in to some decent detail, and should help you get started:
http://www.worldwidedx.com/cb-antennas/31857-dipole-your-citizens-band-radio-base-station.html
 
W5LZ said:
jazzsinger,
Those 'magic' numbers are from common usage and experience. They are derived from the use of 'small' conductors for antennas and are a very general thing. Take a gander at their derivation in that Hand Book in the antenna section, it's there. Has nothing to do with copper, as such. As for rounding up/down, would you rather have it too long, or too short?
- 'Doc
Click to expand...

I don't particularly think looking in a handbook will change the fact that your last number is wrong.I've already told you why its wrong.

I take it you've never heard of the concepts of books being wrong or f@ckwits writing books?

Only the gullible believe what they read in one source,wise men read many sources and seek to educate themselves and question others interpretations.Once they have amassed an array of knowledge then they are in a position to make an informed judgement of facts being presented and whether or not those facts are right or wrong.

It has everything to do with copper as such,and aluminium,because they both have very similar velocity factors when air or other insulator is the dielectric surrounding them.

why not do the mathematics yourself and you will see why its wrong,infact i'll even do that for you to prove it is wrong.

300 speed of light (rounded down as you are using MHZ and not hertz as your frequency measurement) x 3.28 to convert to feet/s = 984 which gives you the number you divide your frequency by for the wavelength in feet in freespace.

984 x .95 the velocity factor of copper/aluminium in air/insulators with similar velocity factor characteristics to air = 934.8 which more often than not is rounded up to 935 or 936 for trimming,infact in most cases a bit extra is added over and above that to allow for the proximity effects of other objects.

try it with the other figures you quoted correctly and you will see they both have .95 velocity factor figured in where your last one doesn't,which makes it WRONG!!!!

I ain't one to be baffled by bull.

As for rounding up I think its particularly obvious which one I prefer,as I clearly rounded them all up to the nearest foot to allow for trimming.As its easier to trim a piece of metal/wire than it is to reattach it.
 
I would suggest you take another look at those numbers in that book. I think you will find that 984 number is actually about 97% of the isotropic, or theoretical full wave length number. It's also one of those numbers that I happened to figure after resonating more than one full wave length antenna, then measuring them. After averaging, it worked out to 984. That's still a 'ball-park' guess for a starting point, just like those other numbers. Certainly works for me, but if it doesn't for you, that's fine too, use your own preferred number.
And contrary to some people's belief, I really have read more than just one book about this sort of stuff. Except for some minor points that are variable anyway, most of them agree about the basics. This sort of thing is a basic thingy. I honestly don't think I've ever said anything on this forum that was not factual and common knowledge. Sure, some of the things I say are pure B.S. but I think it's pretty easy to determine which things those are, and they are not presented in a 'serious' manner to start with. Why should I try to fool anyone? This stuff is just too easy to verify.
- 'Doc
 
W5LZ said:
I would suggest you take another look at those numbers in that book. I think you will find that 984 number is actually about 97% of the isotropic, or theoretical full wave length number. It's also one of those numbers that I happened to figure after resonating more than one full wave length antenna, then measuring them. After averaging, it worked out to 984. That's still a 'ball-park' guess for a starting point, just like those other numbers. Certainly works for me, but if it doesn't for you, that's fine too, use your own preferred number.


This stuff is just too easy to verify.
- 'Doc
Click to expand...

Contrary to what you think,dividing 984 by frequency gives 100% of a wavelength.

Dividing 954.48 by frequency gives 97%.

And like i've said numerous times before dividing 934.8 gives 95%.



If its so easy to verify why not provide the arithmetic that gives you the notion that dividing 984 gives 97% of a wavelength?

Because i'd love to see it!

Seeing as its one of the most basic fundamentals of radio theory/antenna building and your a ham,then you should be able to prove me wrong with the arithmetic,rather than having to quote from books that are wrong.

97% of the isotropic ?

wtf does that mean,isotropic is an imaginery point source antenna that radiates equally in all directions with zero gain,and if it was possible to build one,which it isn't as it defies maxwell's equations it would be spherical,not a longitudinal element.

Dearie me,to argue black is white just because a book says so:confused: makes no sense.
 
W5LZ said:
HiDef,
I'd be willing to bet that almost every antenna you've used has been resonant. Or at least as resonant as you could make it. The definition of resonant is no reactances present. Has little to do with physical length if you load it to neutralize any reactances present.
- 'Doc
Click to expand...


My current H.F. antenna is a 40 meter double extended zepp. It exhibits reactances on every ham band below 29.7 mhz.
 
Hi,

I had a look at those links posted further up the thread. Thanks for those!

I've got a bit better idea for what I want I think.

I read somewhere that you can build a multi-element dipole antenna, but the fact it is multi-element affects its performance. Does it affect it much, or does it depend upon the radio you're using?

Is there a particular way to hang them? e.g. facing East/West, so one end is strung North and the other South?

North

I
I
I
E
I
I
I

South

?

I also want to be able to listen to the majority of frequencies between 150 kHz and 30 MHz.

Chernobyl2.
 

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