The difference an antenna makes on HF

[northern35s]

Isotropic antennas or gain figures can be very misleading. I've found that taking those figures with a large dose of salt is a very good idea. It also helps to keep in mind that those isotropic antennas are being compared to 'perfect' antennas located in a 'perfect' position. It's still a comparison to a 1/2 wave length antenna, so how can they have 'gain' over themselves? Keep things in perspective...
- 'Doc

Not really sure what you mean by 'keep things in perspective', what things are you talking about

We can compare antennas to anything we like, however the normally accepted standards are an isotropic radiator and a dipole in free space, and, just like the isotropic radiator, a dipole in free space doesn't exist.

You take the "isotropic antenna" with a dose of salt as much as you like, be my guest even, but as to what purpose or benefit that serves I don't know.

 

[W5LZ]

"..., however the normally accepted standards are an isotropic radiator and a dipole in free space, and, just like the isotropic radiator, a dipole in free space doesn't exist."

Exactly.
But if you eliminate that "free space" qualifier for a dipole, you have the non-isotropic dipole things are compared to (dBd). To make a valid comparison between a dipole in free space, or an isotropic dipole, and a 'real' dipole, you will need to have access to a very nice antenna range that can get a 'real' dipole to whatever height (if you are comparing in regards to antenna height) and take precise measurements. Then you have a valid comparison.
How about doing that comparing using VHF/UHF antennas instead of HF antennas? That can work under certain conditions, but it's just not going to be a totally accurate comparison without making allowances for the different characteristics between those two frequency ranges, 'path affect' being one of those 'catches'.
I still like a little 'salt' with my isotropic gain figures.
- 'Doc

 

[jazzsinger]

It also helps to keep in mind that those isotropic antennas are being compared to 'perfect' antennas located in a 'perfect' position.
- 'Doc

WTF ? an isotropic antenna is an imaginery antenna which if it existed would radiate equally in all directions. its not compared to anything, it's an imaginery perfect reference point.

 

[W5LZ]

jazzsinger,
Whether you realize it or not, you just confirmed what I said. Think about it...

 

[wavrider]

WTF ? an isotropic antenna is an imaginery antenna which if it existed would radiate equally in all directions. its not compared to anything, it's an imaginery perfect reference point.

So now we should use an imaginary reference point?

This could get good:tongue:

 

[Captain Kilowatt]

WTF ? an isotropic antenna is an imaginery antenna which if it existed would radiate equally in all directions. its not compared to anything, it's an imaginery perfect reference point.

Whether you compare an isotropic source to a real antenna or a real antenna to an isotropic source it ends up being the same thing. It's done all the time and YES an isotropic source is a reference but is that not what is needed to make a comparison? When gain is quoted as dBi is that not comparing it to an isotropic source?

jazzsinger,
Whether you realize it or not, you just confirmed what I said. Think about it...

I thought the same thing.

So now we should use an imaginary reference point?

This could get good:tongue:

We have been doing it almost since the advent of radio.

 

[wavrider]

We have been doing it almost since the advent of radio.

Maybe since the advent of modeling software.

 

[Captain Kilowatt]

Maybe since the advent of modeling software.

You mean we haven't had the slightest idea what gain or F/B ratio was until after the computer was invented??

Gain measurements were made against an isotropic source ever since man learned how to use a slide rule. Seriously, the isotropic antenna was conceived long before modeling software was around. It simply involves a bit of geometry and the application of the inverse cubed law of power density.

 

[wavrider]

I agree, an isotropic antenna is nothing more than a mathematical ideal.

 

[jazzsinger]

jazzsinger,
Whether you realize it or not, you just confirmed what I said. Think about it...

i have thought about it, i still don't see how i confirmed what you said and i quote again:

It also helps to keep in mind that those isotropic antennas are being compared to 'perfect' antennas located in a 'perfect' position.

an isotropic radiator is imaginery, its a standard REAL antennas are judged against, it is not compared to a perfect antenna in a perfect location, infact the exact opposite is true,

if anything it is compared to imperfect antennas in imperfect locations ie REAL antennas, if it existed it WOULD be a far from perfect antenna, which is the reason people design antennas for GAIN, at the compromise of loss in other directions, unless you live in the middle of a multi storey building how can an antenna (albeit one that doesn't exist) that fires rf straight up and straight down be perfect????????????????????


SO WHERE EXACTLY DID I CONFIRM WHAT YOU SAID ??????????

but hey, it really depends on your concept of the word perfect. some people think a rubber duck on a handy is perfect, others think an A99 is perfect.

 

[w8fax]

To prevent errors, always think of what powers are being compared when using antenna gain. The choices for reference power are isotropic antenna (dBi), antenna mainbeam (dB), or some other antenna (dB).
1. dBi is usually a positive number,
2. antenna mainbeam dB are almost always negative,
other antenna comparisons may be positive or negative.
Remember that dBi IS an imaginary reference. dBi refers to an isotropic antenna that radiates equally in all directions. Power cannot be created by an antenna, but it can be re-directed. Gain in one direction will always result in loss of gain in another. Lets say we have a yagi that is claimed have 6 dBi gain. This means that the antenna has a 6 dB gain over an isotrope antenna in space. However, if the comparison is given in dB's, then it relates to ANOTHER existing antenna. So our yagi that has 6 dB gain (NOT dBi) over a dipole at the same condition (height etc) (NOT dBi) is compared to a dipole.

 

[wa4gch]

Heck because of space I'm stick with a R7 but it works .....

R7 front and G7-144 back.

 

[W5LZ]

W8FAX,
It amounts to the antenna used for comparison. If you would add a "d" to the end of that 'dB', I would agree with what you said (dBd versus dBi), since a dipole is a 'standard' reference antenna. With out that 'd' or 'i' used in the measurements units, you have no idea what the standard for comparison is. There are all kinds of non-referenced standards with people making gain statements, and if that reference isn't given, then you can't honestly claim a fair comparison. [A very common practice in advertising, wouldn't you say? Almost any gain figure can be claimed with the right comparison 'antenna'.]
It used to be that gain figures stated with just a plain 'dB' was a comparison with a dipole antenna. But, since isotropic calculations/programs are so easy to use now, you can't really make that assumption anymore. Unless the reference is given, the numbers are meaningless.
- 'Doc

My favorite 'reference' that certainly can explain some of the gain figures you see now is a 'dead' beer can laying in the yard. And if you really wanna get 'picky', it's what brand of beer can!

 

[Quiksilver]

That's just not fair.... :tongue:
How do I compete with those monsterous towers & antennas....

 

[Marconi]

Some have used the definition for dbi as, "...dBi refers to an isotropic antenna that radiates equally in all directions."

It should read with this distinction, "...dBi refers to a isotropic antenna without losses that radiates equally in all directions as a reference.


Jazzinger, RF theories were postulated long before computers, read a little info from one of your own countrymen, James Clerk Maxwell - Wikipedia, the free encyclopedia