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Properly comparing and evaluating antenna question.

P

Palex9

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Dec 23, 2012
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Just got a MFJ 269 analyzer and started measuring everything in sight, like coax db loss, swr, etc.

I don't understand the following: you measure an antenna's resonance at various frequencies. on a ST4 discone I found that it resonates good at 146.00mhz with 1.1 swr for example. But that does not tell me how this antenna may or may not have more 'gain' (not sure if right term) as compared to another antenna that might have the same 1.1 swr at 146.00. any point in comparing db loss feedline/antenna or how to see which is better....

73s

Peter

KK4NUK/AG
 

That analyzer can't tell you anything about 'gain', it can only analyze what the antenna's impedance is and if it's resonant or not. That resonance and impedance matching tells you how efficiently an antenna can radiate, how 'tuned' it is, that's all. You hav to know something about antennas, how they work, and exactly what 'gain' means.
A diskcone antenna has no gain at all. That's because of it's 'length' and the type of antenna it is. It has nbo gain at it's primary resonance point, that 146 Mhz for example. But, at jigher frequencies where it's 'lengths' are more than a 1/4 wave length, the harmonics of that 146 Mhz, it will have some gain. Not much, but some.
So how do you find gain? By measuring the signal strength at a particular distance and comparing that strength to that delivered by a 'standard' antenna, usually a 1/2 wave length at the same distance. Or, if a standard antenna isn't that 1/2 wave length one, then you have to state that the 'gain' realized isn't 'standard' but in relation to whatever antenna you used to make the comparison. If a standard antenna isn't used, then those gain figures aren't 'standard' either so don't amount ot much useful information. "Dang, this antenna's got 50 dB of gain!"... Over what? "Oh, that broken beer bottle over there in the trash.". See how that can go? (Gotta to be a Coors can, right?)
There's more to all this stuff than is readily apparent...
- 'Doc


The operating manual that comes with that analyzer can tell you how to do a lot of different things, so read it. If you don't understand some particular part of it, do the 'digging' required to learn what it's talking about. There really ain't no short cuts till you can do the 'long route' first.
 
Hi Pete,

Like DOC said, all you can measure with the antenna analyzer is VSWR and cable characteristics (ie. cable loss and TDR functions). The VSWR will show you the resistive (Rs) and the reactive (Xs) of your antenna system. What you ideally like to see is Rs = 50, and Xs = 0 at resonance (point of lowest VSWR). Most of the time that won't happen and you will still have a low VSWR. You can then measure your antenna's Bandwidth (BW) over a specified VSWR range such as 1.5:1 or 2.0:1. This way you can determine what the lowest and the highest frequency provides either a 1.5:1 or the 2.0:1 VSWR range.

Antenna gain requires a field measurement at an antenna range. You would have to rotate the reference antenna a full 360 degrees and use that as a reference reading from about 10 wavelengths away. Then you would rotate your other antenna 360 degrees measured from the same distance and there is your antenna gain measured in dBd.

There are three antenna gain values: dBd (gain referenced to a 1/2 wave dipole), dBi (gain referenced to an isotropic source), and just dB (which has been used in the mobile markets as gain referenced to a 1/4 wave ground plane). The best is the dBd reference, as it can be measured easily. The dBi figure is more-or-less a calculated value that is about 2.15 dB higher than a dBd value.

Over simplified, but I hope you get the idea, Pete.

73,
Mike
 
KC9Q said:
The VSWR will show you the resistive (Rs) and the reactive (Xs) of your antenna system. What you ideally like to see is Rs = 50, and Xs = 0 at resonance (point of lowest VSWR).
Click to expand...

Just a note, I believe Pete's analyzer just shows R and X, not Rs and Xs. To avoid confusion, they are pretty much the same thing.


The DB
 
In my understanding that "dB" is a "less meaningfull" term wheter or not it is compared to a mobile antenna (or ground plane)...

One cant extract any knowledge using a term with just "dB".
It could be used in comparasiment to anything ...
Mostly it is used to "cheat" and provide huge gain figures.

I prefer dBI , loads of manufacturers use dBD these days...but tend to add ground gain.
Although dBI is in a "freespace" configuration (software) there is no option to cheat.
And all circumstances are equal to all.
Where in "real live" one migth deal with houses/trees different ground etc.
Besides that one could rotate a dipole 90 degrees and have some serious additional gain compared to it.
Now, for a antenna "freak" this all sounds logical...but for the beginner its just amazing to see how all those dB's look different.

If one measures gain..there are more "things"..of course.
It is NOT easy to measure gain precise.. a rough indiaction is indeed easily obtained.

The resonance "thing" doenst tell you how well a antenna systems radiates.
It just tells you if there will be a miss match or not.
If all the power is "absorbed" by the antenna system (including ground losses)...that doesnt mean
the antenna radiates to maximum "efficientcy.

It is quite possible to "radiate" better while the SWR is worse.
The "prefect ground plane beeing a good example)

Kind regards,
 
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I have to disagree with this,
"The resonance "thing" doenst tell you how well a antenna systems radiates.
It just tells you if there will be a miss match or not.
If all the power is "absorbed" by the antenna system (including ground losses)...that doesnt mean the antenna radiates to maximum "efficientcy."

Resonance -does- tell you if/when an antenna is radiating as efficiently as possible. That's because reactance doesn't contribute to radiation at all, it's a 'negative' factor in that regard, a loss. Resonance is defined as the absence of reactance.
Resonance tells you nothing about any impedance mismatches. It only tells you that what is getting to the antenna is being radiated in the most efficient manner it can be. It tells you nothing about why there may be a loss of some kind, only that the antenna is acting efficiently. If the amount of energy being radiated isn't the same as what's being supplied by the transmitter then the losses are someplace other than the antenna.
That typically means there's an impedance mismatch somewhere. That means that most likely, the feed line doesn't match the antenna's input impedance. That's about as 'common' as it get's. The way to remedy that is by matching impedance with the antenna. And there's where the 'rub' comes in. Impedance is a combination of resistance AND reactance. The likelihood of a resonant antenna having a 50 ohm resistance is very unlikely UNLESS some means is used to make it a resistive 50 ohms impedance. That's one instance where having the proper amount and kind of reactance can come in very handy. But that reactance is in the matching device, not the antenna, and that makes a big difference. Since reactance doesn't produce or transfer power, there will always be some loss associated with that matching device. The important part in that is that even with that loss because of the matching device, the loss will be less than if no matching device was used. When it's done properly there is very little loss in an impedance matching device/circuit. some, sure. But not as much as if that device wasn't used. Reactance isn't always a 'bad' thing, it depends on how/why/where it exists.
- 'Doc
 
Yes, agreed doc...

I should have said it otherwise....still needed a cup of coffee hihi.

You can have a perfect 1;1 SWR though the efficiency is less with a SWR with say 1:1.5

So, why you migth "match" your system it could be you have made it less efficient.

thats why those (ground losses) are writen there..but i think you knew what i ment.

I had that "impedance matching" you wrote in mind..

Thanks for the clarification !

H.
 
My disagreeing concerns only the two things concerning only the antenna, resonance and efficiency, and resonance telling you about impedance matching. It can do the first one, it can not do the second one. If you are talking about the whole antenna system, then those 'efficiency' and 'matching' thingy take on a different meaning, they aren't -specific- anymore.
You are correct in general, I don't disagree in that way. You have a problem with saying what you want because of language differences (and I have the same problem!), that's a sort of 'given' and we both have to contend with it. Oh well...
- 'Doc
 

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