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  #21 (permalink)  
Old 03-06-2018, 1:43 PM
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NO. You're expecting results that kind of say good/no good. As the critical ones are for transmitter matching, and you are only receiving, they're going to be either irrelevant, or un-noticeable.

I can direct you to a page full of the maths if you like, but it is heavy stuff - but you could enter all your data, and get answers for missing boxes - but the important thing is context. Keep in mind, few people until very recently could even measure reactance or radiation resistance.

The answers are not readily understandable. Plus of course, the meter is somewhat unreliable. If you take a measurement, and then change some parameters, like narrow the range, you will find the results change.

The vast majority of these parameters are the result of combinations of the others, so there isn't a range of reactance that is somehow desirable - unless for some technical reason phase errors could be destructive - such as when you are stacking or baying antennas to produce higher gains. For a single antenna, they are unimportant.

I've just connected a random antenna to one of these meters, and it tells you quite a bit. Best VSWR is on a rather low frequency in the UHF band, and it drops off quite quickly either side. The lack of continuous VSWR curves makes me think there are reactive components at play - but very clearly this antenna isn't much use at some parts of the spectrum.

I cannot see the point of having ranges for some antenna design features - I don't know what I'd want them for. However - if you wish to calculate them this trustworthy and scholarly site has what you need to get to the bottom of the features you're interested in - but there is no list of acceptable and unacceptable ranges - as they are context sensitive.
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  #22 (permalink)  
Old 03-06-2018, 2:58 PM
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Thank you paulears.
That puts me in the right direction, which is much more than I had 5 minutes ago.
I'll have to do some more reading on this to fully grasp the concept of how this meter works.
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  #23 (permalink)  
Old 03-06-2018, 3:19 PM
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The one thing that I've learned is that in a way, they tell you too much. An antenna I designed and was quite happy with looks dreadful on the analyser. Another really good result from the analyser got me quite disappointed with the real performance. Low VSWR really isn't everything - some antennas by design have a very narrow band where VSWR is low, but it creeps up either side - but a discone I have is just a wavy line - not brilliant anywhere, but equally not awful either, and it seems to be the best wide band antenna I have. The three main antennas here are a white stick ham dual band ham antenna, a High Band UHF commercial single band antenna and a strange UHF J-pole I bought on Ebay. This on the analyser suggests it's not bad, the Amphenol/Jaybeam commercial antenna out performs them all, but looks average on the meter, and the dual bander has a small amount of gain at the two ham bands, but the VSWR rises sharply outside this area.

I cannot as yet - make the connection between the electrical characteristics and the performance.

I treat the peaks and troughs as indicative evidence of the performance,

The meter is interesting, but doesn't seem to enable any prediction on how well an antenna actually works. I've come to the conclusion that you can have an amazing electrically solid and well matched antenna that's just a rubbish at hearing things a long way away!
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  #24 (permalink)  
Old 03-06-2018, 3:22 PM
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Quote:
Originally Posted by paulears View Post
...I cannot see the point of having ranges for some antenna design features - I don't know what I'd want them for...
(I've been lurking all day while I was at work, not able to chime in).

Well then why are these parameters provided for us to see in the first place ?

Apparently they are provided for some useful purpose?

There must be some useful boundaries' one can use to determine their value or even validate/Justify their existence on the device in the first place?

So far what I have gathered is that "Rs" is representing the total resistance seen by a transmitter, and that assuming you are using a 50 ohm rated coax, values seen as close to 72 ohm's should be what we are looking for. (Why 72 ohms not 50 ohms).

The question that still remains is "what would be an acceptable range for above or below 72 ohms".

As far a "jx" (reactance either capacitive or inductive), "zero"; or as close to it as possible, is the desired value.

Finally "Zx" impedance I'm guessing, ideally, in a perfect world that will never exist, should at least equal to or as close to the coaxial impedance of 50 ohms and transmitter output of 50 ohm's... if not that then what?

Again a boundary as to what an acceptable range for "Zx" is lacking.

(Ok, Flame suit on....) ! LOL

Last edited by Mikejo; 03-06-2018 at 4:05 PM..
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Old 03-06-2018, 4:55 PM
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No need for the flame suit - this is one of those questions with answers that just don't make sense. We are guessing that the resistance parameter is radiation resistance, because plain old resistance, as in DC, won't work - because loads of antennas, by design, have grounded elements, that on a meter read zero - so radiation resistance is the likely version being measured - and 72Ohms is the 'natural' impedance for this.

I'm with you on the Z impedance - 50 Ohms would seem sensible, but we picked 50Ohms as a transmit impedance as a kind of receive/transmit compromise - with the 72Ohm standard actually better on receive, but worse on transmit. If the radiation resistance is 72, the 75Ohm coax would be great - but then we stuff it into a 50Ohm impedance input!

The only one my brain has trouble with is the reactance component - zero does seem the thing to design for - but if you do, the achievement of it also prevents the antenna radiating as there is no current differential - and without current flowing, the RF shouldn't actually get out - which it clearly does.

Physics lets us take some of the known variables and calculate the missing ones, and that's where in my humble view we're chasing our tail. The object is to capture as much of the signal as we can, or at least to capture enough to have sufficient signal to noise. If you can read, or calculate the value of the reactance component - capacitive or inductive, how does that help?

Knowing the reactance is important if you are phasing antennas together, otherwise, it's just an interesting parameter.

When you say what is an acceptable range - what is your definition for 'acceptable' - for receive - enter any two numbers in the possible range - they'll be acceptable? It's a bit like having an acceptable range for volume, or power output, or receiver sensitivity. Is 3W MS output power into a speaker some kind of standard? or X microvolts of signal at the receiver input. This rarely gets talked about - so it's just a line in the spec.

I just don't think there is a magic answer - your acceptable range might be different to mine, but equally valid.
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Old 03-06-2018, 5:24 PM
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SWR is an incredibly complicated subject... It involves using complex mathematics and knowledge of Fourier Transforms. Having a degree in engineering or mathematics helps.

The Zx Rs and jX components are usually displayed on a Smith chart where the relationship becomes apparent. --> Smith Charts
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Old 03-06-2018, 5:53 PM
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Quote:
Originally Posted by paulears View Post
....When you say what is an acceptable range - what is your definition for 'acceptable' - for receive - enter any two numbers in the possible range - they'll be acceptable? ...
Well, I would guess that acceptable parameter might be something that would make the antenna useless like ( not putting out enough energy to get anywhere), or harm the transmitter ?
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Old 03-06-2018, 6:19 PM
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prcguy-
Not to hijack the thread, but I was just checking some ht antennas on a SureCom SA-250, another small Chinese analyzer. VHF checked ok, UHF checked ok, multiple antennas.
But both a BaoFeng and a genuine-from-the-factory Kenwood dual band (2m/70cm) antennas showed as being around 1.4SWR on UHF, and a perplexing 1:19 SWR at 144Mhz.
An SWR of 19?! Really? (Started a "queer analyzer" thread on that.)

Since you've got both the tools and the expertise...Any idea of why this would happen? Some kind of pilot error? I can't see that an SWR of 19 is right. IIRC it was about 1.4 at 159MHz but notched pretty tightly, totally fubar by the ham band. Fortunately, the radios disagree about that and work without complaint, making me all the more certain there's something wrong with this picture.
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Old 03-07-2018, 7:36 AM
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Rred, could it be 1.19:1? If so, that's not bad at all.

If it's 19:1, then use it just for UHF.

I would test it again and double check how you are reading the results.
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  #30 (permalink)  
Old 03-07-2018, 8:00 AM
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I don't think what you got was unusual. When I did my antenna you tube tests recently, some of the results were so bad I had to basically ignore them as they messed up the readings. Some of the antennas had really terrible VSWRs - and when I did the tests using a small groundplane, they all improved - but not a great deal to be honest. 19:1 wasn't the worst one I tested by a long way.
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  #31 (permalink)  
Old 03-07-2018, 9:13 AM
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Doublechecked, it is 1:19 not 1.19:1 and the SWR curve clearly confirms that.

Why on earth would I use it just for UHF, when Kenwood sold it and it has worked as a dual band antenna? Comparable to pretty much every other dual-band antenna?

This is not a case of unknown ebay cheapies, this is as gen-you-whine OEM as it gets.

And I'm trying to understand why that is "testing" as unacceptable to impossible. I don't want to just turn knobs, I want to understand the theory and practice behind what I'm doing.
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Old 03-07-2018, 9:53 AM
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The problem here is that the second number is always "1", so it can't be 19.

you could have a 19:1 reading, but not the other way around. It does not work that way.

Try reading this to help you get a better picture of how SWR is measured.
https://www.arrl.org/files/file/Tech...f/q1106037.pdf
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  #33 (permalink)  
Old 03-07-2018, 10:45 AM
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I would have to see the analyzer display to comment. There are usually several scales to choose from and a scale that would show a 19:1 VSWR would be very coarse and not show fine details very well like a 1.4:1 match. Can you post a picture of the analyzer displaying the bad VSWR in question?

Hand held antennas in general have very poor matches due to lack of ground plane. Its common to see 3:1 and higher VSWR, especially on really shortened HT whips. You also see different VSWR readings on different meters of different sizes with the same antenna where larger meters provide a larger ground plane and actually change the match.

However, a 19:1 match is not something I would ever expect from an antenna operating within its stated range.
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Quote:
Originally Posted by Rred View Post
prcguy-
Not to hijack the thread, but I was just checking some ht antennas on a SureCom SA-250, another small Chinese analyzer. VHF checked ok, UHF checked ok, multiple antennas.
But both a BaoFeng and a genuine-from-the-factory Kenwood dual band (2m/70cm) antennas showed as being around 1.4SWR on UHF, and a perplexing 1:19 SWR at 144Mhz.
An SWR of 19?! Really? (Started a "queer analyzer" thread on that.)

Since you've got both the tools and the expertise...Any idea of why this would happen? Some kind of pilot error? I can't see that an SWR of 19 is right. IIRC it was about 1.4 at 159MHz but notched pretty tightly, totally fubar by the ham band. Fortunately, the radios disagree about that and work without complaint, making me all the more certain there's something wrong with this picture.
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  #34 (permalink)  
Old 03-07-2018, 11:36 AM
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So it's 1.19, so not an issue then?

This is why all the antennas being sold are so unpredictable. Years ago - the only measurements the average person could do were to measure power output, determine the VSWR and then if you wanted to see how much of the input power was actually getting radiated, a field strength meter. Those reactive components mean that it's quite possible for an antenna to have good VSWR a 'normal' impedance and nothing much radiated. Look at those HF tower style antenna systems. The actually antenna is too short by quite a long way to approach a quarter wave, so all kinds of tricks are pulled to make it radiate. All the components we've been talking about play an important part in the effectiveness of the antenna system - antenna, feeder, matching etc. The real engineers who align these things have a need to know exactly how it's working. The maths is complex and not exactly what most of us understand. Some of the IEEC papers are worth reading for the general concepts, but then they move on.

I don't get this bit?
Quote:
1.4SWR on UHF, and a perplexing 1:19
Sounds perfectly normal to me?
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  #35 (permalink)  
Old 03-07-2018, 11:40 AM
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NC1-
"you could have a 19:1 reading, but not the other way around."
Yeah, I could. It is called a brain fart, a typo, dsylexia, or what you will.
As if you've never confused Clark Gable with Clark Kent? (G)

Paul-
No, it is NOT 1.19. The SWR is "19". 1:19 or 19:1, call it what you will, it is 19.0 not 1.19.

PRC-
I'll have to work on that. Getting a clear pic of a tiny screen with single-pixel fonts may be challenging. Doesn't matter how I zoom the scale, it shows a curve for SWR at various frequencies, and that curve looks like a steep notch that showed the lowest SWR on VHF around 155(159?) MHz. There's also a numeric display of SWR. It clearly rises as I go from the 150's to 140, counting from 2 to 3 to 4 to 5 to eventually 19, so there's no way in hell that is "1.9" with a decimal point missing. That would be like a digital speedometer that was going from zero to sixty and then just suddenly popped a "150" in the middle of the progression.(G)

Sounds like no one has actually tested one of the dual band antennas on an analyzer? No comparables?

Last edited by Rred; 03-07-2018 at 11:48 AM..
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  #36 (permalink)  
Old 03-07-2018, 12:34 PM
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Is it possible to have a duel band antenna with a short in on section and no short in the other?
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Old 03-07-2018, 2:49 PM
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AFAIK they are one physical helix winding under the rubber, no sections as such. Simply resonant at both of two different frequencies.

And again, TWO dual band antennas tested out this way, so a fault in "the" antenna is astornomically against the odds, it would have to be faults in TWO antennas. Both of which have been confirmed to be working properly, in any case.

This is either pilot error or instrument error, there's nothing "broken" with the antennas.
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Old 03-07-2018, 4:17 PM
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Just checking, I have the Surecom A-250, and if I do a full scan on any antenna, I will see at least 3 spots over the entire spectrum this analyzer covers (two of which are better than one of the others) so it stands to reason antenna's can be ,and are, marketed for multiple bands.

The reason I had asked is that there are some mobile antenna's that are separated by a coil which is intended to more finely tune one of the sections for a different band and or frequency.

However, I could be wrong... I'm sure I'll find out shortly if so... LOL!
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Old 03-07-2018, 5:28 PM
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I got double confused about the VSWR. In an earlier post, I thought you did mean 19:1, then I noticed it was 1.19, and now I realise you meant the high one. I'll point you to the tests I did recently - oddly, youtube seemed to have lost it, so I have re-uploaded it again. You'll see quite a few have very poor VSWR - and only one I concluded was really faulty.
https://youtu.be/LVzlFTIMV5A
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Old 03-07-2018, 5:45 PM
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Paul-
I've posted screen shots of the analyzer in the other thread.
https://forums.radioreference.com/am...ml#post2894922
The antenna has an SWR of "19.90" or what we'd more properly call 1:19.90 no confusion about it.

I'll check out that Tube video. Funny, the Paul on that has an English accent.(G)

By the way, the orange "Nagoya" that also says "TM" on it? That's good reason to suspect it is counterfeit. By international accord, "TM" is used when a trademark has been applied for. But after the mark has been granted, it is improper to use "TM" and the mark is required to be used. So if the product is genuine, then Nagoya don't know what they are doing. Alternately, it is counterfeit and there's no reason to think they'd know what they are doing.(G)

Last edited by Rred; 03-07-2018 at 5:51 PM..
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