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 Measuring Antenna Wave

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PostSubject: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeSun Feb 07, 2021 1:05 pm

I was wondering which 'tool' can measure an antenna's behaviour in respect of wave.  Something that confirms 0.25 wave if connected to my magmount tankwhip and 0.5 wave if connected to my dipole etc.  Maybe it's an Antenna Analyser or some kind of Wave Meter that does it?

Please don't reply with 'a tape measure' as I already get the theory/physics of it, but was curious how to prove it in the field.  Perhaps it's a measurement that has to be taken externally, away from the transmission site, similar to Field Strength, just guessing...

Not even sure what 'antenna wave' is really called, the closest descriptor I can find is 'electrical length' maybe?  It sure ain't Polarisation, Radiation, Gain, Directionality etc. so not sure what the right terminology even is, lol.  Scuse my ignorance folks Smile

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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeSun Feb 07, 2021 1:50 pm

A NanoVNA maybe.

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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeSun Feb 07, 2021 4:53 pm

The NanoVNA that Alan mentioned is about the cheapest device you can get that can show you the resonant point of an antenna.  And inspite of its price, it is more capable than pretty much all of the more expensive antenna analyzers and VNA's out there.  There are also tons of youtube videos showing how to do various things with it.

And "antenna wave" is "wavelength".  If you see the greek lowercase lamda symbol (this one λ) that is the symbol used by engineers for wavelength, although it isn't often used on the hobby side, some literature you might come across uses it.


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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 7:10 am

If I'm understanding you right, there is no one simple tool that I'm aware of.

An analyser will show impedence and SWR vs frequency, which will assist with determining resonance and bandwidth.  A VNA will also show impedance characteristics/phase via Smith chart from which you can determine the wavelength, along with a lot of other stuff, but I'm not aware of anything you can pop on the feedpoint and have it say "This is a half-wave at 30MHz" for example.

Particularly because a 1/4 vertical on 20m would be a half wave on 10m, etc.

A couple of really good videos, which aren't too heavy, on the VNA here from Alan, W2AEW:

https://www.youtube.com/watch?v=Sb3q8f0NBZc

https://www.youtube.com/watch?v=o1eLK4EMpEQ

(I give it 5 mins after seeing the post and Victor will be watching them both!!)  Very Happy


Last edited by 43CT016 on Mon Feb 08, 2021 10:07 am; edited 1 time in total

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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 10:04 am

It's a long 5 minutes where's our Victor lol.

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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 10:11 am

Alan Pilot wrote:
It's a long 5 minutes where's our Victor lol.
 
Buried in a couple of YouTube vids, I would imagine.  God forbid he checks out that guys channel, as we’ll lose him for days.

One of the best AR/electronics channels on YT.....makes me feel dumb every time I watch him. lol

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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 10:52 am

Not quite cracked the omnipresence skill yet Alan Wink

I looked at buying a nanoVNA and then decided against it. It's indeed a useful tool but it would very soon sit gathering dust along with some of the other test gear.

I've been playing with VHF antennas recently but prefer the simplicity of my MFJ-259B antenna analyser.
You can soon get so wrapped up in numbers, formulas, smith charts, etc. that the information overload can cause confusion. Most people I've seen with a nanoVNA never get past the basic SWR measurements anyway. Wink


If only it was all as simple as plugging in some piece of test gear that had a meter declaring your antenna as "GOOD - BAD" with the needle swinging appropriately. Indeed as already pointed out a 'wave' measurement would only be valid at one particular frequency and as some of us know if you double the frequency you half the wavelength, halve the frequency and double the wavelength.

Even a tape measure or visual estimation could see you wrong if there was a impedance matching coil, loading coil or trap, or even indeed if the whole antenna was coiled(!)

It's a lot more complicated than you think, but you can also make it a lot simpler than it needs to be.

I still think the resonance & impedance arguments are overrated anyway. But it is much simpler Wink


I just know that I can't tell the colour of something simply by touching it with my finger.
(I won't get into quantum level oscillation arguments on that one either Very Happy)


Jeff - Subscribed to W2AEW a while back mate Wink
Always enjoyed his videos. Smile
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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 11:01 am

Just wiped the dust of my nanoVNA lol.
Soon collect some more.
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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 11:06 am

Ha! Ha! Very Happy
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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 12:56 pm

Thanks for all your input folks, it's something I can look further into now you have given me a steer Smile

What a friendly bunch you are, thanks again!

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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 2:01 pm

Seriously Victor it just collects dust the Shark 100 ?? get's more use as the numbers are a bit bigger.
I know the vna does a lot more but working it takes some good memory i don't hav lol.

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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 2:07 pm

LeedsChris wrote:
Thanks for all your input folks, it's something I can look further into now you have given me a steer Smile

What a friendly bunch you are, thanks again!

73's
We all try to help if we can.
If we can't then we might just do a wind up but we don't mean to upset anybody.
Your ok for now it's when we get to know you lol.
Our Victor is the goto guy the Orical font of all knowledge is our Victor top bloke.(and old like most of us ol )...
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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 3:26 pm

LeedsChris,

Every question is a valid one. Smile
We often all do our very best to help each other out, it's the best part about the CT group, all the helpful members.

Sometimes we go off for a banter, as has happened here briefly, (some rib tickling in my direction by some of the lads!), but we all do mean well. None of us individually are 'oracles' as Alan jokingly put it, but when we come together we definitely are the sum of all our parts even if it's just consensus.

It seems the general consensus to your original query lies in a nanoVNA, (Vector Network Analyzer), it probably being the most affordable portable instrument that one can plug into an antenna and give an array of analytical results. The problem only lies in learning to operate the damn thing and interpreting the results, (vector network analysis is a pretty difficult thing to fully master!)

Hence my quip about keeping things simple Wink

We do have some eminently qualified individuals on the group, we also have some serious amateurs with many years experience under their belts. But the best part is all the passionate individuals who readily absorb information about radio and pass it on as best they can.

Keep up your radio interests, it's a hobby that can fill a lifetime with all the different aspects it contains. More importantly, keep on asking questions, it's how we ultimately all learn.


All the best,
Victor
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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 5:01 pm

Never said i could spell.
Just before anybody else get's it in lol.
Most get the jist of what i type but i am still learning.
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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 5:27 pm

I'd be the first to defend you regards spelling Alan Wink

You always do your best to help out where you can and boy have you learned a lot. I still can't get over you with 'passing' the Australian test! Especially after experiencing first hand your initial doubts on your RSGB Foundation.

We've all learned so much because of helpful b*ggers here on CT, even when it's just pointers.
(Myself very much included on that learning stuff front, I'm far from knowing what I need to know!)
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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeMon Feb 08, 2021 6:51 pm

That Australian test was like common sense sort of wrong way to put i think but the answers just seamed to be obvious.
Suppose i have it all locked away in my mind and don't realize i know the answer until i see the question.
I have never done as much reading as i have done in the last few months and still reading so it must be sinking in even tho i don't think it is.

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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeSun Feb 14, 2021 7:50 pm

There is a way to prove it.....

Feedpoint impedance of a dipole is 73 Ohms at resonance.

Feedpoint of a 1/4 wave vertical over a perfect ground is 36.8 Ohms at resonance.

If you're using an electrical half wavelength of coax to connect to the antenna you'd get a usable reading at the radio end to determine which was which.
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PostSubject: Re: Measuring Antenna Wave   antenna - Measuring Antenna Wave Icon_minitimeTue Feb 16, 2021 8:25 am

Northern Crusader wrote:
Feedpoint impedance of a dipole is 73 Ohms at resonance.

Feedpoint of a 1/4 wave vertical over a perfect ground is 36.8 Ohms at resonance.

These numbers are for free space.  Once the antenna is over an earth this may not be true, unless the antenna is multiple wavelengths in height.


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