RigExpert AA-650ZOOM Antenna Readings

[bob85]

338 it does not make you an idiot,
it just means you learned something new today

 

[338_MtRushmore]

When you are measuring antennas through a length of coax the resonance you see on the analyser is very likely not the resonant frequency of the antenna as most think it is,

only under very specific conditions is the resonant point at the rig end the same frequency as the self resonace of the antenna.



Walt maxwells reflections tells us

"Any reactance added to an already resonant (resistive) load of any value for the purpose of com-pensation to reduce the reflection on the line feeding the load will, instead, only increase or worsen the reflection. It is for this reason, although contrary to the teaching of several writers, that the lowest feed-line SWR occurs at the self-resonant frequency of the radiating element it feeds, completely independ-ent of feed-line length. Any measurements that con-tradict this indicate that either the measuring equip-ment or the technique (or both) are in error"

I completely missed this post. What chapter is this in? I only made it a quarter way through the book and life got in the way.

 

[bob85]

REFLECTIONS III
Transmission Lines and Antennas

Chapter 2

Countdown for a Journey from Mythology to Reality

Sec 2.3 True or False?
(24)

 

[bob85]

More from Walt

"Resonances measured at the feedline input are resonances of the combination of the antenna and the feed line, not the resonance of the antenna alone. A change in length of the feed line results in a different resonant frequency of the combination for every different length of feed line. A 1/2wave feed line is not the answer, because since the resonant frequency of the antenna is unknown, the frequency at which the feed line should be 1/2wave is also unknown. If the feed line is 1/2wave at some frequency other than the resonant frequency of the antenna, the resonant frequency measured at the input of the feed line will be different from that of either the antenna or the feed line"

i could post other sources but i think you will get the picture

 

[338_MtRushmore]

You can post all your like, I probably wont be sleeping anyways. Maybe I never even made it to chapter 2, but I did jump around a little to answer some questions.

This mean there is truth to the coax length debate about fooling the radio with a different length coax, only it's not fooling anything, its actually tuning the antenna to resonance.

I really feel like I've been halfway down this road already

 

[bob85]

No its not tuning the antenna or fooling the rig,

while impedance seen looking into a mismatched line will change with line length, repeating the load impedance every electrical halfwave and inverting the load impedance at odd 1/4wave multiples,

vswr does not change significantly with different length coax or when moving the meter to different places along the line

other than going down a little as you make the coax longer or move the meter along the line due to coax loss,

unless you have common mode on the braid due to not having a good choke at the feed-point

common mode impedance of the coax braid is seen in parallel with the load at the antenna end of the coax,

with coax lengths that give you a high common mode impedance say 2000ohms
that high impedance in parallel with the load won't significantly change the load impedance so vswr won't change much,

but if the length gives you a low common mode impedance seen in parallel with the load, that low impedance will change vswr notably,

none of it is fooling the meter, vswr meters cannot be fooled,
only people can be fooled,

people observe an effect but don't understand why its happening so they make myths up like changing coax length is only fooling the meter,

you can read all about it in reflections or other sources

remember this ALL conductors connected to the coax braid antenna junction act the same as the outside of the coax braid, metal masts & ground wires don't escape.

 

[338_MtRushmore]

So if coax length effects swr, it effects it on all frequencies equally, therefore the swr dip will not change frequency, just change magnitude. That makes sense.

I'm just trying to navigate this new world I've been in for a few hours now and my brain is in sensory overload. I need sleep

 

[bob85]

Yes 338

when you see lets say for example adding a 3ft jumper to your coax effects the vswr reading significantly it is effecting all frequencies near to where you are tuning about the same,

elsewhere you may not have as much common mode because the coax braid is a different electrical length at other frequencies & may not be causing much cmc on the braid,

but that's NOT fooling the meter as that well known and often repeated MYTH will tell you,

you are changing the common mode impedance of the braid that is seen in parallel with the load at the antenna end & that is changing the vswr FOR REAL,

You cannot fool the meter, only people are fooled,

if you use a good choke to eliminate most of the current on the braid vswr won't change much when you add or remove a short jumper,

whenever you DO see your vswr change by messing with jumper length that's a very good indicator you have common mode & you should be fixing that issue first.

Not only because it can cause RFI and receive noise on the braid but it also can spoil your radiation pattern,

It may all seem a bit complicated & it is if you go into the equations of it all & your math's algebra aint so good,
I forgot most of that when I left school,

But you don't need any of that math's algebra to understand the basic concepts & how to recognize when you have cmc & what to do to minimize or eliminate those effects,

in the same way that you don't need to understand how oil chemistry, fluid dynamics & hydrodynamic bearings work to know what to do if the low oil light comes on in your truck,

keep reading reflections,
what's in there is the same stuff as Bird & Tellewave & mfj tells you,

People are happy to use their meters & analyzers but make their own shit up about what its telling them,

if anybody mentions fooling the meter you now know they don't know what they are talking about.

If anybody tells you They are tuning their antenna to resonance from the rig end of the coax you now know that's almost certainly not true,

lowest vswr occurs at the self resonant frequency of the antenna.

The resonance you see at the rig end is a combination of antenna reactance been cancelled by reactance in the coax and very well may not be where the antenna is actually resonant ,

 

[338_MtRushmore]

Ha, I never thought about anyone thinking the actual meter reading itself was fooled. It's more of a placing the meter in a location that "fools" the meter by not indicating the problem.

I was trying to debunk the idea that resonance was always at low swr, but all of my arguments snapped right in as a piece of the puzzle. I didn't differentiate between actual swr and common mode problems so the coax length myth fit the puzzle also. I fooled myself on that one.

 

[bob85]

I misunderstood what you asked above & don't want to steer you wrong,

when changing coax length changes vswr notably it will shift where lowest vswr is but its changing vswr everywhere close to the frequency you are using,

you can't fool the meter by moving it further up of down the coax,
if moving it changes vswr reading more than the what can be atributed to the small change in coax loss its indicating you have current on the braid

you can end up in a chasing your tail situation where everytime you change the coax length you have to retune the antenna to get it back where you want it.

 

[338_MtRushmore]

I was referring to fooling the meter into showing a good swr when experiencing common mode problems. I've never really messed with the effects of common mode, its easier to choke it and be done.

I didn't really think about the common mode null changing locations as you did an swr sweep through different frequencies.

 

[338_MtRushmore]

When you are measuring antennas through a length of coax the resonance you see on the analyser is very likely not the resonant frequency of the antenna as most think it is,

only under very specific conditions is the resonant point at the rig end the same frequency as the self resonace of the antenna.



Walt maxwells reflections tells us

"Any reactance added to an already resonant (resistive) load of any value for the purpose of com-pensation to reduce the reflection on the line feeding the load will, instead, only increase or worsen the reflection. It is for this reason, although contrary to the teaching of several writers, that the lowest feed-line SWR occurs at the self-resonant frequency of the radiating element it feeds, completely independ-ent of feed-line length. Any measurements that con-tradict this indicate that either the measuring equip-ment or the technique (or both) are in error"

Doesn't the fact that there is less reflected power mean the antenna itself is radiating more power, therefore it has to be most resonant at the lowest swr? Or did I just oversimplify it in my head and get it wrong again?

 

[bob85]

That's the point 338, yes it does ,

when you add reactance to an already resonant antenna to move resonance measured at the rig through coax to where you want it & see reflect rise thinking resonance is king & vswr is for ducks & turkeys,

you are increasing reflected power & the antenna is no longer resonant even though it looks like it is at the rig end.