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Author Topic: Staions hear me, but I can't hear them  (Read 8497 times)
KD8TZC
Member

Posts: 67




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« Reply #45 on: June 14, 2013, 07:22:49 PM »

Okay, so I did some detailed measurements this evening... didn't seem as bad when I started to look at the raw numbers.

I have uploaded a chart of what my findings were.  The resonant frequency (or most resonant) is down around 24 - 25MHz.  Based on this, do I want the lowest SWR or the lowest Z value?  Lowest SWR (1.6) happens around 25.04 - 25.24, and the lowest Z value (56) happens at 26.54MHz, but the SWR is 2.6 at that Freq.

So based on all of this, I need to shorten the legs of the dipole up a little, but if someone could help me on which frequency would be better I would appreciate it.

Below is a link to a photobucket image of the graph.

http://i879.photobucket.com/albums/ab360/perrybucsdad/20130614_10M_Dipole_zps53a58398.png

Also, I believe (I need to double check, but I am fairly certain this is the measurement) the total length (1/2 Wave) or the antenna is 17' 4" or 8' 8" per leg.  I think I need to shorter than by about 12 inches is the best SWR is 25.14.
« Last Edit: June 14, 2013, 07:30:58 PM by KD8TZC » Logged

John - KD8TZC
W0BTU
Member

Posts: 1621


WWW

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« Reply #46 on: June 14, 2013, 09:14:40 PM »

EZNEC is OK, but we both know that real world results differ. ... I have tested some full sized G5RV installations with an analyzer and I felt that they were way overrated. In fact, dismal. ... 100 watts into a G5RV on most bands will probably radiate in QRP class, the rest is circulating in the tuner.

Respectfully, all those statements are just not accurate. On the bands that a G5RV works well on*, they can radiate just as well as a resonant single-band dipole. Read the section "On The Air Testing" at http://www.w8ji.com/g5rv_facts.htm

* http://www.w0btu.com/g5rv_antenna.html
« Last Edit: June 14, 2013, 09:21:20 PM by W0BTU » Logged

WB6BYU
Member

Posts: 13120




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« Reply #47 on: June 14, 2013, 10:00:12 PM »

Quote from: KD8TZC

I have uploaded a chart of what my findings were.  The resonant frequency (or most resonant) is down around 24 - 25MHz.  Based on this, do I want the lowest SWR or the lowest Z value?  Lowest SWR (1.6) happens around 25.04 - 25.24, and the lowest Z value (56) happens at 26.54MHz, but the SWR is 2.6 at that Freq.



There are two dips in the SWR curve, which leads me to suspect that either the antenna is
coupled to something else in the attic, or the feedline is resonant somewhere close to there.
In that case, I'd recommend shortening the antenna only partly, then rechecking the curves
to make sure that they are moving the way you expect them to.

You can calculate the expected correction this way:  if the desired resonant frequency is
28.5 and it really is resonant at 27.0, then the antenna needs to be 27 / 28.5 = 95% of the
current length.  So multiply the length of each side by 5% or 0.05 and that will give you the
amount to trim off each end.  HOWEVER, that assumes that the antenna isn't coupled
to anything else.  If, for example, there is a metal duct and shortening the antenna pulls the
end further away from it, the shortening effect will be different.  So I'd start with half the
calculated correction and see what happens.
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KD8TZC
Member

Posts: 67




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« Reply #48 on: June 15, 2013, 04:46:56 AM »

Thanks BYU... so basically (Old Freq / Desired Freq) * Current length = New Length?

I don't think I have the antenna touching anything.  I suspended the center section from the ceiling about 1 - 2 feet below, and the antenna lines are then drawn out and held up with string and also suspended.

So that peak in the SWR curve at about 25.84 MHz is from something coupled with the antenna or feedline.  Hmmm... I'll have to give it a look when I get back up there.  I don't know if I should be too concerned about it or not though, but it would be nice to have a wider SWR curve.  f the feedline is resonant, what are my options, shortening/lengthening the feedline and if so, by how much?
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John - KD8TZC
KD2CJJ
Member

Posts: 369




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« Reply #49 on: June 15, 2013, 04:51:59 AM »

The antenna is no where near being resonant on the 10 meter band.  I agree with kd8tdc.  The feedline is what is getting tuned probably.  Your feedline maybe a resonant length and could be culprit or some massive coupling or both.  Try adding 3 or 5 feet of feedline.

Okay, so I did some detailed measurements this evening... didn't seem as bad when I started to look at the raw numbers.

I have uploaded a chart of what my findings were.  The resonant frequency (or most resonant) is down around 24 - 25MHz.  Based on this, do I want the lowest SWR or the lowest Z value?  Lowest SWR (1.6) happens around 25.04 - 25.24, and the lowest Z value (56) happens at 26.54MHz, but the SWR is 2.6 at that Freq.

So based on all of this, I need to shorten the legs of the dipole up a little, but if someone could help me on which frequency would be better I would appreciate it.

Below is a link to a photobucket image of the graph.

http://i879.photobucket.com/albums/ab360/perrybucsdad/20130614_10M_Dipole_zps53a58398.png

Also, I believe (I need to double check, but I am fairly certain this is the measurement) the total length (1/2 Wave) or the antenna is 17' 4" or 8' 8" per leg.  I think I need to shorter than by about 12 inches is the best SWR is 25.14.
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73

Mike
KD2CJJ
WB6BYU
Member

Posts: 13120




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« Reply #50 on: June 15, 2013, 07:54:09 AM »

Quote from: KD8TZC

Thanks BYU... so basically (Old Freq / Desired Freq) * Current length = New Length?



As a first order estimate, assuming no other interactions.  That last clause won't apply
in a typical attic due to metal around it such as ducting, wiring, roof flashing, etc.  After
all, the parasitic elements in a yagi can be 1/4 wave away from the driven element and
still make a big difference in feedpoint impedance, resonant frequency, etc.

Typically a dipole in the attic resonates at a lower frequency than expected due to
capacitive coupling to surrounding objects (though bending it to fit has the opposite
effect.)   That's why it takes some cut and try.  You can't tell which of the dips will
shift as you vary the antenna length, and it may shift more or less than predicted.
So try changing about half the expected correction and see what happens.



Quote

If the feedline is resonant, what are my options, shortening/lengthening the feedline and if so, by how much?



In theory the balun should eliminate the effect of the feedline length.  I've had at least
two cases that I clearly remember when I made a bunch of adjustments to the antenna
length (a dipole without a balun) and the particular resonance I was looking at stayed the
same because it was due to the feedline instead.  It could be due to something else.  You can
try splicing in a 3' patch cable, or whatever else you have on hand, to see if one or the other
points changes on the SWR curve.  (The impedance curve will shift when you change the line
length if the SWR isn't 1 : 1 .) 

But other than knowing which dip is the antenna and which is due to something else, there
really isn't any reason to change the feedline length unless you have significant common
mode current, and we haven't seen evidence of that yet.  That would mean that your
signal would have more of a vertically polarized component to it, rather than horizontal
from the dipole.  I'd recommend getting the antenna tuned first with the balun on it, and
only then changing the feedline length if you are having additional problems.
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AC2EU
Member

Posts: 352


WWW

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« Reply #51 on: June 15, 2013, 09:12:29 AM »

100 watts into a G5RV on most bands will probably radiate in QRP class, ...

Please choose one of the four bands (80m, 40m, 20m, and 12m) where the G5RV is known to work well and show us the math it takes to achieve 13 dB of losses. Such results would be unique in my 60 years as a ham.

Quote
Don't take my word for it, here's another ham who did measurements:
http://www.karinya.net/g3txq/g5rv/

Maybe you should send Steve an email asking him what he thinks about your above posting.

All I'm going to say is that I did empirical measurements on REAL world installations because the Hams who owned them complained that people could not hear them when other Hams in the area could make the contact. The SWR numbers on Steve's site speak for themselves(in fact, they are kind!). Did you read the article? He's not a fan of the G5RV folklore either.
There are also coax loses due to mismatch to consider as well...unless you are using a remote tuner...?

You are using a machine model instead of the real thing. I doubt that that your results are as stellar as you claim, but if you have been comfortable with your G5RV for 60 years, there is no need to change .  like i said before, to each his own.  Grin
I try to avoid high reactive SWR like the plague, but that's my take on it.

I don't want to make this into a big deal, but measurements are measurements and theory is just theory.
I may not have been a Ham for 60 years, but I do know how build, diagnose and repair antenna systems that work.
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KD8TZC
Member

Posts: 67




Ignore
« Reply #52 on: June 15, 2013, 09:19:18 AM »

Quote

If the feedline is resonant, what are my options, shortening/lengthening the feedline and if so, by how much?



In theory the balun should eliminate the effect of the feedline length.  I've had at least
two cases that I clearly remember when I made a bunch of adjustments to the antenna
length (a dipole without a balun) and the particular resonance I was looking at stayed the
same because it was due to the feedline instead.  It could be due to something else.  You can
try splicing in a 3' patch cable, or whatever else you have on hand, to see if one or the other
points changes on the SWR curve.  (The impedance curve will shift when you change the line
length if the SWR isn't 1 : 1 .) 

But other than knowing which dip is the antenna and which is due to something else, there
really isn't any reason to change the feedline length unless you have significant common
mode current, and we haven't seen evidence of that yet.  That would mean that your
signal would have more of a vertically polarized component to it, rather than horizontal
from the dipole.  I'd recommend getting the antenna tuned first with the balun on it, and
only then changing the feedline length if you are having additional problems.
Okay thanks... I was always confused about the feedline length.  Some swear that it has to be some fraction of a wavelength that you are trying to use and then others say it doesn't really matter.  In fact, this morning I was reading in the ARRL Antenna Book (p. 24-4) that changing the length of the feedline is a myth.  They say it does not change the SWR, but then they go on to say that it may because it may change the impedance.

I think I will take the easy route first which is fold back the dipole legs a bit, and then take a reading.  I only have the Ugly Balun on the feedline and I think I will keep it that way for now.  I have the DX Engineering 1:1 current balun and maybe it is working as designed, or maybe it's not.  Field is coming up and I'll bring it along with me and see if one of the moldtimers can tell me in person what is going on and if it is defective or if it is working like it should.
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John - KD8TZC
K7KBN
Member

Posts: 2782




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« Reply #53 on: June 15, 2013, 09:21:29 AM »

No, my analyzer only has Z (which I thought was found by adding reactance to known impedance of the line... so if I measure my line and I know it is 50 ohms, and lets say the Z reading is 350 ohms, then wouldn't the reactace be 300?)

No.  Think of it as a right triangle with one side 50 and hypotenuse 350.  The length of the third side is the reactance.
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73
Pat K7KBN
CWO4 USNR Ret.
WB6BYU
Member

Posts: 13120




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« Reply #54 on: June 15, 2013, 09:44:07 AM »

Quote from: K7KBN
Quote from: KD8TZC
No, my analyzer only has Z (which I thought was found by adding reactance to known impedance of the line... so if I measure my line and I know it is 50 ohms, and lets say the Z reading is 350 ohms, then wouldn't the reactace be 300?)

No.  Think of it as a right triangle with one side 50 and hypotenuse 350.  The length of the third side is the reactance.


And that still isn't quite right.  There seems to be confusion among impedance (a complex or
vector quantity), the magnitude of impedance (which is what the analyzer reads), and the
expected values of those along a feedline.

Impedance has two components:  either resistance and reactance in rectangular coordinates, or
magnitude and phase in polar coordinates.  It's like looking at a map, where you have both
north/south and east/west dimensions.

Z is the magnitude of the reactance:  equivalent to the distance from the center of a map.
It doesn't tell you which direction you went in, or how far you are away from a particular point.
It would be more useful if we were trying to match an antenna to 0 ohms rather than 50 ohms.
But you can get a 50 ohm Z value with a perfect 50 ohm load, in case the SWR would be 1 : 1,
with perfectly reactive loads, inductive or capacitive (where the SWR is infinite), or some other
combination thereof.  There may be a few cases where Z is a useful value in antenna work,
but not as many as one might think.

The impedance on a feedline is determined by the load impedance at the far end, the
characteristic impedance of the line, and the electrical length of the line.  You can't assume
that the R value of a complex impedance is the same as the characteristic impedance of
the line:  R will vary along the line.  About the best you can say is that, if the SWR on the
line isn't 1 : 1, then at no point along the line will the (complex) impedance be equal to the
characteristic impedance of the line:  you can have the resistive component equal in some
spots along the line, but those points will have a reactive component such that the SWR is
constant along the line (except for line losses.)

So, yes, the magnitude of impedance ( Z ) is given by the Pythagorean theorem using the
R and X components of impedance, but R is usually NOT equal to the characteristic impedance
of the line.
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AC2EU
Member

Posts: 352


WWW

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« Reply #55 on: June 15, 2013, 09:54:28 AM »

Okay, so I did some detailed measurements this evening... didn't seem as bad when I started to look at the raw numbers.

I have uploaded a chart of what my findings were.  The resonant frequency (or most resonant) is down around 24 - 25MHz.  Based on this, do I want the lowest SWR or the lowest Z value?  Lowest SWR (1.6) happens around 25.04 - 25.24, and the lowest Z value (56) happens at 26.54MHz, but the SWR is 2.6 at that Freq.

So based on all of this, I need to shorten the legs of the dipole up a little, but if someone could help me on which frequency would be better I would appreciate it.

Below is a link to a photobucket image of the graph.

http://i879.photobucket.com/albums/ab360/perrybucsdad/20130614_10M_Dipole_zps53a58398.png

Also, I believe (I need to double check, but I am fairly certain this is the measurement) the total length (1/2 Wave) or the antenna is 17' 4" or 8' 8" per leg.  I think I need to shorter than by about 12 inches is the best SWR is 25.14.


According to your graph, you have a pretty good 12 meter antenna!
As others have said , your resonance appears to be too low, but unfortunately, your meter only measures total Z so you don't know how much of it is reactive. True resonance is where the reactance is = 0 .However to take a leap of faith and say that the low Z/low SWR is the resonant frequency, you would have to do some pruning. There does seem to be other coupling going there as well. Only cut it if the attic is going to be it's permanent home.  

The antenna and coax are a system, there are interactions due to frequency coax length and match.

Reversing the magic dipole formula by dividing by feet, your antenna in free space should resonate at 468/17.3 feet =27.5 MHZ
You apparently have influences there that are bringing it down substantially lower than expected. Theory gets one just so far...then the work begins...


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W5DXP
Member

Posts: 3551


WWW

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« Reply #56 on: June 15, 2013, 09:55:13 AM »

Z is the magnitude of the reactance:

Dale, you obviously meant to type something else above.
Z is the magnitude of the impedance, X is the magnitude of the reactance.
« Last Edit: June 15, 2013, 10:01:01 AM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
Member

Posts: 13120




Ignore
« Reply #57 on: June 15, 2013, 10:03:22 AM »

Yes, you're right.  I'm glad someone is catching my mistakes!  I was in a hurry.
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KD8TZC
Member

Posts: 67




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« Reply #58 on: June 15, 2013, 03:38:42 PM »

Quote from: K7KBN
Quote from: KD8TZC
No, my analyzer only has Z (which I thought was found by adding reactance to known impedance of the line... so if I measure my line and I know it is 50 ohms, and lets say the Z reading is 350 ohms, then wouldn't the reactace be 300?)

No.  Think of it as a right triangle with one side 50 and hypotenuse 350.  The length of the third side is the reactance.


And that still isn't quite right.  There seems to be confusion among impedance (a complex or
vector quantity), the magnitude of impedance (which is what the analyzer reads), and the
expected values of those along a feedline.

Impedance has two components:  either resistance and reactance in rectangular coordinates, or
magnitude and phase in polar coordinates.  It's like looking at a map, where you have both
north/south and east/west dimensions.

Z is the magnitude of the reactance:  equivalent to the distance from the center of a map.
It doesn't tell you which direction you went in, or how far you are away from a particular point.
It would be more useful if we were trying to match an antenna to 0 ohms rather than 50 ohms.
But you can get a 50 ohm Z value with a perfect 50 ohm load, in case the SWR would be 1 : 1,
with perfectly reactive loads, inductive or capacitive (where the SWR is infinite), or some other
combination thereof.  There may be a few cases where Z is a useful value in antenna work,
but not as many as one might think.

The impedance on a feedline is determined by the load impedance at the far end, the
characteristic impedance of the line, and the electrical length of the line.  You can't assume
that the R value of a complex impedance is the same as the characteristic impedance of
the line:  R will vary along the line.  About the best you can say is that, if the SWR on the
line isn't 1 : 1, then at no point along the line will the (complex) impedance be equal to the
characteristic impedance of the line:  you can have the resistive component equal in some
spots along the line, but those points will have a reactive component such that the SWR is
constant along the line (except for line losses.)

So, yes, the magnitude of impedance ( Z ) is given by the Pythagorean theorem using the
R and X components of impedance, but R is usually NOT equal to the characteristic impedance
of the line.

There seems to be a wealth of rich information here, but I don't even know what I should be looking at... I apologize.  I'm just a tech, and this seem really far above my head, but I need to learn it.  Anyway you can dumb this down so this dumb Hungarian can understand it... I apologize for asking, but I didn't want to let this information that you are trying to share with me go to waste.

On a different note, I ordered a Rigexpert AA-54 yesterday that should be here on Tuesday or Wed, so hopefully I can give more information than just the Z value in a few days.
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John - KD8TZC
VE3TTF
Member

Posts: 1




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« Reply #59 on: June 15, 2013, 05:29:45 PM »

RCS-4 Switch will handle hf only not vhf and uhf try taking your 140- 440 line off the switch head try that 
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