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Author Topic: External antenna tuners and ZS6BKW and G5RV antennas  (Read 7331 times)
K5KNE
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Posts: 65




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« Reply #30 on: September 18, 2012, 04:16:22 AM »

You need to start out with something simple.  Try a dipole on one band and see if you like it.
You can change it to another band or put up another one on the second band later.

Those SWR numbers you quoted are not anywhere near where your antenna should be. No one has SWR that high unless they are trying to load a dead short or open line.

Look up "dipole antenna" on Youtube and look at the tutorials about making antennas.

Good Luck    K5KNE
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AA5WG
Member

Posts: 498




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« Reply #31 on: September 18, 2012, 06:37:53 AM »

The multi-band antenna for decades has been the center-fed Zepp.  It is simple, multi-functional, efficient  and effective.

If you home brew a link antenna coupler, ladder line and a random length horizontal antenna for the lowest band you should have much enjoyment for years to come.

Chuck
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AK4YA
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Posts: 106




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« Reply #32 on: September 18, 2012, 06:54:12 AM »

Ive been looking around for feedpoint Z of dipole antennas, it appears that it is very low, around 75 ohms, for frequencies that it is resonant at.  Will an all band doublet show the same relatively Z for when it is used for other bands?  Just trying to run some modeling calculations.

I understand the height affects antenna input Z as well.  Are there any charts or calculators that are useful?
« Last Edit: September 18, 2012, 07:32:21 AM by AK4YA » Logged
AA5WG
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Posts: 498




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« Reply #33 on: September 18, 2012, 08:01:01 AM »

When a dipole is "cut" for a specific frequency then it will have a specific length and impedance.  Yes, 75 ohms resistive (non reactive) is considered and average or ballpark figure that represents this "cut" antenna impedance, in theory.

I say in theory because many items effect your antenna system impedance, i.e. antenna coupling to home wiring, tower, sappy trees, power line, steel rod in concrete, rotor cable and more have influence on your antenna.  These items act as part of your antenna system.  

And, when you change your dial on the radio away from the original "cut" frequency then you will not have the above theoretical 75 ohm resistive component impedance.  Your dial settings have influence on your antenna system.

All the above have and effect on your antenna system.  This effect is the addition of reactance, either capacitive or inductive.  Reactance is not bad it is just an added component that has to be considered.

If you transmit up the band, that is, higher in frequency from your "cut" antenna length, the antenna now appears long or inductively reactive.  If you tune down the band, lower in frequency, then your antenna appears short or capacitive reactive.

Having an antenna system (antenna length, transmission line length and antenna coupler type)  that is built to work with multi-band impedance changes is important.

The impedance changes at the antenna coupler go up and down and down and up depending on the band and frequency in use.  

Having an antenna coupler that is built for these changes is important.  

The link antenna coupler allows you to select either the series circuit for low impedance's or parallel circuit to accommodate high impedance's which appear at the coupler.  Back in the day series tuning was called current feeding and parallel tuning was called voltage feeding.

The antenna system length is one half the horizontal antenna length added to the ladder line length measured in 1/4 wavelengths for the band in use.  Ideally one tries to stay close to quarter wavelength multiples for the total antenna system lengths.  

The total antenna system is comprised of the antenna, ladder line and link antenna coupler.  

The link antenna coupler is built to accommodate changes in the above system lengths.  It is simple to use and is used in the comfort of your shack.  It it  not limited to center fed antennas.  However, it shines when used with center fed antennas that are use for all band wide band operations.

Chuck
« Last Edit: September 18, 2012, 08:26:01 AM by AA5WG » Logged
W5DXP
Member

Posts: 3622


WWW

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« Reply #34 on: September 18, 2012, 10:40:26 AM »

Will an all band doublet show the same relatively Z for when it is used for other bands?  

The short answer is NO! Let's say the feedpoint impedance of a 1/2WL dipole is 50 ohms on 80m. The feedpoint impedance is likely to be 100 times that value on 40m, i.e. about 5000 ohms. Feedpoint impedances of 2000 ohms of resistance plus 2000 ohms of reactance are not uncommon.

I would suggest downloading the free demo version of EZNEC from:

http://www.eznec.com

and taking a look at the dipole feedpoint impedances as the frequency is changed.
« Last Edit: September 18, 2012, 10:54:40 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.
AK4YA
Member

Posts: 106




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« Reply #35 on: September 18, 2012, 12:13:34 PM »

<SNIP>
I would suggest downloading the free demo version of EZNEC from:

http://www.eznec.com

and taking a look at the dipole feedpoint impedances as the frequency is changed.

very slick.  I can get it to plot freq vs. swr, but cannot seem to get it to chart freq versus impedance.  I suppose I could just multiply swr by whatever value I used for feedline Z?

Now there are 4 values listed as "Z" for the antenna at each swr point plotted but they seem to be plus or minus either an angle or j.  I do understand the concept of current leading voltage and what the + or - angle/j figure means electrically, but not sure how to plot that as how to measure for a mismatch against an arbitrary feedline characteristic Z.

ETA:  nevermind, lightbulb just went on.  assuming a tuner can cancel out the j part, the number to the left of the j is the antenna Z.  got it.

Is it even possible to use EZNEC to plot Z as a function of Freq?
« Last Edit: September 18, 2012, 12:45:19 PM by AK4YA » Logged
WB6BYU
Member

Posts: 13341




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« Reply #36 on: September 18, 2012, 01:14:27 PM »

If you click on any point in the SWR graph it will show you the impedance at that point.
(Probably in the numbers below the window somewhere.)

When calculating SWR you really do need to use the +/-j value as well as R.  An impedance
of 50 - j200 gives an SWR of something like 18 : 1 in spite of R = 50.  Think of it as a two-
dimensional space, with R as one coordinate and X as the other.   SWR is a measure of how
close you are to a target impedance (commonly 50 + j0) and the difference in both R and X
factor into that.


There are some various plot capabilities:  think you'd find them under "Options" in the main
window.  I don't have a copy in front of me, but I think you can have it plot R and X.
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WB6BYU
Member

Posts: 13341




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« Reply #37 on: September 18, 2012, 01:17:14 PM »

Quote from: AK4YA

... not sure how to plot that as how to measure for a mismatch against an arbitrary feedline characteristic Z.



Set the reference feedline impedance in the main menu, then it appears as an option beside the SWR
plot and you can click on it there.  (You can also change the default to something other than 50 ohms
in the options menu, but having the second one available eliminates that need most of the time.)  That
gives you a plot of SWR relative to the specified feedline impedance.
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