Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1] 2 3 4 5 6 ... 8 Next   Go Down
  Print  
Author Topic: EFHW impedance  (Read 9779 times)
WD8KNI
Member

Posts: 173




Ignore
« on: October 14, 2017, 04:58:41 AM »

Can anyone point me to a source showing the impedance of a resonate EFHW  based on height above ground for the ham bands.  If it's in wavelength above ground that would be great.  Just thought I would ask before modeling each one and changing height for each .1 wavelength to .5 wavelength.  I thinking someone must have done this already.
Thanks Fred
Logged
W1VT
Member

Posts: 2493




Ignore
« Reply #1 on: October 14, 2017, 05:50:04 AM »

One approximation may be to model a wire twice as long as you intend to use, and feed it at the center.  Half the impedance at the center may be the number you are looking for.  This takes feedline out of the picture--I don't know whether you want to do that or not.  But, it is a very simple model that shows the effect of height above ground.

Zack W1VT
Logged
WD8KNI
Member

Posts: 173




Ignore
« Reply #2 on: October 14, 2017, 05:53:19 AM »

That will give me the impedance of a dipole, not end fed, and end fed is in the thousands of ohms not hundreds.   That information is already in any antenna manual, Thanks Fred

Logged
W1VT
Member

Posts: 2493




Ignore
« Reply #3 on: October 14, 2017, 06:22:26 AM »

I modeled at 66 ft dipole at 17 feet over perfect ground. #14 wire. This is roughly one wavelength long, to model a half wave end fed 20M dipole.
The impedance of the full wave dipole at 13.75 MHz is 4672-j18 ohms.  Half of that is 2336-j9 ohms.

Zack W1VT
« Last Edit: October 14, 2017, 06:28:30 AM by W1VT » Logged
WD8KNI
Member

Posts: 173




Ignore
« Reply #4 on: October 14, 2017, 06:38:27 AM »

Yes, you are absolutely correct Zac, however I think that model is upset by the ground loss of the second half of the antenna you are modeling.  I answered you last time too quickly, my bad, my brain said two ¼ waves when I answered and you said two half waves. Sometimes I read too quickly.

It's also very easy to model just a half wave over ground, just make make the ½ wave line 5000 segments and feed it at segment 1 since most versions of modeling software won't let you feed at an end of an odd number segmented line, feeding at segment 1 of a huge number of segments gives you a very very close reading and would be within a couple of ohms. 
What I really want is a chart showing the impedance of each band center frequency based on height above ground in tenths of a wavelength.  In free space it would be the same impedance for all frequencies, it's when ground resistance gets involved that modeling changes..  I can develop the table, just wanted to save some myself some time if it had already been done.  Thank you very much for doing what you did, I am concerned about ground effect of another ½ wave of wire with your model.. Regards.. Fred
Logged
G8HQP
Member

Posts: 596




Ignore
« Reply #5 on: October 14, 2017, 06:53:51 AM »

The table you request would need to specify the wire size, as this is the crucial issue for antennas fed at a high impedance point. It will probably change the result more than height above ground.
Logged
AC9RN
Member

Posts: 173




Ignore
« Reply #6 on: October 14, 2017, 07:22:33 AM »

Many commercially available EFHW and balun / unun suppliers are utilizing a 9:1 unun at the feedpoint of an EFHW.  That is what I will be putting up myself, very soon.  I also have 4:1 unun on hand, just in case, and to experiment with.  But I have seen the 9:1 recommended for EFHW antennas several different times.
Logged

Lenny, AC9RN (Formerly KB2NYA)
K4SAV
Member

Posts: 2388




Ignore
« Reply #7 on: October 14, 2017, 07:54:24 AM »

It's also very easy to model just a half wave over ground, just make make the ½ wave line 5000 segments and feed it at segment 1 since most versions of modeling software won't let you feed at an end of an odd number segmented line, feeding at segment 1 of a huge number of segments gives you a very very close reading and would be within a couple of ohms.  

If you do that and keep increasing the number of segments, you will see that the impedance becomes huge, and would ultimately approach infinity if you had enough segments.  That should tell you that end feeding a wire is not possible.  Then you can examine practical ways of approximately end feeding a wire, which will result in a much lower impedance, and that will actually allow some current to flow in the wire.  Wire diameter will be a big player in the answer, as well as the counterpoise (there is always one). Height above ground will be a minor contributor.  

Jerry, K4SAV
« Last Edit: October 14, 2017, 07:56:29 AM by K4SAV » Logged
WD8KNI
Member

Posts: 173




Ignore
« Reply #8 on: October 14, 2017, 10:24:35 AM »

Many commercially available EFHW and balun / unun suppliers are utilizing a 9:1 unun at the feedpoint of an EFHW.  That is what I will be putting up myself, very soon.  I also have 4:1 unun on hand, just in case, and to experiment with.  But I have seen the 9:1 recommended for EFHW antennas several different times.

You need a round 49:1 to feed an EFHW, as the feed point will be in the neighborhood of 2500-3000 ohms  a 9:1 will not feed a resonate wire that is in the neighborhood of 450 ohms.  The 9:1 is recommended for nonresident which are not half wave antenna's, they have to resonate to be a half wave.  Second I don't want this data to figure out anything about any feedline. 
Logged
WD8KNI
Member

Posts: 173




Ignore
« Reply #9 on: October 14, 2017, 10:27:05 AM »

It's also very easy to model just a half wave over ground, just make make the ½ wave line 5000 segments and feed it at segment 1 since most versions of modeling software won't let you feed at an end of an odd number segmented line, feeding at segment 1 of a huge number of segments gives you a very very close reading and would be within a couple of ohms.  

If you do that and keep increasing the number of segments, you will see that the impedance becomes huge, and would ultimately approach infinity if you had enough segments.  That should tell you that end feeding a wire is not possible.  Then you can examine practical ways of approximately end feeding a wire, which will result in a much lower impedance, and that will actually allow some current to flow in the wire.  Wire diameter will be a big player in the answer, as well as the counterpoise (there is always one). Height above ground will be a minor contributor.  

Jerry, I have increased the number of segments many many times in my life, never has it had the effect you are talking about.. 
I will just model what I need, thanks for the input guys....

Jerry, K4SAV
Logged
E73M
Member

Posts: 147


WWW

Ignore
« Reply #10 on: October 14, 2017, 11:24:33 AM »

It's also very easy to model just a half wave over ground, just make make the ½ wave line 5000 segments and feed it at segment 1 since most versions of modeling software won't let you feed at an end of an odd number segmented line, feeding at segment 1 of a huge number of segments gives you a very very close reading and would be within a couple of ohms.  

If you do that and keep increasing the number of segments, you will see that the impedance becomes huge, and would ultimately approach infinity if you had enough segments.  That should tell you that end feeding a wire is not possible.  Then you can examine practical ways of approximately end feeding a wire, which will result in a much lower impedance, and that will actually allow some current to flow in the wire.  Wire diameter will be a big player in the answer, as well as the counterpoise (there is always one). Height above ground will be a minor contributor.  

Jerry, I have increased the number of segments many many times in my life, never has it had the effect you are talking about.. 
I will just model what I need, thanks for the input guys....

Jerry, K4SAV

The real world antenna will never match the model regardless of how many segments you choose to implement.
Logged

Daniel "Danny" Horvat, N4EXA aka E73M
www.E73M.com & www.MyAntennas.com
K4SAV
Member

Posts: 2388




Ignore
« Reply #11 on: October 14, 2017, 12:01:14 PM »

Jerry, I have increased the number of segments many many times in my life, never has it had the effect you are talking about.. 

You must be looking at the real part of the impedance and not at the total impedance of the wire.  You can investigate both of those with NEC and also how they change with counterpoise length.  It's also interesting to look at the resonant frequency of the wire as a function of counterpoise length.  Be careful to recognize the difference between theoretical values produced by NEC and what you measure on an actual antenna.  NEC can analyze configurations that you can't build.

A practical implementation of an end fed wire would have a counterpoise and a method of cancelling the reactance of the antenna.  That would result in a more useful feedpoint impedance and closer to what most people think of as the feedpoint impedance of a half wavelength wire.  But you have to define those other things before you can figure out the real feedpoint impedance.  The counterpoise will have more effect on the feedpoint impedance than the height above ground.

Jerry, K4SAV
Logged
WD8KNI
Member

Posts: 173




Ignore
« Reply #12 on: October 14, 2017, 12:28:56 PM »

Jerry, I have increased the number of segments many many times in my life, never has it had the effect you are talking about.. 

You must be looking at the real part of the impedance and not at the total impedance of the wire.  You can investigate both of those with NEC and also how they change with counterpoise length.  It's also interesting to look at the resonant frequency of the wire as a function of counterpoise length.  Be careful to recognize the difference between theoretical values produced by NEC and what you measure on an actual antenna.  NEC can analyze configurations that you can't build.

A practical implementation of an end fed wire would have a counterpoise and a method of cancelling the reactance of the antenna.  That would result in a more useful feedpoint impedance and closer to what most people think of as the feedpoint impedance of a half wavelength wire.  But you have to define those other things before you can figure out the real feedpoint impedance.  The counterpoise will have more effect on the feedpoint impedance than the height above ground.

Jerry, K4SAV

Jerry thank you very much for the thoughtfull comments to this thread, I wanted this information as input to a problem I am working on understanding which has nothing to do with the actual building or troubleshooting of an antenna.  Your comments are 100% spot on as to counterpoise, height etc...   As for reactance I was carefull to specify in my original question "Resonate" meaning that the reactance was zero.  At this point I will just generate what I need from NEC and move on with understand my problem.   

Thanks to others also who chimed trying to help, with one exception the guy who owns the "Crappy Service" antenna company... who implied I was wasting my time as it won't match the real world.  I was lucky you were out of the country working a show and cancelled my order.  I and others seem to have results in modeling that very closely to the real world, amazing that you as an "antenna design engineer" don't.
  Regards.. Fred
Logged
E73M
Member

Posts: 147


WWW

Ignore
« Reply #13 on: October 14, 2017, 01:08:24 PM »

Jerry, I have increased the number of segments many many times in my life, never has it had the effect you are talking about.. 

You must be looking at the real part of the impedance and not at the total impedance of the wire.  You can investigate both of those with NEC and also how they change with counterpoise length.  It's also interesting to look at the resonant frequency of the wire as a function of counterpoise length.  Be careful to recognize the difference between theoretical values produced by NEC and what you measure on an actual antenna.  NEC can analyze configurations that you can't build.

A practical implementation of an end fed wire would have a counterpoise and a method of cancelling the reactance of the antenna.  That would result in a more useful feedpoint impedance and closer to what most people think of as the feedpoint impedance of a half wavelength wire.  But you have to define those other things before you can figure out the real feedpoint impedance.  The counterpoise will have more effect on the feedpoint impedance than the height above ground.

Jerry, K4SAV

Jerry thank you very much for the thoughtfull comments to this thread, I wanted this information as input to a problem I am working on understanding which has nothing to do with the actual building or troubleshooting of an antenna.  Your comments are 100% spot on as to counterpoise, height etc...   As for reactance I was carefull to specify in my original question "Resonate" meaning that the reactance was zero.  At this point I will just generate what I need from NEC and move on with understand my problem.   

Thanks to others also who chimed trying to help, with one exception the guy who owns the "Crappy Service" antenna company... who implied I was wasting my time as it won't match the real world.  I was lucky you were out of the country working a show and cancelled my order.  I and others seem to have results in modeling that very closely to the real world, amazing that you as an "antenna design engineer" don't.
  Regards.. Fred


Thank you for kind words, Fred,  I should have known that I can never please everybody and I am really sorry that you feel that way about me and my company.

 Regarding modeling EFHW antenna, My EZNEC model almost perfectly matches the real world antenna and I already shared that file with many HAM on some other forums.

 Good luck with your modeling and again am sorry about your "bad" shopping experiance while we were on vacation back in July...

 Best 73 Danny
Logged

Daniel "Danny" Horvat, N4EXA aka E73M
www.E73M.com & www.MyAntennas.com
KD1I
Member

Posts: 396




Ignore
« Reply #14 on: October 14, 2017, 04:11:07 PM »

At the risk of making myself a target of a flame, I will state my experiences with Danny's antennas. I have both his EFHW for 80 M and the OCFD for 80 M.    Both work as advertised and have very usable SWR across all their intended bands. I'm very pleased with the performance as well as the service Danny provides. I had an e-mail exchange with him on some technical points and he replied to all my messages in a timely manner. These are, of course, just my observations based upon my own personal experience.

All antennas, 'store bought' or home made will vary with configuration and location. My EFHW is mounted as an inverted 'L' with the transformer very near ground and rises to about 50 feet or so to the horizontal component. I'm at the very peak of a 1380 Ft ridge in North Georgia in a heavily wooded area. Rather than a traditional radial system, I'm connected to a series of perimeter grounds with #6 wire connecting the ground rods.

The OCFD is at right angles to the EFHW and is a similar height with the ground falling away on 3 sides about 150 to 200 feet depending on the side.
The OCFD gives a consistently better report by about 2 'S' units for most contacts.

My point is like the auto companies say: 1. "Your actual mileage may vary" and 2. Danny was a pleasure to work with.       

73, Jim
Logged
Pages: [1] 2 3 4 5 6 ... 8 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!