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impedance of end fed half wave antenna



 
 
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  #1  
Old May 4th 05, 02:43 PM
Andreas Magun
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Default impedance of end fed half wave antenna

I am planning to use QRP with a half wave length end fed antenna at 40 m
on a trip into desert country. What range of impedances is to be
expected with the fed end near ground and the other end between
3 and 10 m above ground? Another option would be an end fed inverted V
with the same range of center heights.

Or would be better to use a center fed inverted V with its low
impedance that can be mathed with smaller losses to 50 Ohms?

What would be the best (mimimum loss) tuner for an end fed antenna?

Thanks for your help

Andreas
  #2  
Old May 5th 05, 12:31 AM
Joel Kolstad
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Default

"Andreas Magun" wrote in message
...
I am planning to use QRP with a half wave length end fed antenna at 40 m
on a trip into desert country. What range of impedances is to be
expected with the fed end near ground and the other end between
3 and 10 m above ground?


See he http://webpages.charter.net/aa5tb/efha.html ... lots of good links!


  #3  
Old May 5th 05, 12:59 AM
David.Shrader
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Default

More than 1000 ohms. You will need a tuner

Joel Kolstad wrote:
"Andreas Magun" wrote in message
...

I am planning to use QRP with a half wave length end fed antenna at 40 m
on a trip into desert country. What range of impedances is to be
expected with the fed end near ground and the other end between
3 and 10 m above ground?



See he http://webpages.charter.net/aa5tb/efha.html ... lots of good links!



  #4  
Old May 5th 05, 04:24 AM
Hal Rosser
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Posts: n/a
Default

This page - from that web site - looks like the reference he needs
http://www.qsl.net/oe3mzc/hlfewve.htm


"Joel Kolstad" wrote in message
...
"Andreas Magun" wrote in message
...
I am planning to use QRP with a half wave length end fed antenna at 40 m
on a trip into desert country. What range of impedances is to be
expected with the fed end near ground and the other end between
3 and 10 m above ground?


See he http://webpages.charter.net/aa5tb/efha.html ... lots of good

links!




  #5  
Old May 6th 05, 05:55 AM
[email protected]
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Posts: n/a
Default

Or would be better to use a center fed inverted V with its low
impedance that can be mathed with smaller losses to 50 Ohms?

Pass go and collect $200.... Way better than any other
method...Scrap the tuner...Thats the last thing you want
for QRP...Center feed with coax...Can be lightweight rg-58...
You really don't even need a balun...In the unlikely event rf
at the rig is a problem, just roll a few turns of coax at the feed,
and tie wrap it...As a bonus, you can add wire, or insulators,
and use for other bands, or make it multiband, with insulators,
and jumpers to change bands...No tuner needed anywhere,
once it's set up. You could make one for all bands...
Would look kinda like this for a 20-40-80m version.....
-----------------o---------o-------00--------o----------o-----------------
You jumper all the insulators for 80..Only the inner ones for 40,
and none for 20...In that pix, you can't see the jumpers.....
Can be wires with gator clips, or whatever....I've used that
portable many times...Usually built on site from wire and stuff
I carry...But I usually just make them for 80/40....
MK

  #6  
Old May 7th 05, 10:10 PM
[email protected]
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Posts: n/a
Default

Basically, either antenna can be fine. Just try to make sure that the
section that carries maximum current (and thus does most of the
radiation) is as high and clear of conducting objects as possible.
Also, avoid bending that part of the antenna much more than 90 degrees.
For example, if you make a 180 degree bend, radiation due to currents
going in opposite directions will largely cancel out.

For your half wave long wire the place of maximum radiation is at the
wire center, and then tapering off towards the ends. So, keep the
center of that antenna relatively straight and high up. Don't let
strict theoretical assumptions hold you back from trying out what one
might think is impossible. For example, I have frequently made QSOs
for over 4000 miles with 1W (one watt) and a horizontal half wave wire
only 8-10 ft above ground on the 20m band. And that at the poorer half
of the solar cycle.

I suggest you keep your end fed half wave wire and use a small tuner
with it, such as the ZM-2 (http://emtech.steadynet.com/), and feed the
antenna wire directly from the tuner (no coax). It is very small,
versatile and has built-in an SWR indicator. It takes power up to
approx. 15W and has so far tuned into just about anything for me.

73 - Kris, TF3KX

  #7  
Old May 10th 05, 05:11 PM
andreas magun
Guest
 
Posts: n/a
Default

Thanks for all the answers. Intrigued by the lack of information
on Internet on the impedance of efhw-antennas (one value I found was
round 5kohm) I took the opportunity to become acquainted with the
EZNEC4-Demo and ran a few simulations of the efhwa as a straight wire
with the fed end near ground and an inverted V also. Considering
different geometries that could be realized by simple means I found a
large range of impedances between a few kohms up to 18kohm, probably
giving rise to high losses when matched to 50 Ohms - I have found
information on a "Z-Match using a toroidal Core Coil" by VK5BR that
shows efficiencies below 70% at 2kOhms for frequencies between 3.5
and 21 MHz, efficiency falling rapidly with increasing impedance.

On the other hand, a center fed inverted V has impedances between 36 and
52 Ohms, depending on the height (angle between wires) of the center. So
I am taking the advice of MK using this kind of antenna without any tuner.

Cheers
Andreas


wrote:
Or would be better to use a center fed inverted V with its low
impedance that can be mathed with smaller losses to 50 Ohms?

Pass go and collect $200.... Way better than any other
method...Scrap the tuner...Thats the last thing you want
for QRP...Center feed with coax...Can be lightweight rg-58...
You really don't even need a balun...In the unlikely event rf
at the rig is a problem, just roll a few turns of coax at the feed,
and tie wrap it...As a bonus, you can add wire, or insulators,
and use for other bands, or make it multiband, with insulators,
and jumpers to change bands...No tuner needed anywhere,
once it's set up. You could make one for all bands...
Would look kinda like this for a 20-40-80m version.....
-----------------o---------o-------00--------o----------o-----------------
You jumper all the insulators for 80..Only the inner ones for 40,
and none for 20...In that pix, you can't see the jumpers.....
Can be wires with gator clips, or whatever....I've used that
portable many times...Usually built on site from wire and stuff
I carry...But I usually just make them for 80/40....
MK

 




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