[Elecraft] EFHW vs Off-Center Fed Dipole

[David Gilbert]

You can use whatever terms you want, but physically and electrically you 
are turning the EFHW into an off-center fed dipole when you add the 
"counterpoise" wire beyond the feedpoint.  If you don't agree with that 
you are merely adding to the confusion.

If you add 3.5 feet to a half wave 40 meter dipole it already is no 
longer a true half wave anyway, and you might as well just feed the half 
wave antenna 3.5 feet from one end.

Don't believe me?  I modeled a half wave 40m dipole at 70 feet in 
EZNEC+.  At 67 feet long and fed at the center the feedpoint impedance 
was 66 - j4 ohms.  When I simply moved the feedpoint out to 3.5 feet 
from one end (keeping the total length at 67 feet) the feedpoint 
impedance became 1509 - j1202.  I then added 3.5 feet to the antenna 
(total of 70.5 feet) and fed it 3.5 feet from one end, which in your 
world would be the EFHW with a 3.5 foot "counterpoise."  The feedpoint 
impedance changed to 3317 - j3115 ohms.  Which do you think would be 
easier to match (or easier to understand)?   The 3D radiation pattern 
was indistinguishable from each other in all three cases, of course.

It doesn't matter what you call it, but if you take a piece of wire and 
feed it some distance from the end you now have an off-center fed dipole 
... an extreme version possibly, but an off-center fed dipole nonetheless.

In my opinion, the term "counterpoise" is mostly a silly obfuscation.

Dave   AB7E



On 1/13/2018 6:44 AM, Don Wilhelm wrote:
> Dave,
>
> All information I have seen says that the counterpoise needed for an 
> EFHW is 0.05 wavelength - at 40 meters, that is about 3.5 feet.
>
> If you make it longer than that, it becomes an offset center fed 
> antenna, longer than a halfwave, in other words, it is a random length 
> wire.  Both the half wavelength wire and the counterpoise wire will 
> radiate.
>
> 73,
> Don W3FPR
>
> On 1/12/2018 7:43 PM, David Gilbert wrote:
>> Well, since you brought up EFHW there is a relevant comment I've 
>> wanted to make for a while.
>>
>> An EFHW with a counterpoise wire (which everyone seems to claim is 
>> important to have) is basically just an extreme version of an 
>> off-center fed dipole.  A half wave dipole has its lowest impedance 
>> at the center, where the current is high and the voltage is low.  As 
>> you move out away from the center the current decreases and the 
>> voltage increases, which is equivalent to saying that the impedance 
>> increases.  As you get to the end of the wire the current obviously 
>> goes to near zero except for capacitive currents while the voltage 
>> goes very high ... meaning high impedance.  The "counterpoise" for an 
>> EFHW is merely an extension that puts the feedpoint back toward the 
>> center where the impedance isn't quite as high.  And as with any 
>> dipole, it isn't critical how that "counterpoise" is physically 
>> arrayed because the current there is small so it doesn't affect the 
>> pattern much ... just as is the case with a dipole with drooping ends.
>>
>> I think if everyone viewed EFHW antennas as off-center-fed dipoles 
>> there would be a lot less confusion about how they work. 
>