[Elecraft] Long wire antennas MORE

Fri Jan 12 19:43:38 EST 2018

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.  Just as with an 
off-center fed dipole, an EFHW of the right length would have low 
reactance and high impedance that could be properly matched with the 
right transformer, and you'd need common mode chokes for both to keep 
currents off the shield of the feedline.  The two antennas are different 
purely in terms, not in physical reality or in the radiation patterns 
they produce.

73,
Dave   AB7E

On 1/12/2018 4:33 PM, Bill Johnson wrote:
> Ron, love your comment.  I use EFHW and it is amazing the confusion over how they work.  Got to have a great transformer and a chosen antenna length and stick to it and perhaps a multiple that fits.  My 160 EFHW is made for 1.900 and works well at 3.800 in certain directions on either band.  I have do use a remote tuner for slight deviations and also compare two different dipoles to pick the best one to use.  The 160 is a c shaped set in the trees.  Insulated wire.  Now to get it isolated with insulators. Next spring... too much cold weather and snow to deal with.  :-)
>
> 73,
> Bill
> K9YEQ
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ron D'Eau Claire
> Sent: Friday, January 12, 2018 4:11 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] Long wire antennas MORE
>
> A mistaken idea that many Hams get is that a wire antenna has no radiation in the "nulls". For example, a half-wave wire is often thought of as having no energy radiated off of its ends. There is LESS off the ends, but a real-world wire has some radiation in ALL directions as Dave notes. It's just stronger radiation in some directions.
>
> A real long wire (many wavelengths) is easy to match since the longer a wire is, the smaller the impedance excursions across the RF spectrum. The hardest to match are wires a half wavelength (or less) long. However, most compact ATUs are limited in matching range based on simple physics. Their small size cannot tolerate the huge RF currents and voltages frequently encountered even at moderate power levels. There's a good reason why the old time "antenna tuners" (matching networks) were so huge. It's just a matter of basic physics. But today most of us use antennas that offer an feed point impedance limited to the range our ultra-fast, super-smart "automatic antenna tuners" can handle.
>
> 73, Ron AC7AC
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of David Gilbert
> Sent: Thursday, January 11, 2018 11:41 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] Long wire antennas MORE
>
>
> There is only a fixed amount of total energy contained in all the lobes of an antenna.  You almost definitely did get lots of lobes ... but you also got lots of nulls that exactly offset all those lobes.  You just never heard the the hams that were in those nulls and they never heard you.   Whatever you gain in one or more directions is sacrificed in one one or more other directions.  This is basic physics.
>
> More lobes is not necessarily better.  In fact, taken to the extreme it is self defeating because a very large number of lobes (assuming they were somehow all of equal strength as you stated) begins to approximate a unidirectional antenna with no azimuth gain in any direction.
>
> Just for grins I modeled your 700 foot antenna in EZNEC+ and on 20m it gave a maximum gain of about 9 dbi in a fairly narrow lobe at 16 degree elevation in both directions along the axis of the wire.  It also gave a total of 36 other sharply narrow lobes arrayed symmetrically in all other directions, each with a gain of about 6 dbi.  Between each lobe was a deep null of around minus 10 dbi. This was all at the same 16 degree elevation angle ... there were literally too many lobes to count on the 3D pattern, with lots of lobes and nulls at every azimuth and elevation angle.
>
> A simple dipole at the same 40 foot height would have given similar gain with a much broader lobe (both azimuth and elevation) in the two main directions, but of course without the multiple smaller side lobes. Three poles and two perpendicular dipoles would have given better overall single band results ... the only advantage of the long wire being that it gives a similar pattern along with similarly ugly match on multiple bands.
>
> Dave   AB7E
>
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