[Lowfer] Antenna.

[Steve Dove]

Hi Kurt,

Mostly agreement with Dale on his comments on antennas;  additionally, the 'gain' and 
'low radiation angle' of a 5/8 wave particularly at HF are practically illusive.  In order to 
get even close to the pretty pictures modelling shows, the ground plane beneath a 5/8 
has to be *huge*, extending at least a few wavelengths out;  the initial ground reflection is 
all important, and of course the lower the take-off angle, the further out that takes place, 
especially from an antenna where the current maximum is so high above ground.  Try 
doing the modelling again in free-space against radials, and see the difference.

A 1/4 wave against ground  -  where the current maximum is low down -  'merely' gets 
progressively less efficient as the ground screen gets smaller;  the vertical take-off 
characteristics change surprisingly little.

A 1/2 wave vertical fed against ground actually has the same problems as the 5/8 in 
terms of needing a large screen;  it is a fallacious old-wive's tale that a 1/2 wave doesn't 
need a ground screen 'because it's high impedance, and so the ground loss is relatively 
small it doesn't matter'.  The antenna *still* needs screening from lossy dirt, and *still* 
needs a means of efficiently collecting the return currents.  However, since the metric is 
field strength, one can always crank the power to compensate;  just don't expect 
fabulous low-angle performance.

An elevated 1/2 wave end-fed (a la 'slim jim') can work, but is a pig to tune, match and 
feed, and unless one is careful the feedline does a goodly amount of the radiating  -  a 
current balun at the feedpoint (and perhaps another a bit further down) is a must.  A 
(30m.) version I once built had the half-wave radiator in the form of a 1/4 wave 'T', i.e. a 
half-wave horizontal top section with a 1/4 wave descender from the middle of it, fed at 
the bottom.  This allowed the whole thing to sit higher in the air with no compromise in 
performance. 

Actually, the feedline radiation thing is true of the (recommended) humble elevated 1/4 
wave with radials, too.

Counter-intuitively, the Part 15 rules' field-strength based limits do negate any advantage 
to antenna gain, as much as ordinarily we would strive to persuade the RF to go where 
we think it'd do most good.  Equal-opportunity interference.

        Cheers,

                Steve        W3EEE


 
9/2/2003 3:50:44 AM, "KD7JYK" <kd7jyk@ross-technologies.com> wrote:

>Ran some antenna numbers, I like the low radiation angle of a 5/8 wave
>antenna but 43' is a bit high, the 1/4 wave vertical with radials looks
>really nice and only about 18' high so real easy to build a whip.
>
>What do you guys think?
>
>Kurt