[Antennas] Short ground radials

[Charles Greene]

Alex,

I'm sorry but I respectfully have to disagree.  The radial can be a little 
longer or shorter than 1/4 wave length, and are only slightly less 
efficient if they are a little shorter than 1/4 wave length.  However, 1/4, 
or 90 electrical degrees is to be avoided for elevated radials.  The reason 
is that at 90 electrical degrees, the impedance is near zero ohms, and all 
the radials due to slightly different environmental conditions will have a 
little more or less impedance than the others.  The radial with the lowest 
impedance will hog the current, the next lowest, the next most current 
etc.  The different currents distort your radiation pattern.  This has been 
written up extensively in the amateur radio literature.  Therefore, you 
should make your radials either more than 90 electrical degrees, or less 
than 90 electrical degrees.  One author suggested either 60 electrical 
degrees or 120 electrical degrees.  You then need a matching network at the 
antenna base, or loading coil in the radiator.  The entire system is tuned 
to resonance, with the single matching network or the loading coil 
supplying the necessary   X to balance the X of the combination of all the 
radials in parallel.  One trick is to make the radials just a little short 
so they have an Xc reactance, and balance it by making the radiator just a 
little long to give it an Xl reactance.  I tried it and it works well for a 
longer radiator and shorter radials, and a little less well if the radials 
are a little long and the radiator is a little short.  A little less well 
means the antenna efficiency is not quite as high.  A little long or little 
short means 2 feet or so for the radiator on 20 meters, and a little less 
for the radials, as they are in parallel.  The exact length depends upon 
their height above ground and number of radials.

Concerning ground losses, even with the elevated radial, you cannot 
eliminate ground losses or even cut it down much.  The RF current returns 
through the ground as well as through the elevated radials, and any current 
through the lossy ground will cause losses.  Most of the top band operators 
say that an installation with a lot of 1/4 wavelength buried radials beats 
an installation with 4 elevated radials.

Now if you bury the radials, it's a different story.  The ground changes 
the electrical length of the radials.  Actually, they can be reduced to 
about 0.1 wave length without effecting the performance of the antenna much 
at all.  The key is to use a lot of them.  On the other hand, there is not 
much point in using more than 4 elevated radials, and using only 2 will 
give you nearly the same results as 4.

At 03:08 PM 1/12/2003 +0200, alexander eban wrote:
>   Hi guys! A propos radials: radials are suppoesd to extend from the antenna
>base to a minimum distance of one quarter wavelength. It doesn't have
>anything to do with electrical lenfth, but with free space dimensions.
>Ground laid radials are intended to separate between the lossy ground and
>the antenna, by providing a lower loss return path for the antenna currents,
>thus free space dimensions. For an elevated radial system shortening the
>radials will incur some losses, but smaller that for a ground level
>installation. All in all, if you can't avoid it use it, but with tongue in
>cheek.
>                 73 and all the best:
>                         Alex    4Z5KS
>
>
>-----Original Message-----
>From: Charles Greene [mailto:W1CG@QSL.net]
>Sent: Sunday, January 12, 2003 2:37 PM
>To: ka4inm@tampabay.rr.com
>Cc: a
>Subject: Re: [Antennas] Short ground radials
>
>
>Ron and All.
>
>You can use shortened radials and load them with an inductor.  Ref: Moxon,
>HF Antennas for All Locations, or model it using EZNEC.  A single inductor
>will resonate all the radials which can be connected together but not
>connected to the coax shield.  In fact, if you structure the antenna
>properly, a single coil at the base will resonate both the radials and the
>radiator.  I modelled it and then built one.
>
>At 03:21 AM 1/12/2003 +0000, Ronald KA4INM Youvan wrote:
> >>I was wondering... If one can make a "short dipole" with the use of
> >>loading coils.
> >
> >
> >   Yes, a stinger at the end should increase efficiency.
> >  (not all coil, see ham stick)  (twice in this case)
> >
> >>Could one also use loading coils to shorten the ground radials of an
> >>vertical antenna?
> >
> >
> >   No, the radials are more reflectors.
>
>
>73, Chas, W1CG
>K2 #462


73, Chas, W1CG
K2 #462