Hi, Tzikas
Short version:
Your six elevated 1/8 wave radials at 9 meter height will do better
than the plan in your picture with the long 1/4 wave radials in only
one direction. Make sure you have a good, real common mode current
blocking device on the feed coax at the antenna feedpoint to keep the
antenna from using the feedline and everything in your shack as its
only radial. If you add a common mode blocking device and the SWR
changes, you were losing power and picking up noise on the feedline
shield.
Long version:
However many elevated radials you put up, elevated radials need to be
the same length and approximately equally distributed around the
compass. (This is what you have now.) This is different than buried,
bare wire radials. Buried radials will conduct to the ground which
carries current and tends to equalize current. Buried radials can be
miscellaneous lengths, as long and as dense as practical. This is not
true with raised radials.
The current will favor the raised radials with the lowest impedance
presented to the center, and proportionally disfavor radials with
higher impedance. You will do better, loss-wise, to have raised
radials equally distributed to all directions. ON4UN describes a
radial system that is 4 elevated 1/8 wave radials, with the reactance
tuned out with a single inductor in series at the joining point. Your
system is very close to this. There is research that suggests that
the main disadvantage to the shorter version of a small number raised
radials seems to be narrower bandwidth than the same number of longer
raised radials, NOT lesser performance. A tuner can deal with a narrow
natural bandwidth.
NEAR THE GROUND, e.g. 2-3 meters, 1/4 wave elevated radials seem to
become more effective than the same number of shorter radials starting
at roughly 16 radials peaking around 32 and above. The issue is the
total induced ground loss underneath. The current in a radial produces
a field mostly reverse to the vertical radiator in the ground
underneath which may or may not cancel out the vertical radiator's
field, depending on the details.
Using just a few long radials does several unhelpful things. A
majority of the ground around the vertical radiator effectively has no
radial overhead. Worse, four raised radials will each be carrying
four or eight times the current of raised radials in a 16 or 32 raised
radial system, which is too much to cancel out the vertical radiators
field underneath, and results in lossy induced current underneath. In
your case the four 1/4 wave elevated radials so close together will
behave like a single 1/4 wave elevated radial.
The ideal "ground shield" effect of a raised radial system occurs
where the sum of all current in the ground underneath the radials is
at the theoretical minimum (complete cancellation is not possible).
In the diagram you supplied, look at the layout as if only the long
raised radials are conducting and you will see that in spite of all
the short wire in the air, you are effectively only working with one
raised radial.
A raised radial system is performing TWO tasks: 1) providing a current
sink, somewhere for the current on the shield side of the feed to go,
and 2) providing a "ground effect" or "reflection loss" shield
underneath the raised radials which may be viewed as minimizing
induced current in the lossy ground underneath.
If one cannot place 16 or 32 raised quarter wave radials then one is
effectively giving up on doing much for task number two. Then one
just tries to create an efficient current sink with a small footprint
and where the total induced current in the ground is as minimal as
achievable. This is the strategy in ON4UN's tuned 4 times 1/8 wave
radial system.
There is another solution, a single tuned folded raised "radial" as
counterpoise, a folded counterpoise which minimizes ground induction,
which goes 32 feet or so either side of center. It works by folding
back and forth in such a way as to mostly cancel the field underneath.
It gives up on doing anything about "reflection loss" but is efficient
and has minimal ground induction and performs the necessary current
sink function. This is under investigation and we will publish results
here by the middle of September or so. It may even be possible to
"double fold" it, giving a possible not-so-bad alternative for those
for whom traditional radials are simply out of the question.
Many objections have been raised to schemes of this sort in the past,
comparing them unfavorably to the commercial broadcast gold standard.
Of course, they compare unfavorably. But some also gloss over the
weaknesses of attempted alternatives with poor results.
The problem with this objection, comparing to the BC standard, is that
it does nothing for the "little guy", people in situations where the
idea needs to be how to accurately capture AS MUCH AS POSSIBLE of the
gold standard for the circumstances. Models and some testing suggest
that performance of a minimal radial/counterpoise system can approach
the commercial BC quarterwave within a few dB, taking certain required
precautions. This is in comparison to a pair of raised quarter wave
radials showing ten dB or more below the BC standard.
Some have tried to dodge the miscellaneous raised radial length
problems by INDIVIDUALLY TUNING each radial to zero out reactance.
This is a difficult thing to do given the interaction of adjacent
radials, involves varied inductor values per radial, and resulted in
scattered success.
You are very close to ON4UN's method, and just need to make sure you
have a GOOD common mode current block on the feedline at the antenna
feedpoint. If your feed impedance or SWR changes when you put one on,
that is a sure sign that the antenna would rather use your feedline,
and whatever is attached to the shield in your shack, as a single
radial, than use your actual radials. It would seem normal to me that
an unblocked feedline would shunt short elevated radials every time,
most likely losing you six dB or more in the process.
73, and good luck,
Guy
On Wed, Aug 25, 2010 at 12:44 PM, tzikas tzik <tzitzikas77@gmail.com> wrote:
> Hi all. At this link
> http://tzitzikas.webs.com/radials_direction.JPG
> you can see my plan for radials installation.The sort radials are 1/8 and
> the longer 1/4 wave length. Radials are with blue color. Because of the
> buildings i can't install radials at all directions.
> Now i am using only 6 radials. The four of them are at the left direction
> and the other two at the opposite direction. What do you think about my
> plan? This radial installation what effect it will have to my antenna
> efficiency? It will help, or do you propose to keep my current installation
> with only six radials? I must tell you here, that if i will install all
> these radials, many of them will be at higher altitude as regards the
> antenna's feedpoint.
> The transmitter and the radials are about 9 meters above the ground surface
> on the floor of a building. My antenna is inverted L 5/16.
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|
