On Nov 14, 9:36*am, wrote:
On Nov 13, 10:21*pm, JIMMIE wrote:





On Nov 13, 9:11*pm, "Peter" wrote:

Why do you call it a 1/8 wave loading coil? It wouldn't be along the
lines of the flawed "loading coil replaces the missing degrees" concept
would it?

I referred to the 1/8 wave loading coil without really thinking about it. I
was unsure of the loading coil dimensions, so I simple tried a 1/8 wave
length wire formed into a coil. This is for the simple series arrangement
5/8 radiator. This created a load coil that appeared to have a little too
much L so I have removed one turn, seems to load up ok after a little
trimming of the radiator. Keen to hear how too determine the
value/dimensions for the loading coil.

Having said that I'm not sure what so wrong with missing degrees" concept.

A 5/8 monopole's performance is quite senstive to the ground plane
implementation. The behavior of a 5/8 monopole over a perfect ground is
not replicated over real radial systems or car roofs, yet people compare
antennas based on the perfect ground plane environment.

As the length of the radiator is increased beyone a half wave, low angle
gain increaeses until about 0.6 wavelengths when power is shifted into a
developing upper lobe. The optimum length over a perfect ground is
probably just a little less than 5/8, and less still over practical
ground planes.

The other dimension is feedpoint impedance. For a simple series L
matching arrangement, R is a little high and the optimum length is
typically longer than 5/8.

So, for optimum pattern, and low VSWR, a better solution is a tapped base
coil with 0.6 wavelength vertical... but that doesn't play well with the
simplest of mobile antenna bases that provide only one connection to the
screw on antenna.

My current 5/8 wave ground plan project is simply to get something on air,
however I plans to construct an improved version with the tapped coil
approach.

I may be looking in the wrong places, but I have been surprised at how
little information there is on the net regarding 5/8 wave ground plan..

Thanks Owen for the above over view of the 5/8 wave ground plan.

Cheers

Peter VK6YSF

http://members.optushome.com.au/vk6y...htm-Hidequoted text -

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The way you did it works pretty good. A lot of practical antenna work
is estimate and trim.There is or used to be a site that goes into a
lot of detail on the 5/8ths. I had it in my bookmarks for a long time
but lost it in my last computer crash. Compares 5/8ths with 1/4 wave
radial 5/8 radials *horizontal and drooping radials and much more.
Sorry but I cant remember who had the site bet someone here does.

Jimmie

I modeled a few of the usual versions.http://home.comcast.net/~nm5k/acompari.htm
I had thought I had also modeled a few using resonant
3/4 wave radials, but I guess I had found better modeled
results using the 5/8 radials. *But I know the 3/4 wave
radials give a much better pattern than the 1/4 wave
radials, but maybe a tad less gain than 5/8 radials.
But these show why I don't like 1/4 wave radials for
a 5/8 radiator. And Richard may have a point about it
it being an "OCF" antenna. This is why I consider it
perverted. I don't like horizontal OCF antennas either.. *
Through the years of modeling these, and playing with
them in the real world, I've noticed a few things about
the radials.
I prefer sloping 1/4 wave radials when used with a 1/4
wave radiator. The performance difference between
"straight out" radials is not large, but is about .3 db
or so better with the sloping variety. And you get a
bit better match.
But sloping 1/4 radials with a 5/8 radiator is bad news.
The pattern is even worse than when they are straight out.
So if one were to use 1/4 radials on a 5/8 GP, they should
be straight out for the best results.
But I much prefer using either 3/4 or 5/8 radials with
a 5/8 radiator, and the plots show why.
The pattern is cleaned up, and the high angle lobe
does a vanishing act. *You then start to see the
comparative textbook gains at the horizon when comparing
to shorter antennas.
IE: most books will claim a 5/8 antenna to have appx
3 db gain vs the 1/4 wave.
But you won't see that with the short radial version.
The gain is there, but it's not on the horizon where you
want it.
If you look at the azimuth plot for each, note the 1/4 GP
shows about 1.8 dbi, and the 1/2 about 2.1 dbi.
As they should..
But look at the perverted 5/8 version.. A lowly 1.1 dbi
at the horizon, with most of the real gain shooting off
to venus at about 45 degrees.. The antenna is sad, and
needs therapy.. * *Where is the appx 3 dbi we are
supposed to be seeing?
But if you check the version with sloping 5/8 radials,
we see our expected gain on the horizon. About 3.1
dbi in this plot. That's pretty close to the theoretical
expectations.
But if you make the long radials even steeper to
more closely resemble the collinear, the gain increases
to 4.25 dbi.
You are starting to approach the gain territory of the
dual 5/8 collinear which will show about 5.1 dbi on
the horizon. Assuming good decoupling from the
feed line of course... Decoupling is half the battle,
and if it is ignored, one might as well hang a wet
noodle on the roof, and be done with it.
This explains why I have such a negative view of
1/4 wave radials under a 5/8 whip. It's like using
a band aid to deal with severe chainsaw lacerations.
The blood with still spew, and it will be spewing
up into the air at about 45 degrees from the horizon.
Chortle..
All the speculation about matching seems silly to
me. The matching coil is so simple to apply, it's a
non issue. I've built so many of them, I can tell
you about how many turns to use for any particular
band.. I can usually just guess, and get pretty close.
Maybe tweak a turn or two to get just right..
It's simple, and any matching schemes should
not interfere with the lengths of the elements
if you want the most gain at low angles.
It's like matching a yagi.. I don't alter the element
lengths of a yagi to get a batter match. I use the
appropriate matching scheme, and leave the elements
the length they were designed to be for the gain/fb
the antenna was designed to produce.- Hide quoted text -

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Thanks thats the data I was looking for but I dont believe it is the
same site. More than good enough.