Vertical anttenas ,IS it the same

[Gito]

Hi


Since I work at low frequency  AM Band,the problem I encountered is ,the radial system in a vertical Antenna,Say when we work at 160 meter band ,we need 40 meter radial.

So when I think of it,I think I found out that a vertical with radial has less gain with vertical (dipole}
 compare pic A and pic B,at A the radial does not ad to the radiation gain to the antenna,
because the current flow in the radial is flowing against each other direction (canceling it's radiation .)

At picture B It's a vertical dipole,the lower part act as a "radial" but since it's only one it also had it's own radiation resistant (1/4 wave antenna)


Now look at picture C ,the vertical Antenna is only 1/8 wave long,of course it has a low Radiation Resistance,and also has loses in the loading coil,
By using a fat vertical and making a good loading coil ,we can minimize ,the coil loses.
But the radiation resistance is low.(1/8 wave)

Looking again at picture C since the "vertical ground radial" is 1/4  wave long ,it has it's own radiation resistance

So this Antenna must have  much more gain than the antenna at picture D.
and no need of ground radials

It's only a though


Gito

[Opcom]

That's very interesting. For A and B comparison, I always thought the radials acted to reflect the waves out. But I want to see what the experts around here have to say.

[Gito]

Hi

Working with small Medium Wave transmitter,the problem I have is building an Vertical Antenna with their radials,since at ground level  we need 120 radial 1/4 wave long,it's out off the question ( need a lot of land and money).

As I know(maybe I'm wrong) the purpose of the radials ,is to reduce the Ground Resistance for RF.make it a perfect RF ground and also make a return passage for the RF current ,That's why We need only 4 radial ,when the vertical Antenna is high in the air(high above ground).

And since this radials runs opposite direction ,so does the RF current flow in this radials and canceling the radiation in it.So it does not contribute for the gain of the Antenna

In a dipole ,the upper half is 1/4 wave  and the bottom half is also 1/4 wave as a whole it's a half wave Antenna,so I think It must have a higher gain than the grounded 1/4 antenna with radials.

What I'm trying to do is making an unsymitricall  Dipole  ,with the bottom half a quarter wave length, and making the upper part a small part off a wave .

With a capacitive head and a fat pole ,we can resonate it with a coil with less looses {uper part of the dipole)
The aim is not the Radiation resistance of this upper half,since it must be small.
The aim is used the Botom part  as the true radiator which is a quarter wave long.

So as a whole this Antenna must have around 35 ohm radiation resistance (1/4 wave , the bottom half of the Antenna))

Say when the looses is 5 ohm (in the loading coil) than the efficiency of the Antenna is 35/40 X 100% =87.5%.

Building a 40 meter to 60 meter vertical,needs less land ,than building 120 radial ,with 40 meter long each.

Of course I can be wrong ,it's just a though that came into me.


Gito

[ke7trp]

I just installed Antenna c last week.  I cant compare it to antenna A since I have not owned one.  However,  Antenna c works very well for DX and beats my horizontal dipole on stations past 800 miles.

C

[KA2DZT]

I think antenna A and B would work better than antenna C or D simple because there are no loading coils.  I also think antenna C would work better than antenna D because there are no ground losses.  I think the same is true with antennas A and B, antenna B working better than A.  So, it seems that antenna B would work better than the other three.

The only problem I see with antennas B and C is feeding the antenna a 1/4 wavelength up the radiator.  Your feed line should run at right angles to the radiator or run up through the center of lower 1/4 wavelength of the radiator provided it is made from some sort of tubing.  If the lower 1/4 wavelength is a tower structure the coax should be run inside the tower framework.

Fred

[Gito]

Fred You are right.

But why did I "made" the other Antenna design,As I wrote before,it's because the Frequency's of the transmitter ,like 160 meter transmitter ,to make an antenna like A ,it uses 120 ground radial with 40 meter each long so we must used a land with R =40 meter ,or diameter of 80 meter,so it needs  a lot of land and fund to make this Antenna.

At B we need to make a  80 meter long vertical half wave antenna (a vertical dipole).
So it's difficult to make this antenna.

So I try to figure out to make a shorter Antenna( unsymitricall dipole) ,like C ,of course the radiation resistance is lower than a full dipole.

But when we can get /design the upper half antenna (C) short and fat dipole with a capacity hat,we can resonate it with a small Loading/matching coil (henry} ,so that the RF Resitance of this coil  is small

The purpose is to resonate the upper half but have  small RF resistance(coil),and not the radiation resistance of it,because it 's very small.

But the other half/bottom half of this "dipole" is 1/4 wave long ,and gave around 35 Ohm radiation resistance.

So as a whole the radiation resistance is around 35 ohm.
If we can make the loading/matching coil(upper half antenna)  RF resistance low like 5 ohm.

Than the efficiency of this antenna(as a whole)   35/ (35 + 5) X 100% = 87.5%.

But it's only my theory, It can be wrong.

Gito





 

[Steve - WB3HUZ]

Some well proven facts for verticals.

• Any vertical antenna when placed near the ground (regardless of the size or how and where it is fed) has ground losses.
• The amount of ground loss in the near field is directly related to the quality of the radial system - there is no way around this.
• A one-half wave vertical will have more ground loss than a one-quarter wave vertical.
• Coil loading will have more loss than capacitive top-hat loading.
• Elevated radials will radiate unless they are carefully trimmed or tuned.

If you can make a one-eight wave vertical (as in C or D), just us a top-hat or make it a Tee antenna to resonate. This will be the lowest loss approach. But, as noted above, you will still need a good ground radial system.

[Gito]

Hi

Steve, you are right

,but I think that ,since we feed the antenna at 40 meter of ground than highest the RF current is at that point and demising at this antenna  end /near ground  ( feeding 1/4 wave antenna in reverse/}.At this end it's consist of High voltage and "zero" current.
And since the current  it's high above ground ,I think it has little effect on the ground looses.
This current make a complete cycle with the upper resonated  pole.which is much  closer to this end/feed point.(more closer than the ground 40 meter below)

To make it short a dipole does not need a ground radial ,See it this way ,
it's a "modified dipole" which one of the End is near ground.(Highest voltage and "Zero current"  at this end)}.

What I meant  by a modified dipole is the upper leg is the  1/8 wave antenna ,loaded and resonated, and the other leg (in this Case the bottom leg with 1/4 wave long] an unsymitricall dipole.
 
A grounded vertical needs a ground radial ,since the current flow from the antenna  through space to ground and back to the antenna ( complete cycle).
To minimize this looses ,we make a "perfect ground" that's the radials.

When I wrote a 80 meter vertical  Antenna,I also wrote  ( a vertical  dipole  },not a half wave vertical, but like picture B, center fed dipole  with  each leg 40 meter long but in vertical position,maybe I didn't wrote it clearly


Steve off course my analysis ,can be wrong.

Gito

[Steve - WB3HUZ]

There's only one way to know for sure - build it and test. Sounds like an interesting project.