LF: Horizontally polarised radiation

["Dick Rollema" <d.w.rollema@freeler.nl>]

To All from PA0SE,

In an earlier e-mail I wrote:

----- Original Message -----

From: Dick Rollema

To: rsgb_lf_group@blacksheep.org

Sent: Sunday, January 04, 2004 3:50 PM

Subject: Re: LF: Re: "T" versus "L"aerial

Dear John,

Computer simulation takes place over perfect ground and shows appreciable horizontally polarised radiation under high elevation angles for the inverted-L aerial .  But even at say 20m height of the horizontal part of the aerial this is only 0.0009 of a wavelength so the aerial is almost on the ground.  This means that over real earth such strong currents will be induced in the earth under the aerial that almost all power in the horizontally polarised field will be converted into heat in its resistance.

Mal, G3KEV commented:

Dick

You are saying that my 4 x inv L antennas each at 120 ft vertical and 400 ft horizontal are only burning up the field under the array.

My signal is reported throughout EU and beyond to Russia and the USA as the strongest on the air only surpassed by MM0ALM when he was active. As far as I can remember he was using 2 x inv L antennas at over 140 ft high using two masts.

I have never used a T antenna, my logic is that the more independent wire in the air the better, you have the benefit in my case of 4 top loaded verticals in parallel. All the bottom ends of the verticals are connected to the top of the loading coil of about 0.3 mh. The top of each L is about 10 feet out from the mast and the drop wires taper to the bottom reducing capacitance to ground.

An inv L antenna is really a top loaded vertical.

I hope you understand what I mean.

Calculations and theoretical assumptions are often way of the mark when it comes to small antenna systems like radio amateurs use on LF.

The only solution is the practical approach, common sense gained with experience and put up the best antenna you can then judge the results. As a matter of interest I have never burnt out any insulators, antenna wires, ground radials or any vegetation underneath the antenna array.

If you are working worldwide on normal CW then you have got it right !!!!!!!

73 de Mal/G3KEV

 

What I said was that the radiation from the horizontal part of a top loaded aerial is largely absorbed in the earth under it because the wire is so near to the surface.
To have a closer look I modeled a 40m long dipole for 136kHz, made of 1.5mm copper wire over real ground at a height  h.  Ground constants are dielectric constant 13; conductivity 5mS/m.

The simulation shows that the maximum of radiation is under an elevation of 90 degrees. The gain is referenced to a half wave dipole in free space.

h in metres    gain in dBd

500                   -2.29
200                   -6.65
100                 -11.84
 50                  -17.44
 20                  -25.40
 10                  -31.48

The numbers speak for themselves.

The radiation by the vertical part of the aerial, that does the work, is not affected by this of course.

73, Dick, PA0SE