[Fwd: LF: Efficient antenna]
Thu, 27 May 1999 07:37:02 -0400
Rik Strobbe wrote:
> At 09:05 27/05/99 +-200, you wrote:
> >Hello LowFers!
> >As a newcomer I hope I have right to ask silly questions.
> >1. How efficient is actually capacitancy hat of Marconi "T" ant? In other
> >words, if there is space 40 meters horizontaly and 15 meters verticaly, I
> >can build either T ant with heigth of 15 m and length of hat of 40 meters,
> >or inverted L ant, 15 m long vertical part and 40 m long horizontal part.
> Compared to a 'pure' vertical a sufficient horizotal top section can
> quadruple the radiation resistance and so increase a 6dB improvement.
> Furthermore you can bring the antenna to resonance with a smaller loading
> coil, also reducing the losses in the coil, this can eventually give an
> additional 1 or 2 dB gain.
> An example (I skip the detailled calculations here, but I mail then to you
> if you want) :
> A 15m vertical (not top-hat) will have a radiation resistance of about 18
> mOhm and a capacitance of about 90pF. You will need a loading coil of 15mH
> you bring it to resonance. Assuming a Q of 300 the loss in th coil will be
> 43 Ohm.
> The 'ground loss' will be between 30 Ohm (very good) to 200 Ohm (very
> poor), so let's assume it will be somewhere inbetween = 60 Ohm. So the
> total loss resistance will be 103 Ohm. If you put 100W RF power into this
> antenna the antennacurrent will be 0.99A and your ERP will be about 18mW.
> Now you add the 40m top-load section to the antenna. This will increase the
> radiation resistance to about 50 mOhm. Compared to the 18 mOhm of the pure
> vertical this gives you a gain of about 4.4dB. Due to the top-load the
> antenna capacitance will increase to about 290pF, so you will need a
> loading coil of only 4.7mH. Assuming a Q of 300 this means a coil loss of
> about 13 Ohm, so your total loss will be 73 Ohm now. If you put 100W into
> this system the antennacurrent will be 1.17A and the ERP will be about 68mW.
> So compared to the 18mW you got with the pure vertical you have an
> 'overall' gain of 5.8dB !
> >Is it sure that T antenna would be more efficient than that inverted L ant?
> Personally I don't think that there is a real difference and that the
> selection between T and inv-L is mostly done based on the location.
> 73, Rik
> Rik Strobbe ON7YD
> Villadreef 14 B-3128 Baal BELGIUM (JO20IX)