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
[email protected]
Villadreef 14 B-3128 Baal BELGIUM (JO20IX)
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