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HB9ABX > TECH 01.09.05 14:08l 65 Lines 2256 Bytes #999 (0) @ WW
BID : IM503KHB9EAS
Read: DJ9UI GUEST DJ4KI DL1LCA OE7FMI
Subj: Knowhow for good antennas
Path: DB0FHN<DB0MRW<DB0RGB<DB0IGL<DB0FSG<OE7XLR<OE9XPI<HB9EAS
Sent: 050901/1203z @:HB9EAS.CHE.EU [Basel JN37TM] DP6.00 $:IM503KHB9EAS
From: HB9ABX @ HB9EAS.CHE.EU (Felix)
To: TECH @ WW
The good antenna and the Q
13.5.2003 (translation 30.8.05)
Copyright Felix Meyer (c) 2005
In different mailings I found the opinion of many OMs, that the antenna with the
higher Q is the better antenna.
But is that really true ???
Many times, the Q is higher, but the antenna is worse ...
At least for me, the better antenna is this one, which radiates more energy into
space instead of converting RF energy into useless heat.
The Q-factor of a coil is the relation of Inductance to Resistance or
Q = (2 Pi f L) / R
An antenna, or in our case the magnetic loop, is more than a coil, the
term R consists of the sum of series loss resistance Rloss and
series radiation resistance Rrad.
For an antenna, Q becomes (at resonance)
Q = (2 Pi f L) / (Rloss + Rrad)
Furthermore, Q is the relation of resonance frequency F0 to bandwidth B
Q = F0 / B
However, the factor Q says nothing about the amount of power radiated
by the antenna as RF wave.
How good the power is radiated by the antenna is told by the efficiency Eta
which expresses the relation of the radiated power to the antenna input power.
Eta = Rrad / (Rrad + Rloss)
Here we are:
For magnetic loop antennas Q is generally between 500 and 1000, but
the efficiency may vary from below 1 percent up to over 75 percent.
Note: At 1 % efficiency you convert from 100 W input power 99 W into heat and radiate
1 W as radio wave!
What do you prefer ? To have more heat or to have a stronger signal ?
Conclusion:
Analyzing these formulas one can see, that high efficiency is obtained
through a high relation of radiation resistance to loss resistance.
If the loss resistance increases, the Q is lower and efficiency is lower.
If the radiation resistance increases, the Q is lower and efficiency is higher.
It is important to design your antenne such, that the relation Rrad to Rloss
is highest.
This is the full secret of magnetic loop antennas.
How to calculate the radiation resistance and the loss resistance is
found in the formula section of the magnetic loop text on my homepage.
Now, all should be clear for a GOOD MAGNETIC LOOP ANTENNA.
My Homepage: http://home.tiscali.ch/hb9abx
best 73s
Felix HB9ABX
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