I did some EZNEC modeling of a planned inverted-L, and came up with some
results that surprised me, and caused me to re-think my understanding of
Jerry Sevick's work.
I modeled an inverted L over perfect ground and got a feedpoint impedance of
about 13 ohms. I then switched to real ground model, and added a few
radials. The feedpoint impedance went to about 50 ohms.
This is expected. I've heard it explained as, effectively, a series circuit
of the 13 ohm antenna impedance, and 37 ohms of return path resistance
through the ground. (Three-fourths of the TX power is 'warming the worms').
So far so good.
I then repeated the exercise with a folded-inverted-L (i.e., with twinlead).
As is the case with a folded dipole, the feedpoint impedance of the folded
antenna over perfect ground is about 4 times the impedance of the antenna
with a single wire, in this case 50 ohms. Again no surprise.
What then surprised me was the feedpoint impedance when I switched to real
ground, and the same few radials. Instead of going to about 87 ohms (50
ohms ideal antenna impedance plus 37 ohms of return path resistance), it
went to about 200 ohms.
Thus, with the higher impedance antenna, the return path resistance (ground
loss) now looks like 150 ohms, instead of 37 ohms.
In one way, this makes sense. The radials don't do their job better simply
because they are used with an antenna having a higher nominal feedpoint
impedance; there must still be a return path for all the antenna current.
The same three-fourths of the TX power still warms the worms with the same
radial system.
But it contradicts an earlier impression I'd had - that ground loss becomes
more of an issue with lower impedance antennas. I'd had this impression
since Jerry Sevick's 1970s articles about really short verticals, where he
stressed the importance of really good radial systems because of the
antennas' low antenna radiation resistances.
His articles give a familiar calculation of antenna efficiency as the
quotient of radiation resistance divided by (radiation resistance + ground
loss + ohmic loss in antenna). This formula assumes ground loss is a fixed
ohmic value. But the EZNEC data suggest this view is simplistic. While
ground loss with one antenna may look like 37 ohms; with another antenna it
may look like 150 ohms - all with the same radial system.
So now I'm thinking that my earlier understanding of Jerry's work, that
antennas with lower radiation resistances require better ground systems, is
wrong. At least if the EZNEC models are to be believed.
Can anyone give the technical reason that ground return path resistance
seems to vary in proportion with antenna radiation resistance?
Tnx,
/Bill, K2PO
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