Yup, Thanks Stewart, caught out !, I was assuming the PA would deliver the
same voltage but of course that would mean it was supplying twice the power.
I have intended to consider an equal power in both states so your 3dB is
correct....even so a doubling of ERP for just putting wire in the air is not
to be sneezed at !! We know from hard experience 3dB on QRSS is the
difference between "T" and "O" reports.
We have come so far in our understnding including the effects of loops in
combating local "environmental losses" I really feel we have made some
contributions to some areas of LF. mainly because of our real estate and
financial limitations compared with the professionals.
Cheers de Alan G3NYK
----- Original Message -----
From: "Stewart Nelson" <[email protected]>
To: <[email protected]>
Sent: 01 August 2005 00:13
Subject: LF: Re: Re: Suitable ground? Measure it !!
Hi Alan and all,
> However in practice
> the capacity of the antenna continues to increase at about 5 to 6 pF per
> metre. This turns out to have two useful effects. First is reduces the
size
> of the loading coil required to resonate the antenna, and hence reduces
> tuning losses. Secondly it reduces the current density flowing into the
> ground. Measurements show, and they are supported by some theory from
Alex,
> that if you double the antenna capacity you half the ground loss. Since
ERP
> is current squared, halving the ground loss doubles the antenna current,
> giving a four-fold, 6dB, more ERP ( a cheap "amplifier").
IMHO that is misleading. If you halve the ground loss (and also halve the
loading coil loss resistance), you could have twice the antenna current,
only if the transmitter can deliver it at the original feedpoint voltage.
But most of us have PAs with limited output power, and would need to
adjust the matching transformer for the new impedance (reduce turns ratio
by sqrt(2)). So, we would "only" gain 3 dB more ERP from the improved
antenna.
73,
Stewart KK7KA
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