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Message-ID: <008d01c6e897$e4f7f940$0300a8c0@lark>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <001301c6e829$a8a69370$6801a8c0@Radio> <004901c6e881$7ad2d320$0300a8c0@lark> <005101c6e888$76be7f30$6501a8c0@eagles>
Date: Thu, 5 Oct 2006 17:04:05 +0100
MIME-Version: 1.0
Subject: LF: Re: Math & advice
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Status: O

Hi again J. Being cynical I would use whatever calculation seems to give the
"lowest" ERP.  The idea that the effective height is reduced to the level of
the lowest sag in the top wires is attractive !!. In fact the actual
reduction for long and widely separated wires like yours is probably much
less (30%), but that is to your advanatge over the "regulations". ERP is a
very difficult thing to measure or estimate, and most attempts at
measurement , Laurie G3AQC, Jim MoBMU and  Christer SM6PZJ gave values
around 6dB below the theoretical values.  It would be better to be radiating
and arguable 2W ERP than a conservative 0.5W ERP !!

Top capacity does not affect the effective height once the top-wire length
exceeds the vertical height (h). With no top wire the heff is =h/2 with a
long wire
parallel to the ground, it becomes about 0.95*h. Extending the wire or
putting more up does not affect this. What it does do at LF is to increase
the capacity so you need a smaller coil for resonance, and it spreads out
the ground currents so the ground loss is much reduced (hence you 19ish ohms
figure).

I am not sure about running the PA at a lower voltage. PA efficiency is
usually better at higher supply voltages. I am not sure what the effects of
running a Class-D design is, but I know you design a Class-E stage for the
power required at the voltage selected. The key with a Class-E stage is the
FET shunt capacitor selection, maybe the same thing works with Class D
stages but ypu will have to rely on the exponents of those for an answer
(Dave G0MRF, Jim M0BMU, Dave G3YXM) For you application you want a minimum
stress situation where the PA will run 24/7 without any grief.

Cheers de Alan G3NYK


----- Original Message -----
From: J. Allen <vy1ja@northwestel.net>
To: <rsgb_lf_group@blacksheep.org>
Sent: 05 October 2006 15:13
Subject: LF: Math & advice


> Alan, and All,
>
> The top atttachment for the tower end of the horizontal wires at is at 100
> feet, these 390 foot long wires fan out from the vertical portion at a ~55
> degree angle to eachother and the other two ends are at ~63 and ~70 feet
> respectively.  There is ~10 ft of sag in the wires.
>
> One person who gave me early help with the calculation said that the lower
> attachment points of  the wires out away from the tower also lowered the
> effective height of the vertical portion of the antenna significantly,
being
> as low as the lowest part of the horizontal wires, and that is how I ended
> up with the number 2.85 for the antenna current for 1 W ierp.
>
> Is capacity hat wire elevation significant in the calculations?
>
> There are now four different numbers from as many people, and I am unsure
> how to proceed.  LF seems to be significantly art and partially science.
> :o)
>
> I moved the thermocouple type RF ammeter to the line between the matching
> transformer and the loading coil and it reads exactly the same, 2.8 Amps.
>
> Would it work better for me to wind a new matching transformer at the
> antenna on an iron or ferite core.  Is a metallic core likely to improve
> what I am seeing?   If so, would a TV core do the job?   The existing XER
is
> a bifilar wound 1:1 air solenoid type.
>
> J.
> VY1JA
>
>
>
>
>
>
>