[Lf] Higher L - higher ERP]

[Andre' Kesteloot]

Brian Rogerson wrote:

> Hi Rik,
>
> I have been following the folklore around antennas since I joined the
> reflector and have many printed out and and in a folder.  I am currently
> trying to make sense of my own situation.
>
> My antenna is a conventional Marconi T, very similar to the ones I see
> at the Portuguese Naval Radio station which I pass quite frequently.
> With 23m overall height, the top is an 80m folded dipole and the vertical
> is a 300ohm quad transmission line, all constructed in 2.5mm aluminium
> wire.  I think you wrote a little note on alumium wire.  Unfortunately I
> have many trees and the vertical was shaddowed closely up to about 20m
> with separation of only 2m in places.  For 136 all four wires of the feeder
> are shorted together and the system is resonated by by a coil with spaced
> turns also in aluminium wire.
>
> Under these conditions I measured the feed point resistance in the region
> of 140 to 150 ohms using a  noise bridge and managed about 0.8-0.9A aerial
> current using the appropriate tapping on the matching transformer.  The
> output of the tx was 200V peak to peak or about 100W.  Ignoring cable
> losses R=100/(0.85)^2=138ohms and there is rough confirmation between
> the measurements.  I have had the trees reduced in height.  The new
> measurement for feedpoint resistance is around 70ohms with a very
> wide flat minima on the noise bridge.  The tx is now one module of the
> Decca with 54v ht and 8A or 432W input when transmitting.  I measured
> 400W into a 50ohm dummy load so efficiency is 92.6%!  The best match
> on the matching transformer is between 40 and 50ohms.  Aerial current
> is now 3A so R=400/(3)^2=44ohm and there is a big difference between
> noise bridge and power measurement.  I cannot think why.  Of course
> when I was carrying out the last measurements everything was soaking as
> it had been raining for months and maybe this is the answer.  I shall have
> to repeat the measurements when everything has thoroughly dried out.
>
> This may give you another set of data for European soil and of course
> any comments would be gratefully received.
>
> 73, Brian
>
> At 09:20 23/02/01, you wrote:
> >Hi John,
> >
> >I believe that the 'basic rule' with an elevated loading coil is that the
> >current remains constant from the bottom end (feeding point) up to the coil
> >and will drop linear from the coil toward the end of the antenna.
> >So if you have the coil at the top of the vertical section you will get a
> >constant current over this section, the same that you would get with a
> >infinite topload (that is Andy's theory in other words ..).
> >
> >But I am 'triggered' by something else :
> >
> >>I base this on measurements under our US no-license regulations at 1750
> >>meters.  One of my last attempts at operation from this qth, surrounded by
> >>trees, resulted in 200ma at the base and roughly 150 to 170ma entering the
> >>top hat with base tuning only.  (I say "roughly" because I could only get
> >>myself to about half the height of the vertical run and had to sight
> through
> >>binoculars.  Also, my notes have long since gone missing, so I'm having to
> >>depend a lot on my write-only memory.  This was all with a vertical run
> of a
> >>little over 8m and a top radius a little less than 7m.)
> >
> >At a rather poor location (trees) you manage to get 0.2A with 1W input,
> >what means a loss resistance of less than 25 Ohm.
> >When I read the results similar of other lowfers (antenna currents up to
> >0.4A, what would mean a loss of only 6 Ohm) I am always suprised by the low
> >losses they achieve.
> >>From what I know from European stations on 136kHz the losses go from +/- 30
> >Ohm (those with a close-to-perfect location) to 150 Ohm at a poor location.
> >
> >So either :
> >1. ground losses are very frequency dependent
> >2. American soil is so much better (for LF) than European soil
> >3. the conversion rate from American Watt to European Watt is not 1/1
> >
> >Assuming that :
> >1. most lowfers stick to the 1 Watt input rule
> >2. soil can be different, but not over an entire continent
> >I believe that ground loss must be very frequency dependent
> >
> >73, Rik  ON7YD
> >
> >
> >
> >
> 73 Brian     CT1DRP     IN51QD     41 09 58N  08 39 11W
> http://homepage.esoterica.pt/~brian