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
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