Hi Alan and Rik,
Thanks for the comments. I was hoping to take advantage of the
'footprint' effect by the longer top loading, but it seems
this may be pulling down Rrad. Perhaps some pruning may be
in order for a better erp.
Thanks for the model calculations Rik. The 50 uH is a very ballpark
figure; I'll see if I can get a better value. If Rrad ~1.5 ohm and
the current is about the same as that for a 50 ohm load, it seems
eta~1.5/50=3% which is not very flattering for the ground system,
is it? Perhaps extending the radial under the wire from 30 to
100m may be a simple way to get eta up. Putting more wire in the air
is asking for more trouble from Mother Nature.
The CW beacon is on tonigt, but I'll certainly QSX for anyone who
wants to TX.
73
Joe VO1NA
On Thu, 29 Jan 2015, Rik Strobbe wrote:
Hello Joe,
I just simulated the antenna with MMANA-GAL and it resulted in Z = 1.57-j185.5
Ohm.
So the antenna C would be about 1.8 nF.
However a 50 uH loading coil point to 2.2 nF antenna capacitance.
The extra antenna C may be caused by obstacles close to the antenna (buildings,
trees, ...).
If this is the case it will also lower the radiation resistance a bit, so 1.4
Ohm might be a good guess.
73, Rik ON7YD
________________________________
Van: [email protected] [[email protected]]
namens [email protected] [[email protected]]
Verzonden: donderdag 29 januari 2015 2:09
Aan: [email protected]
Onderwerp: LF: RL aerial
Dear Group,
Rik and Jean Pierre were asking about the new aerial here.
Jean-Pierre sent yesterday a series of captures from 2208-0730
showing much variation in the signal levels on 477.7 kHz.
They were quite strong between 0330 and 0500 though there
were other peaks and complete fades during the night.
Many thanks Jean-Pierre.
Here are the details about the "Rotated L" now on the air on 477.7 kHz.
Comments and critiques are most welcome.
The current was read with a clamp-on ammeter which was then
calibrated by measuring current through a 50 ohm load. The
wattmeter power was 70 watts but the scope was showing 200V p/p
which is 100 watts. Perhaps the wattmeter is reading low.
It had a nasty accident with a Decca, was repaired but not properly
recalibrated. Assuming the scope is the more accurate, the current
was I=P/V=100/71=1.4 A. Scaling the aerial current accordingly
yields 1.6 amps.
The RL extends from the feedpoint up 27 metres and slopes down to
12 metres over a 100m span. The mean height is about 18 metres.
It is fed with ~50 uH of tuning; no R matching was needed.
A 30m vertical has Rrad~1 ohm on 630 metres. The ON7YD page gives
an expression for Rrad for a horizontal top load, but the RL is
sloping this will reduce Rrad. Let's guess that it's 2 ohms
So... ERP=I^2R= 2.6 R or about 5 watts.
I'd best crank the variac down a few notches to keep it legal!
73
Joe VO1NA
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