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Re: LF: Re: Increased ERP on 137.5kHz - WSPR reports please

To: [email protected]
Subject: Re: LF: Re: Increased ERP on 137.5kHz - WSPR reports please
From: Roger Lapthorn <[email protected]>
Date: Tue, 11 Jan 2011 19:34:03 +0000
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The ferrite rod is tuned with a parallel capacitance of around 1400pF. The antenna is connected to the top of the tuned circuit and the antenna current meter is in series with the antenna. A 50 ohm tap point was found empirically low down the coil.

Roger G3XBM

On 11 January 2011 19:19, Andy Talbot <[email protected]> wrote:
If I understand corrrctly, you're measuring the current into the tap then?
In any case, I doubt a ferrite rod even at that power could be linear as a loading coil
For example ::
Using the approximation of 6pF /metre, antenna cap = 18pF.  Lets call it 25pF to allows for strays.  
That will need 53mH to resonate, and Xc / Xl =  j46k ohms.     
If you were to have 180mA flowing in the antenna, it would therefore have around 8kV on it. 
If we assume a ferrite rod 8mm diameter,  X sectional area = 50mm^2, and allow a Bmax of 0.2 Tesla
Using V = 4.44.F,N.A.B  it will need 270 turns to avoid saturation.   
Which I'm guessing it hasn't got.  If it has, it'll be very thin wire that can't take 8kV

On 11 January 2011 19:04, Roger Lapthorn <[email protected]> wrote:
My Marconi is just my 28MHz halo and its short coax feed (strapped) fed via an indoors ferrite rod inductor which is tuned and tapped. So most of the current is NOT flowing in the vertical section. I need to set to work on erecting a "real" vertical and a real matching network. There is no excuse.

The loop produces a signal that has been received at 148km (and quite consistently at 46km) despite the low ERP (recalculated as around 200uW based on the current flowing in the loop), so if I can get a more effective vertical, rather than my poor attempt today, there is hope.

Roger G3XBM

2011/1/11 Stefan Schäfer <[email protected]>


Maybe there was a kind of  "utilities assisted earth mode" on the loop at 3 km distance, as you call it? How high is the antenna and where is is placed? I hope its on the roof of the house :-) If its near a wet tree and just 3m long, it may be worse than the loop. How do you measure the resonance? If you measure the antenna current, how high is it?
I think you are on the right way to make your signal detectable in higher distances! Good luck!

73, Stefan/DK7FC

Am 11.01.2011 19:26, schrieb Roger Lapthorn:
So much for the theory then....

The signal level at G6ALB in the next village (3km although WSPR shows it as 0km) is some 10dB down on my loop antenna. Not sure if I got my sums wrong or there is some unexplained loss. The RX system at G6ALB is unchanged.

Roger G3XBM

On 11 January 2011 17:32, Roger Lapthorn <[email protected]> wrote:
Dear All,

Today I rigged up and matched a simple, small, Marconi vertical on 136kHz instead of the vertical loop. Based on measurements of antenna current and some reasonable assumptions of effective height, I now believe I have around 200uW ERP on the band - heck this is high power for me!

From now until late this evening I will run the WSPR beacon (136kHz USB dial) in the hope that a few more people will be able to copy the QRPp signal. Please have a go and let me have any reports.

73s and good luck

Roger G3XBM

G3XBM   GQRP 1678    ISWL G11088

G3XBM   GQRP 1678    ISWL G11088


G3XBM   GQRP 1678    ISWL G11088

G3XBM   GQRP 1678    ISWL G11088
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