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Message-ID: <5A39905CAF744124A2560F59835F8821@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <C10E0C39C85E4C0583562B56DB0056DE@JimPC>
In-Reply-To: <C10E0C39C85E4C0583562B56DB0056DE@JimPC>
Date: Tue, 9 Jun 2009 23:20:42 +0100
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DomainKey-Status: good (testing) 
Subject: LF: Re: M0BMU small antenna test beacon
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Dear Dave, Ken, Dave, Rik, Pete, LF Group,

Thanks for the reports on the beacon signal. If nothing else, it is at least 
a viable 500kHz TX antenna - as well as the reports, it is showing up 
reasonably well on the G3YXM and G4WGT grabbers. I have now closed down at 
2206utc.

The TX power was about 70W maximum, dropping to about 0.3W at the minimum 
power level. I haven't  done detailed measurements yet, but the loss 
resistance is about 70 - 80 ohms, and the antenna current just under an amp 
at maximum power. My "guesstimate" of ERP is 100mW at maximum power, down to 
0.4mW at the minimum level - I am allowing -6dB for the fact that the 
antenna is completely screened by the surrounding trees. The loss resistance 
is about 3 x that of my normal antenna, which has much more wire. The high 
resistance isn't a great suprise, as the ends of the top load wires are only 
a couple of metres from the trees and bushes. From previous experience, in 
an actual open /P location the loss resistance should be much lower, and the 
ERP significantly higher for a given antenna current.

The TX signal is a .wav file of CW and QRSS recorded from the "digimodes 
terminal" in Spectrum Lab, and played back through the linear transverter. 
The software generates a smoothly rounded raised-cosine keying envelope 
which minimises the bandwidth and results in the "soft" keying. The 5s 
dashes have a proportionally slower rise/fall - I left it with the default 
settings to see if anyone would notice, I think on balance a faster rising, 
sharper keying envelope would be better for a listening test. The variable 
power level was obtained by editing the audio level in the .wav file using 
some sound editing software.

As Rik observed, I did briefly operate last night as well - power was 
slightly lower then.

Cheers, Jim Moritz
73 de M0BMU