Date: Thu, 1 Nov 2007 14:32:41 +0000
From: John GM4SLV <gm4slv@sighthound.demon.co.uk>
To: rsgb_lf_group@blacksheep.org
Message-ID: <20071101143241.7af5f29e@saluki.ngw.uk.com>
In-Reply-To: <000d01c81c8e$85911230$0d00000a@AGB>
References: <001c01c81c0b$ef981a80$2201a8c0@PC2>
	<E1InMCb-0006LO-Ae@post.thorcom.com>
	<47297997.1208.1CCA4C@dave.davesergeant.com>
	<20071101082740.18172494@lurcher.twatt.local>
	<000d01c81c8e$85911230$0d00000a@AGB>
Mime-Version: 1.0
Subject: Re: LF: RE: 500 kHz report /  Great expectations
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


On Thu, 1 Nov 2007 13:53:02 -0000
"Graham" <g8fzk@g8fzk.fsnet.co.uk> wrote:

> 
> >>>>.....  No-one will hear you if you can't radiate a
> >>>>signal! .....<<<<

> 'Steady on old boy', J ,   On the contrary, we can all radiate
> signals. its just that the receiving stations are in the wrong
> place's ,


Hello Graham, LF,

Well...yes I suppose we can all radiate something! A VFO connected to a
piece of wet string will radiate. I've just turned on an old AR88D in
my office (under the desk - keeps the feet warm) and to find the start
of 40m on its old dial I turned on a sig. gen in the workshop, stuck
2" of solder in the o/p connector, returned to the office, found my
marker signal....

However...on 500kHz the receiving stations are where they are! If you
want to work what you hear you must approach reciprosity of ERP,
considerations of local noise floors aside.

I'm sure you understans that I meant :

"No-one will hear you if you can't radiate a signal... of sufficient
strength to reach the desired location"

My point was that Dave has said more than once that he struggles to be
heard, despite receiving other more distant stations with seeming ease.

The reason for the disparity is that he isn't radiating enough ERP to
make his signal heard at distance. Given that Dave's chosen mode is
CW then we can discount the fancy DSP datamodes and notions of negative
S/N ratios.

If his (and mine too) ground losses are high then to get enough current
flowing in the antenna he'll need more TX power, not a new house! 


My point was not to say "we need more ERP to make this work" (although
I happen to think that we do....) it was to say "if your antenna system
is inefficient you can still achieve the necessary, modest, ERP  by
compensating with more TX power". 

We shouldn't be afraid of the engineering needed to generate highish
powers, nor feel that it's against the spirit of QRP to do so.

As you and everyone else will know, it's simple to get "lots" of power
on 500kHz. By "lots" I mean > 100W, which is "lots" to QRPers like Dave
and myself.

My TX uses 3 active devices (discounting the external VFO). A divider
to get 500kHz from 8MHz VFO, a MOSFET gate drive chip (TC4427 ?) and a
single IRF150. At 30V DC supply to the FET it produces 140W. This is
adequate to get 100mW ERP (just) from a simple 10m vertical wire. With
more capacity/height I can happily turn down the wick. 

I can get my 100mW quota with my current antenna with about 25W TX
power but I have power in hand if necessary - eg alternative antennas
or if (fingers crossed) we get a power increase from OFCOM.


There doesn't seem much point in complaining that you can't get out
when the answer is easy enough, and shouldn't involve the drastic
step of moving house!

Cheers all,

John