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From: "John W Gould" <jwgould@iee.org.uk>
To: <rsgb_lf_group@blacksheep.org>
Date: Thu, 22 Feb 2007 13:36:43 -0000
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Subject: RE: LF: Special Research Permits (501-504kHz) for UK Amateurs
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Status: O

I should have added that it might be reasonable to add a separate sheet =
with
the issue about field-strength, assumptions and how you arrived at the
figure in the RFI assessment box

73 John, G3WKL

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of John W Gould
Sent: 22 February 2007 13:21
To: rsgb_lf_group@blacksheep.org
Subject: RE: LF: Special Research Permits (501-504kHz) for UK Amateurs


Jim, thanks for raising this issue. =20

The SRP is a generic form that often is related to "experiments" with =
people
asking for permission to run high power (for EME, MS, HF, etc), where =
the
field-strength would be more meaningful.   I will probably be vetting =
the
applications obo RSGB, so I suggest that one keeps it short and simple;
mention where applicable that the neighbour(s) are in the near zone and
summarise the calculations.  It might be worth stating that this would =
be an
absolute maximum figure as you may experiment with erp less that the =
-10dBW
maximum stipulated.

73 John, G3WKL

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of james moritz
Sent: 22 February 2007 12:18
To: rsgb_lf_group@blacksheep.org
Subject: RE: LF: Special Research Permits (501-504kHz) for UK Amateurs


Dear Dave, LF Group,

I don't believe it is possible to give an accurate value for field =
strength
at points close to the antenna - it will depend on too many variable
factors, such as the position, geometry, conductivity and dielectric
properties of all nearby objects. One can come up with a reasonable sort =
of
"worst-case" figure, however; this is my attempt:

-Assume a long-wire antenna with an effective height of 8m. With lambda =
=3D
596m, radiation resistance Rrad will be 0.285ohms.=20

-To achieve 0.1W ERP, Iant =3D sqrt(Perp/1.8xRrad) =3D 0.44A
(I am assuming directional gain of 2.6dB, i.e. 1.8 for a short vertical
antenna over a dipole)

-The antenna voltage depends on the current and the impedance of the
antenna. Most of the impedance is due to the capacitive reactance of the
antenna. The measured antenna capacitance is about 340pF, giving a =
reactance
of 931ohms at 503kHz. Therefore the antenna voltage will be about 411V.

(Also, the loss resistance of this antenna at 500kHz is around 20ohms; =
the
TX power required to obtain 0.44A therefore works out to be about 4W)

-If as a worst case we assume the antenna and ground acts as a sort of
parallel plate capacitor with the physical height of 9m being the =
spacing
between the two plates, the E field strength will be about 46V/m under =
the
antenna.=20

In reality, the field gradient will be higher close to the antenna wire =
than
it is at ground level, so the field strength will be lower than this at
ground level. Also, at positions not directly under the antenna the =
field
will be further reduced, and trees, buildings, bushes, cats etc. in the
vicinity will have a partial screening effect that will also reduce the
field strength. So I think 46V/m is a very conservative maximum figure =
for
this antenna. A bigger antenna will have lower reactance, require less
current due to higher Rrad, and be further away from potential =
"victims", so
would result in lower FS. The converse applies to small antennas.

As you can see, the above figures are pretty tiddly compared to what we =
are
used to at 136k, or at HF for that matter. At my QTH, the FS due to the
nearby broadcast transmitters is around 20V/m. Does anyone know offhand =
what
Ofcom might regard as an unacceptable field strength?

I guess one reason Ofcom might be nervous about permitting higher ERPs =
is
the possible effect on MF broadcast receivers with both RF and IF
frequencies fairly close to the band. But this does not seem to be a =
huge
problem with 160m, or MF beacons. Another possibility might be the =
Navtex
marine safety broadcasts on 490 and 518kHz, whose receivers might get
swamped - but I would think the likelihood of this would be pretty low =
also.

BTW - what are the correct emission codes for modes like PSK31 or Wolf, =
and
FSK / mFSK signals like DFCW and Jason?

Cheers, Jim Moritz
73 de M0BMU


-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Dave G3WCB
Sent: 21 February 2007 17:14
To: rsgb_lf_group@blacksheep.org
Cc: John W Gould
Subject: RE: LF: Special Research Permits (501-504kHz) for UK Amateurs

The first UK applicant to get a 500kHz SRP has to buy the beers. (Won't =
be
me...I haven't yet figured out how to measure the field strength inside =
my
neighbour's siting-room, will it interfere with his toaster, etc)

73, Dave G3WCB IO91RM

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org]On Behalf Of =
k2ors@comcast.net
Sent: 21 February 2007 13:17
To: rsgb_lf_group@blacksheep.org; rsgb_lf_group@blacksheep.org
Cc: John W Gould
Subject: Re: LF: Special Research Permits (501-504kHz) for UK Amateurs


Hi John and the list,

     Congratulations on your new experimental allocation!   I see that =
Ofcom
is restricting your experiment to very low power levels:

"In general, considerably lower e.r.p. levels than that permitted in the =
136
kHz band are likely to be favourable and this will only be permitted =
where
it can be demonstrated that increased powers are necessary for research. =
On
no account will an e.r.p. level of greater than - 10 dBW be permitted =
under
any circumstance."

  This will pretty much preclude transatlantic work, except possibly for
QRS, which is a pity.  I wonder what it is that Ofcom is worried about
interfering with?  The U.S. experimental stations are limited to 20 =
watts
erp, which is 23dB more than Ofcom is allow U.K. amateurs.


--
73 Warren K2ORS/WD2XGJ/WD2XSH/23
FN42hi
http://www.w4dex.com/wd2xgj.htm

 -------------- Original message ----------------------
From: "John W Gould" <jwgould@iee.org.uk>
> Ofcom has announced today that a limited number of SRPs will me made
> available for 12 months from 1st March. See
>
>
http://www.ofcom.org.uk/radiocomms/ifi/licensing/classes/amateur/Notices/=
200
> 70221
>
> and the RSGB and Spectrum Forum websites.
>
> 73 John, G3WKL
> RSGB HF Manager
>
>
>