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 =
596m, radiation resistance Rrad will be 0.285ohms.
-To achieve 0.1W ERP, Iant = sqrt(Perp/1.8xRrad) = 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.
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: [email protected]
[mailto:[email protected]] On Behalf Of Dave G3WCB
Sent: 21 February 2007 17:14
To: [email protected]
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: [email protected]
[mailto:[email protected]]On Behalf Of
[email protected]
Sent: 21 February 2007 13:17
To: [email protected]; [email protected]
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" <[email protected]>
> 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
>
>
>
|
