Peter, I may be being obstinate but the answer to question one is "reduce
the transmitter power until the aerial current criterion is met". I think
most home built PAs will be capable of running more than the limit, but
going into too much detail may be a delusion....if you play around with the
supply voltage, for instance, the impedance, efficiency change, and
harmonic/spurious content may change.
On the second item send them details of your aerial by all means, but
indicate that as this is an experimental licence that may not remain the
prefered solution (particularly as you have a difficult site) and any new
arrangement will be assessed for its Rrad and the transmitter adjusted to
ensure that the radiation with the new arrangement stays within the permit
ERP limit. As full licencees we are supposed to be able to assess whether or
not we are operating within the terms and conditions of the licence/permit.
This is a basic tenet of Amateur Radio ! All I think you need to do is show
you understand how to do it, otherwise why are Ofcom moving the goalposts,
or are they too atuned to appliance operators! The alternative to this is
Ofcom flooded with requests to ammend height by 14.75 inches etc!! which is
patently silly.
In your case a plan view of the aerial will not tell them very much, because
the position with respect to the sloping ground will determine the effective
height, or Rrad. The ground quality wont matter as you will just use more
power to overcome the loss resistance. which is in series with the Rrad. The
only sensible check of whether you are getting right will be a field
strength measurement at about 1km. My guess because of the trees is that you
will be under the limit even when running a best-estimate aerial current.
The spreadsheet only "works" for an inverted L (not a V) over flat
unobstructed ground ....not a description of the average garden. One can
them make informed guesses as to what the effect of a sloping wire or
sloping ground might be. .... and then the aerial will not radiate like the
ideal specimen assumed in the ERP definition !! In general I think it is
safe to say the actual ERP will be less than the theoretical.
I feel the above may not be helpful. Give them what they ask for but no
more, otherwise you will be binding yourself to conditions you may not be
able to work under. I dont think it is necessary to tell them HOW you are
going to adjust the TX power, it depend on what rig you choose to use.
Cheers de Alan G3NYK
----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: 28 April 2007 08:45
Subject: LF: 500KHz NOV
> From G3JFS
>
> Hullo All,
>
> In my application for an NOV I indicated that erp would be assessed using
> the table published by Alan G3NYK and measuring the aerial current. I have
> received the following request from OFCOM:
>
>
> "Thank you for your recent application for a Special Research Permit to
> operate in the bands 501 -504kHz. Your application is currently being
processed
> pending authorisation.
> Whilst this is in motion I would just like to request further
information.
> i) Could you please provide a statement on how you
> propose to control your transmissions to within the maximum limit of
<-10dBW
> ii) Also please provide a plan view of the aerial
system
> along with complete details of wire lengths and heights. Also giving
> relationships to boundary fences, neighbouring buildings etc
> I appreciate that this was not an explicit requirement in the original
> application however we do still need this information."
> The site plan is no problem, just a hassle, though the RSGB guidelines
> indicated it was not necessary because of the distance between my aerial
and
> boundaries/neighbours.
> To avoid more delay any suggestions as to how best to respond? Also as a
> matter of interest what sort of figures are current users getting in the
way of
> efficiency, aerial current and transmitter power output to achieve 100mW
> erp? I shall be using a 120 ft end fed wire, 30 ft high on a steeply
sloping
> site surrounded on three sides by very tall trees over very poor (RF)
ground.
> 73 Peter G3JFS
>
>
>
>
>
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