----- Original Message -----
From: "Mike McCarthy, W1NR" <lists@w1nr.net>
To: <amps@contesting.com>
Sent: Wednesday, April 06, 2011 8:42 AM
Subject: Re: [Amps] Where is power measured ?
> On 04/06/2011 08:38 AM, Roger D Johnson wrote:
>> On 4/6/2011 8:11 AM, Jim Thomson wrote:
>>> Someone mentioned that in the US, the power is measured at the back of
>>> the amp.
>>> Your rules don't stipulate where it's actually measured. Dave Leeson,
>>> and other's,
>>> contacted FCC officials, and the FCC said it was reasonable to measure
>>> the power at the
>>> load, IE: ant feed-point. US broadcast TX are measured at the
>>> feed-point, or close to it, not the
>>> output of the amp at base of tower.
>>>
>> FCC rules 97.313
>>
>> (b) No station may transmit with a transmitter power exceeding 1.5 kW PEP
>>
>> 73, Roger
>>
>>
>
> Broadcast stations power in many cases, especially digital TV, by ERP
> rather than TX power.
>
> I knew a QRP'er who had 5 watt amps at the antenna feedpoints to
> maximize ERP and eliminate feedline loss.
>
> Mike, W1NR
I believe that the 1500W into the feedline is in lieu of the alternative of
measuring at the antenna. Its been awhile since I dug thru the regulations
but my recollection is that it is buried elsewhere in the fine print. Now if
youre running 5KW on 75 into 300' of hardline the losses are inconsequential
and it is seriously illegal power. OTOH with RG-213 running about 1.2dB/100'
on 10M it doesnt take much to lose 50% of the power which is also reciprocal
in the receiving direction. SWR loss is also added in when applicable.
The LF and MF experimenters bands are expressed in ERP and antenna
efficiency calculations are considered OK as there is free software
available.
Broadcast stations power is based upon their license specifications and
field strength measurements are taken at the time of certification. The FCC
also requires regular field strength measurements to certify that you are
both still maintaing the power level and that nulls are as required in
directional arrays. Running a too low a power level due to deteriorated
radial fields or feedlines is just as illegal as running too much power.
Carl
KM1H
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