Re: Allston-Brighton Free Radio

[dan.Strassberg@att.net]

I hate to rain on Steve Provizer's parade but I believe 
that he has some very strange substance in the pipe he's 
smoking. I just made a simple calculation that, if 
correct, shows that his 100 mW station on 1580 will be 
doing very well indeed to be heard as far as 1 km (about 
0.6 miles) on a _good_ radio (such as a GE Super Radio 
III).

Assuming that his makeshift transmitting antenna meets 
the FCC's minimum efficiency requirements for Class B AM 
stations (an extremely optimistic assumption for a 
rooftop long-wire antenna), Provizer's station will have 
an inverse-distance field of 189 microvolts/meter at 1 
km. The inverse-distance field is the field that would 
be achieved if the ground were perfectly conductive. At 
those short distances, salt water would be a reasonable 
approximation, but I don't know of the existence of any 
salt water near Brighton Center--unless its in the 
lobster tank of some restaurant.

Moreover, 189 microvolts/meter is a very weak signal. 
The FCC calls 500 microvolts/meter the limit of an AM 
station's primary daytime service area. In a low 
electrical-noise environment, a 500 microvolt/meter AM 
signal is clearly audible on a decent radio, but 
Brighton is not a low-noise environmant. Moreover, at 
1.5 miles, the signal would be only about 75 
microvolts/meter. At 2.5 miles, the signal would be less 
than 50 microvolts/meter. To receive a signal of that 
level, you need a communications receiver--or something 
close to it. And, again, those figures assume a perfect 
ground.

In an urban environment, the ground is not highly 
conductive. And it seems most unlikely that the 
transmitting antenna will come anywhere near the minimum 
efficiency for a Class B AM. Such an antenna would 
require a tower slightly over 100' high (somewhat 
shorter with top loading) and a ground system of 120 
radials extending about 150' in all directions from the 
antenna or an equivalent electrically resonant 
(counterpoise) ground. There are other unusual 
constructions, such as so-called Paran antennas, that 
provide the same efficiency with much less height (about 
40' at 1580 kHz) but, contrary to what the zealots 
claim, these structures do require good grounds to work 
properly.

> Just a reminder that Allston-Brighton Free Radio
> goes on the air tomorrow at noon (AM 1580). There was
> an article about it in yesterday's Globe. Their
> studios will be at 105 Brighton Ave. and the antenna
> will be on Braintree Street, a few blocks away; the
> anticipated range will be a mile and a half, but it
> may reach further on car radios.
> 
> The A-B Free Radio site is http://www.abfreeradio.org
> 
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