[BC] Antenna Question

[Richard Fry]

The basic antenna referred to in Paul's original post is a "j-pole," which 
is popular with amateur radio operators.  While the overall length is about 
3/4-lambda, almost all of the radiation is produced by the uppermost 
1/2-wave section.  The lower, 1/4-wave "J" section is used for matching the 
transmission line to the antenna feedpoint.  And unless some means are used 
to prevent it, r-f current will flow down the outside of the outer 
conductor of the coax transmission line, which will change the overall 
pattern/gain of the antenna.

NEC analysis of this design shows a peak, free-space gain varying between 2 
dBi and 2.5 dBi, depending on azimuth.  A 2-section, collinear j-pole form 
would have at most another 3 dB of gain, for a maximum of 5.5 dBi.

So, regardless of whether or not the FCC permits them for licensed FM 
broadcast use, it would be good for potential users to ask the manufacturer 
for complete, factual data, including measured patterns -- especially when 
a gain of 8.15 dBi is claimed, such as for the Phazor®.

A claim of unrealistically high antenna gain will result in stations 
running less ERP than they are licensed for.  In the case here their ERP 
would be about 54% of their licensed value.

It could even be possible that stations bought a transmitter that isn't 
capable of the power output needed to _really_ produce their licensed ERP, 
expecting that the gain of their antenna was as high as advertised.

RF