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From: n7ws@azstarnet.com
Subject: re: Dayton Johnson's 1296 double rhombic.
Date: Sat, 30 Nov 1996 09:23:47 +0000
At 08:43 PM 11/29/96 -0600, you wrote: [snip]

>G/T is Gain over Temperature (ie, noise temperature). At HF, where external
>noise is the limiting factor, antenna temperature is not very important. At
>VHF, it becomes more important but at UHF and above, the noise temperature
>of an antenna, coax, power combiner, etc. start limiting the weakest signal
>you can hear regardless of the gain of the antenna. Noise Figure is really
>a spin off of Noise Temperature.

Are you saying that at some point, it is wasted effort to increase antenna gain because something else is noise limiting the system?

>The problem with a Rhombic for receiving is that since it uses a termination
>resistor, it will never be below about 300 degrees Kelvin (room
>temperature). When aimed at the horizon, this may be acceptable since you
>are seeing the earth anyway (which is 300K, of course), but when aimed at
>the moon, a satellite, etc., better performance is possible. The typical
>TVRO antenna, for example, has a Noise Temperature below 120 degrees Kelvin
>and many modern ones are 50-75K.
>Thus, to transmit, the Rhombic would have merit but for receiving it's gain
>wouldn't necessarily improve an incoming station's signal to noise ratio and
>that's what we're really after.
>Jim WB6JNN/9

Consider this... The terminating resistor can be considered a noise source at ambient temperature. As such, it has a calculatable noise power output. This power is applied to the antenna terminals and radiated in a direction toward the normal feed point. If the gain is high, much of the power is lost (radiated) before reaching the feed point. THe effective noise temperature is then reduced by the ratio of lost to generated power. Therefore, the noise temperature of the antenna is not quite as high as one might assume.

73, Wes -- N7WS

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