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Author Topic: Settle a Playful Disagreement  (Read 1192 times)
KI6DYR
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Posts: 227




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« on: January 29, 2010, 04:49:38 PM »

I won't say which side I am on of this in case I am wrong <grin>.

The premise: a 1/2 wave coaxial antenna for 2 meters has unity gain and a donut-like pattern. More or less.

We know that to achive gain we modify that pattern generally towards the horizon.

Now then, if we place a 6dB preamp on a unity gain antenna without modifying its pattern can we now say that the antenna "system" has 6dB gain? Nothing about increasing the noise floor. Just the basic question. I almost left out "system" and that would change the question because we would still have a unity gain "antenna." Yousse guys are sticklers for minute detail.

This is a 12 pack of Bud question.
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WB6BYU
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Posts: 13040




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« Reply #1 on: January 29, 2010, 05:22:28 PM »

Most people would not consider the preamp to be part of the antenna system.
It is part of the receive system.  An antenna is reciprocal - it works the same
on transmit and receive.  If you try transmitting into your antenna with sufficient
power it will no longer work on receive either.
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WB2WIK
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Posts: 20547




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« Reply #2 on: January 29, 2010, 06:08:35 PM »

I'd say "absolutely not."

Unless a VHF receiver has a high noise figure and the preamp has a lower noise figure and sufficient gain to overcome the difference, preamps do absolutely "nothing" to improve station performance (even on receive).

On 2 meters, ambient (atmospheric) noise in most locations is high enough that a 3-4 dB noise figure is sufficient; if this is the case, adding a 6 dB preamp, or a 60 dB preamp, with a lower noise figure does absolutely zero to improve sensitivity.

When low noise preamps are most helpful is usually when using highly directional antennas that can be aimed away from noise sources, such as when working e.m.e. and aiming into deep space away from galactic and sun noise.
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KL7AJ
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Posts: 329


WWW

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« Reply #3 on: March 09, 2010, 10:24:07 AM »

Hi Steve:

  I'm SO glad you mention the ambient noise issue on 2 meters.  So many current hams assume the ambient noise on 2 meters is nil....they've obviously never operated 2 meter A.M.!  All us old geezers remember when the two meter horizontal halo was king...because it was horizontally polarized and more immune to noise.  In fact, I still have an ancient 2 meter "big wheel".  I should mount that  on my Escape just for effect. Smiley

Eric
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WM9V
Member

Posts: 106




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« Reply #4 on: March 28, 2010, 11:33:06 PM »

there is some insertion loss with line Receive preamps
so it could be said that some power is lost going thru the line amp
that would necessarily indicate that whatever gain
the antenna is spec'ed for would be less due to the insertion loss
better make that a 30 pack and a bottle of Jack
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AA4PB
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Posts: 12700




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« Reply #5 on: March 29, 2010, 05:39:37 AM »

"there is some insertion loss with line Receive preamps"

A poor preamp indeed. The idea of most preamps is to provide gain, not loss.

The preamp does not change the gain of the antenna. You can't ignore noise level either. Provided that your receiver has a decent front end, the only benefit of a receive preamp is to place it at the antenna to provide enough gain to make up for feed line loss. You raise the level of the signal and the noise leaving the antenna so that when they are attenuated by the feed line loss you wind up with the same signal and noise level at the input to the receiver as you have at the antenna.
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WM9V
Member

Posts: 106




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« Reply #6 on: April 07, 2010, 11:56:38 PM »

the perception that transmit and receive are one and the same systems cannot be overcome until one realizes that
most antenna systems are designed to be efficient radiators and are passive receivers
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KI6DYR
Member

Posts: 227




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« Reply #7 on: April 08, 2010, 01:01:56 AM »

Although it is true that an antenna needs to be an efficient radiator, it cannot be said that the same antenna is not an efficient receiver. We get gain from an antenna from length and narrowing its aperature. I can prove it every day up here at 6,3400 feet working on 146.52 using a unity gain stick, a 6dB stick and a 9dB stick. I can carry on three separate conversations at once utilizing the antenna necessary for the job.
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AA4PB
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Posts: 12700




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« Reply #8 on: April 08, 2010, 12:25:50 PM »

Antennas are reciprical, meaning that they have the same efficiency, pattern and gain on receive as they do on transmit. On receive (as opposed to transmit) however, the important thing is signal to noise ratio. It is quite possible for a less efficient or lower gain antenna to produce a better signal to noise ratio than a more efficient or higher gain antenna. All it takes is for the antenna to attenuate the noise more than it does the signal. An example of that is an antenna whose pattern is directed away from the noise source.

You find that in use most often on 160M where a station uses a low wire antenna for receive and a good vertical for transmit. The received signal level is less on the low wire than the vertical, but the noise is reduced more than the signal so the signal to noise ratio improves.
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NI3S
Member

Posts: 67




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« Reply #9 on: April 09, 2010, 05:07:22 PM »

"I can prove it every day up here at 6,3400 feet"

Air a tad thin?  

It's been my history that preamps are only useful on directional antennas, and are used to drive the feedline on receive.
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KI6DYR
Member

Posts: 227




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« Reply #10 on: April 09, 2010, 05:15:05 PM »

Thin air has nothing to do with it. The statement made before mine was that antennas are designed to be efficient radiators. Regardless of a preamp I simply made the statement that antennas designed for gain are also efficient receivers. From this altitude I can easily demonstrate how gain (without a preamp) exists.
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N8EKT
Member

Posts: 371




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« Reply #11 on: April 11, 2010, 06:35:27 PM »

Unfortuately, if it were that simple, we would all be using 1/4 wave groundplanes with 30db preamps instead of using a dish to pick up our tv reception. LOL
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N8EKT
Member

Posts: 371




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« Reply #12 on: April 11, 2010, 06:36:53 PM »

PS
Did this question come up BEFORE or AFTER the 12 pack!

LOL!
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KI6DYR
Member

Posts: 227




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« Reply #13 on: April 11, 2010, 06:41:15 PM »

DURING the 12 pack Smiley I came somewhere after the "discussion" that humidity needs to be 100% for it to rain but maybe the rain never hits the ground.

For the record, my position was that real gain can only be achieved by narrowing the aperature of the receive antenna.
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