Use of 4-Way Combiner in Receive Only antenna

[Sckotty-Boy]

Will the use of a 4-Way power combiner in a "receive only" Quad-Helix antenna degrad the antenna receive capabilities, due to the added insertion loss of the device (noise figure), or enhance its ability to receive, due to the phase gain provided by the combiner? Assume perfect phase alignment of the 4 helical antenna inputs to the combiner.

Thanks for you assistance, opinon's and help...:?:

 

[WimRFP]

Hello,

What is the noise temperature of the antenna alone (does it look into the sky, what is frequency, etc)?

When the antenn looks at the horizon, and has high radiation efficiency, the noise temperature will be more then 150k.

Adding 4 of them (no mutual interaction) will give 6 dB increase (theoretically). Assuming 1 dB loss gives 5 dB signal increase. Assuming room temperature for the combiner, overall noise temperature will be 270K (increase of 2.5 dB), so the overall S/N ratio (assuming good LNA), will increase by 2.5 dB.

when your LNA is not so good and noise temperature of one antenna is more then 150K, then the increase in S/N will be more (with te combiner).

 

[Sckotty-Boy]

I'm not sure what the noise temp of the antenna would be. The antenna has an average gain of about 15dB. The frequency band is approximately 200 to 500MHz and the amplifier has a noise figure of about 1dB.

I'm assuming from what you've stated that the gain achieved by the phase combining, to some degree offsets the noise attributable to the increase in insertion loss?

Thanks for the feedback.

 

[Filip]

It's not possible to have phase matched antenna system within 500-200=300MHz bandwidth in UHF. Some frequencies will be attenuated.

 

[biff44]

When you say quad-helix, are you talking about an array of 4 helix antennas, or a quadrifilar wound single helix antenna?

 

[WimRFP]

Hello,

The noise temperature of the antennas depends on the radiation efficiency and whether or not they look into space.

When you antenna looks into space (so doesn't "see") warm earth, the noise temperature can be significantly below ambient temperature (in particular for the high end of your frequency range). Your combiner and cables are at room temperature (I suppose). As soon as these introduce loss, they also add thermal noise.

When your antenna looks to the horizon (land based communication), you can use 150 K as first guess. When man-made noise dominates (so the equivalent noise temperature is far higher), then this noise is dominant and noise from combiners/cables/LNA will no longer be important. Increase in S/N ratio will be closer to 6dB – (combiner loss).

Important issue in case of ground-ground communication: Actual increase can change significantly when dominant part of man made noise is within or outside the main lobe of the 4 element array.

 

[Sckotty-Boy]

Hey biff44
It's an array of 4-helical antennas. I'm trying to determine the most efficient method (device) with which to combine these antenna. Any ideas?

Added after 7 minutes:

Hey biff44
It's an array of 4-helical antennas. I'm trying to determine the most efficient method (device) with which to combine these antenna. Any ideas?

 

[WimRFP]

Hello,

Besides the combiner, you also have to consider the distance between the antennas. As you said to have 15 dBi for a single antenna, you probably need more then 1 lambda distance between the individual antennas at the lowest frequency to get near 6 dB gain increase.

The half wave distance as mentioned all around is not sufficient when each individual antenna has significant gain.