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Received power vs frequency graph of horn antenna.

Received power vs frequency graph of horn antenna.

Received power vs frequency graph of horn antenna.

(OP)
Hello Sir,

I would be very grateful if  engineers could  offer any explanations as to why I have found that the received_power versus frequency graph of  “standard gain horns” is very jagged indeed. –It is not anything like a straight line (not even a curved line). –Rather, this graph is extremely “zig-zagging” (without graphical means at my disposal here, may I be forgiven for describing it like a graph of a volatile stock-exchange on a nervy day). –There are deep downward spikes where the received_power suddenly goes down as much as 15dB.

My test set-up involves two identical rectangular horn antennas (designed for 2 to 4 GHz). One horn transmits a known power to the other which acts as a receiver.  The horns have apertures of about 20cm by 30cm. They are 4 metres apart and  aimed to point in each others boresight (i.e. they are ‘peaked’). The frequency of the transmit horn is swept from 2 to 4 GHz.

The horns are placed about 2 metres above grassy ground. Could it be possible that the “dips” in received_power are caused by ground reflections? –I would not expect ground reflection to give such sharp dips in received_power –since the ground reflected ray has travelled further than the direct one (some 40% further) and would also surely have suffered some ground_attenuation and also some change in polarisation. Also, the ground_reflected_ray is surely approaching the aperture of the receive horn reasonably far out of its boresight (and indeed was emitted from the transmit antenna in a direction which is quite far out of boresight) and so would surely not be high-powered enough to cause such dips in received power(?)

Is it possible that the dips are caused by some resonant effect at the frequencies at which the many “dips” occur?

Alternatively, do you know of any other reason for the very jagged appearance of the received_power versus frequency line? I have found that in my test set-up, this “jagged” effect occurs with all the other horns that I use too.

RE: Received power vs frequency graph of horn antenna.

Simplest explanation is reflections. Compare the relative path lengths (direct versus simple one-bounce reflection) against the frequency dependence. Simple math/geometry can confirm or refute the reflection theory.

In other words, examine the two paths, predict the frequency delta where it would go in and out of phase. Compare to the graph. It either matches or not even close.

RE: Received power vs frequency graph of horn antenna.

PS: I'd use MS-Excel - maybe ten or twenty minutes work for the first cut analysis. Nothing but first principles.

PS2: There was a discussion here earlier about how to reduce reflections.

RE: Received power vs frequency graph of horn antenna.

phased,
the dips are likely caused by the ground although smaller dips are probably VSWR interactions between your cable and antenna and other components.

A simple formula whereby one signal interferes with another from a ground bounce or similar is the distance between the direct ray and reflected ray is d"=6"/[frequency (Ghz) from peak to peak or null to null].

Hence I would guess your ripple measurments are maybe 6"/72" or every 80 Mhz, maybe a bit finer.
What is your ripple spacing?

Your antenna -3 dB beamwidth is 40 degrees at 2 ghz and 20 degrees at 4 Ghz, your reflection from the ground is at 45 degrees off boresight. That should make your ripple fairly low, one to two dB.

Suggestions, point your antennas up 10 degrees in the air and see the result, if the ripple drops alot, you know it's the ground.

kch

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