LPDA Performance Measurement - Confusing Results

[Paul F]

I bought a Betagear LPDA (470-950MHz). I made measurements to compare it's performance with the 1/4 wave whip. With the 1/4 wave whip I had a reading of -38db. With the LPDA I had a reading of -14.5db. The delta seems too large. Is this what is expected?

 

Test setup

- Indoors, shop. Normally, I would do this outside, but it was raining.

- Wireless transmitter with whip antenna suspended at 56"

- LPDA antenna at 76" held up by a metal post with a 14" Delrin extension

- Whip at same height, hand held with spectrum analyzer

- Transmitter/Antenna separation - 11'

- Transmitter transmitting @574.450

- Transmitter antenna length - 10.66cm (manufacturer's antenna)

 

1/4 wave test

- 1/4 wave whip connected directly to an Anritsu MS2711D

 

LPDA test

- LPDA connected to a Bias-t. Bias-t connected to Anritsu.

- Bias power on

- LPDA amplifier switch set to 0db (no active amplification)

[LarryF]

You are correct in that the delta is too large. You should see an LPDA improvement of only 4 to 5 dB. To correctly measure an antenna's gain, you need an RF anechoic chamber ($100k+) or an outdoor site far from any reflecting objects with a large metallic ground plane in or on the ground. The antenna is placed on a wooden pole with a rope so it can be moved up and down. The antenna is moved up and down so the reflection from the ground plane is in phase and maximises the signal. You are always going to have some reflection from the ground  (unless you have an anechoic chamber) so you simply maximise the received signal with a perfect ground plane and then subtract 3 dB.

 

Your measurements are compromised by many reflections either adding to the signal level of the LPDA or more likely canceling out the signal at the 1/4 wave antenna. Basically you have a "dropout" at one antenna. Trying to measure a 4 dB delta indoors is impossible.

Best Regards,

Larry Fisher 

[Paul F]

Thanks Larry.  I'll give it another go outside. I live in a rural residential area so I can get at least 100' away from buildings. The spectrum in my area is pretty quiet. Hey, maybe I'll set up an Part 15 test site! ... no.

 

If nothing else then, it is demonstrating the importance of antenna placement. This result makes me want to test more indoors just to learn how an actor's movements might affect the signal.

[LarryF]

2 hours ago, Paul F said:

Thanks Larry.  I'll give it another go outside. I live in a rural residential area so I can get at least 100' away from buildings. The spectrum in my area is pretty quiet. Hey, maybe I'll set up an Part 15 test site! ... no.

 

If nothing else then, it is demonstrating the importance of antenna placement. This result makes me want to test more indoors just to learn how an actor's movements might affect the signal.

You probably can do the equivalent of moving the receiving antenna, as described above, by moving the transmitter up and down until you get a peak RF level. Again, the reason for looking for a peak is to insure you haven't created an interference at the receiving antenna, i.e., a dropout. I would think you could get a reasonable comparison between the quarter wave and the LPDA. Also, the quarter wave antenna needs a ground plane to work against, such as a receiver housing (not a good ground plane but better than nothing). It sounds like you just have it at the end of cable. Any bags of water (people) that are close to the antennas will change the results also. 

Best Regards,

Larry Fisher 

[Paul F]

Thanks for the tips. The 1/4 wave is connected directly to the spectrum analyzer, which I'm assuming has some sort of metal frame inside.

[LarryF]

1 hour ago, Paul F said:

Thanks for the tips. The 1/4 wave is connected directly to the spectrum analyzer, which I'm assuming has some sort of metal frame inside.

That should imitate a receiver case.

LEF