vertical and horizontal induced measurements

[EngrN]

Hi,

I am trying to measure vertical and horizontal induced voltages of a monopole antenna made out of a measuring tape. Since the antenna is vertically polarized there should be at least 10dB difference but at times it is not - any advise what i might be doing wrong?

Thanks in advance

 

[FvM]

Please describe the antenna geometry including feed line. Without a large ground plane, the feed line will work as antenna and possible change the polarization.

 

[EngrN]

It is a monopole antenna made up of measuring tape with a pin at one end which is epoxied (conductive) to a sma connector feed by an RG58 cable through a signal generator. The antenna is mounted on a plastic housing and kept on a 1m diameter ground plane. I did get reflections from the wire - shielded it to by sticking copper sheets and tape to make it fix to the ground plane. This thing worked at some angles only.

But a concern - is width of the monopole effecting. Simulation results show around 20dB axial ratio which is quite sufficient.

 

[FvM]

"Shielding" won't stop the cable from acting as an antenna. Better use ferrite tubes or rings to isolate common mode currents.

 

[BMR]

Might be the plastic casing around the antenna. Did you try without any enclosures?

 

[volker@muehlhaus]

Are you measuring in the far field, i.e. some wavelength away from the antenna?
Are there reflections from walls etc than can affect polarization?

 

[EngrN]

Thanks for replying guys.
FvM: I do not have ferrite but can make arrangements to place the wires under the ground plane.

BMR: Unfortunately, the casing is required.

Volker: there are definitly reflections but I tested a wired monopole - the results were fine, so I am expecting atleast that accuracy.

I simulated the assembly with antenna on the ground and discovered that if the antenna ground and the casing ground are not at same potential than the ground of the casing starts radiating. I think this is probably whats going on. I am thinking of actually keeping the antenna in a normal vertical position with ground horizontally placed on the virtual ground. And rotate the measuring antenna (dipole) vertical and horizontal to measure the two induced voltages - as done in an anechoic chamber. Am I thinking correctly?

 

[EngrN]

Thank you everyone for your help and suggestions. So the problem got rectified using few precautions

1. Placing the feeding wire/line underneath the group plane.
2. Connecting the antenna ground to the measurement setup ground ( there was a large ground plane on which the antenna was placed)
3. Orienting the reference antenna instead of the Monopole for measuring the two induced voltages.

Hope this helps/guides some other in trouble.

Thanks!

 

[biff44]

you have a metal piece of tape...but what is the "ground plane" for your monopole antenna? The ground plane is probably your hand, the spring mechanism in the measuring tape, or like said above, the shield on the coax cable leading away from it.

If you really need crosspol rejection, try making the measuring tape into a center-fed dipole. That way nearby pieces of metal will NOT significantly impact the radiation patterns.

 

[EngrN]

Thanks for your comment Biff44 - unfortunately the antenna can not be modified. But the problem got sorted out.