Identifying the antennas

[bugsbunnyboss]

Is it possible to look at the antenna and tell if its a dipole or monopole antenna? If so,how?Pls see the attached pic and answer.

 

[AndreyG]

The one on the top is probably helix.

The middle antenna is what sometimes called 'sleeve dipole': coaxial cable in the middle, center conductor extends up (thin top part of the antenna) and forms one side of the dipole. The braid soldered to section of tube, forming second electrode. the thicker short section at the bottom, right before the connector is likely ferrite choke.

The bottom antenna is most likely a coil loaded whip. (The bottom thick section is a coil)

Strictly speaking 'monopole' is artificial construct: all the antennas are dipoles in a sense they all have two electrodes.

Structurally antenna element can be single conductor. Then the second part of the antenna is somewhere below the point where this antenna element is connected. It can be conductive case of the device, buried radials or even conductive peg hammered into ground. ut there is always second element.

 

[bugsbunnyboss]

From the reply I take it that these antennas can be used directly without any necessity for the ground plane. Please correct me if I'm wrong.

Also when I measured the swr reading for these antennas, the bottom antenna's reading was fine. But the other two antenna's swr reading was more than 5 for its specified frequency (Frequency specified by the manufacturer). When I made a ground plane (pasted copper sheet in a cardboard and mounted the antennas on top of it), the swr reading started improving, and with more copper sheet, I was able to bring the swr to less than 2. Can someone explain what happened and why it happened?

 

[AndreyG]

>> From the reply I take it that these antennas can be used directly without any necessity for the ground plane.

What made you think that? And how you can imagine 'without the ground plane'. Say you connect device to the antenna on your picture. The outside of the connector is touching other conductive objects. All these objects comprise your ground plane.

This is conceptual level. In practice you just want to make sure your antenna has low return loss so energy goes into the antenna and radiates.
Measuring any of the antennas you show on the end of Network Analyzer cable is poor practice.
Simulate dimensions of the device you plan using this antenna to. Decouple the Network Analyzer cable past the extend of our device. Then you get measurements close to what you deice will see.

 

[bugsbunnyboss]

''Strictly speaking 'monopole' is artificial construct: all the antennas are dipoles in a sense they all have two electrodes.''
Reading this, I misunderstood that the two electrodes will be in the antenna itself and thats why I thought there was no need for an external ground plane.
I am a beginner, so please excuse me for all the silly questions!

What made you think that? And how you can imagine 'without the ground plane'. Say you connect device to the antenna on your picture. The outside of the connector is touching other conductive objects. All these objects comprise your ground plane.
Do you say that the connector touching other conductive objects can substitute the ground plane? or should I make an external ground plane?
I want to connect these antennas in a RC plane.If I have to make an external ground plane, what should be its dimension?

Measuring any of the antennas you show on the end of Network Analyzer cable is poor practice.
Is it a bad practice to connect the antenna to a swr meter and check for its working frequency? Is there any other way to check for the antenna's working frequency?

Also what about the last question about the ground plane that I asked. Is it a correct way to make a ground plane?

 

[AndreyG]

And your RC plane is non conductive?
I recommend then adding ground plane.
How big is your radio? If your radio has large enough metal case this can be your ground plane. In this case radio should be very close to the antenna. Think hand-held radio - antenna mounted right on top of it.

As for the size of ground plane, think how big is your dipole.
Your dipole (= half wavelength) will be approximately 35 cm long.
Say you use your top antenna on the picture I posted. I would make your ground plane as wide as you can and about 20 - 25 cm long.
I would recommend it to be 20 x 20 cm but this probably will not fit in your plane.
You can use foil to make ground plane and protrude connector through it.
Foil will make it conformal with the surface of your plane.

So you make the ground plane, install connector, mount antenna and connect your SWR meter to the connector. Right inside your plane. Then you get realistic reading.
Next, you adjust size of your ground plane to tune your antenna system to required frequency.
The shorter the ground plane the higher the frequency.

This should give you efficient radiator.

 

[bugsbunnyboss]

Yes! The body is non conductive.
The radio is very small, less than 10 cm. and it is in the center of the plane. So I cannot mount the antenna on top of it. I have used a cable from the transmitter to the outside of the plane, then connected the antenna there.

The foil is a good idea! I'll try using it.