Can I take apart this antenna?

[Joe Sar]

Hi,

I am taking apart a 900Mhz antenna whip so that it should not be so rigid.

I want to install it in a quadcopter and need to
flex it to install it alongside one of the legs.

The antenna (picture attached) consists of:

outer plastic tube
braided shield
inner plastic tube
center conductor

They have soldered the shield to a hollow rigid metal tube. I want to know if I can remove this metal tube, and have just a shield.

Since it is not connected to the center conductor it should not affect SWR, or is the larger amount of metal
acting as a ground plane of sorts, and I cannot remove it?

Thanks for your help.

 

[JimB]

I want to know if I can remove this metal tube, and have just a shield.

No, you cannot.

Since it is not connected to the center conductor it should not affect SWR, or is the larger amount of metal
acting as a ground plane of sorts,

Oh yes it will!
That form of antenna is known as a sleeved dipole.
The metal tube is effectively one of the legs of the dipole folded back around the sheath of the coax.

I have no time to discuss this just now, it is 02:00 and I have to be up at 06:00 to go and do a days work.

JimB

 

[Joe Sar]

Ok, so if instead of the tube, I peel back the sheath on itself to an exact equivalent length of the center conductor protrusion, I will have the exact same thing, and it will be flexible?

Thanks so much for your help.

 

[JimB]

OK, I have had a few thoughts about this antenna today and will just ramble on a bit, I hope this makes sense.

I want to install it in a quadcopter and need to
flex it to install it alongside one of the legs.

What is the quadcopter leg made from?
Metal? or plastic with a metal insert? If there is a significant amount of metal, that would most likely have an adverse effect on the antenna performance.

I am assuming that the quadcopter will be in the same attitude during normal operation, but its heading relative to the ground station can be anything from 0 through to 360 degrees, so ideally the antenna needs to be omni-directional in the horizontal plane.
Your cut-up sleeved dipole, attached to the leg will most likely be at some angle and there will be a deep null in the response off the end of the antenna.
This of course is all theoretical, but in practice you can get away with all kinds of theoretically bad things depending on the application.
So with that in mind, a few questions for you:

What will the antenna be used for?
Transmitting?
Receiving? or both?

What equipment connects to the antenna?

What range are you hoping for on the radio link?

JimB

 

[Joe Sar]

Hi Jim,

Firstly, thanks so much for your help.

The copter uses 2.4ghz for video, and 5.8ghz for control. The 900mhz module is an alternate module for control, which has more range and more features in its software and hardware implementation.

The quadcopter legs are plastic only.
However, 3 legs contain various things:
2 contain 5.8ghz control antennas so I definitely don't want to put another antenna there
1 contains a compass module - same story


Optimally, the antenna should go into the leg that does not contain anything, however this means I would need to run the coax a bit. I can only mount the transmitter in the back of the copter (as the camera is in the front), and the free leg is in the front.

Others who have done this modification have used the compass leg. They report that the range of this transmitter is amazing, and basically exceeds the limits of the on board battery. The antenna is used both for transmitting and receiving control signals. You use it to plan missions for search and rescue work, repetitive flights, etc.

They removed the tube without peeling back the sheath, and I now know this means they have removed half of the dipole. If they get such good range when they messed the antenna, then I am not so worried about the slight angular slant of the leg.

(FYI- One discussion is at: **broken link removed**
)

In practice, the null off the end affects you if you fly directly overhead, and generally you are quite low when directly overhead and this does not present any problems.

The issue basically is, that at 900Mhz the antennas are larger. The 900mhz transmitter is 1000mw, and I would also gain that there would be a little better vegetation penetration at this frequency.

My other choice is to mount an antenna at the top center of the quadcopter. The exact top center would not interfere with the propellers. This would mean mounting a sma female through connector at the top, and threading an adapter cable through the unit.

This would give me the additional flexibility of trying different antennas, like a short quarter wave stub when flying close, and for farther flights using a pinwheel, or larger whips like: https://www.l-com.com/wireless-antenna-900-mhz-5dbi-rubber-duck-antenna-rp-sma-plug-connector

The downside to putting it on top is some wind loading, but my larger concern is that the gps module is located along the inside top of the unit, and I am concerned that it might somehow be affected if there is an RF wire there and antenna on top. Some users have reported that if they put foil strips around the gps module they pick up more satellites. This leaves me to believe they did a poor job of shielding it from below, and I am thinking if this means I should not put anything above it either.

These are my thoughts, your comments and time are most appreciated.

Joe

 

[JimB]

OK, I understand what you are doing.
There are obviously going to be a lot of engineering compromises in a situation like this.

Would it be possible to bend the tube part of the antenna to fit the 'copter leg?
This would be the easiest thing to try, but maybe difficult to achieve without a proper tube bending tool. Even then the shape may have to change a bit as the antenna is fitted.

You say that others have simply removed the tube and had reasonable results. This sounds like a bad idea from an rf engineering point of view, it is just one of those things which you can get away with.

A few years ago I did some half hearted experiments in making a sleeved dipole at a much lower frequency (145MHz) and was not impressed with the results as measured using a fairly sophisticated antenna analyser and just gave up without trying it with a real transmitter/receiver (It was a half hearted experiment after all!).
What I did to make this sleeved dipole was roll the screen braid back over the outer plastic sheath of the coax cable, effectively creating other element of the dipole by exposing the center conductor.
You could try something like this, but it would probably be quite difficult with thin coax.

Alternatively, why not try removing the solid tube and replacing it with several thin wires of the same length?
I suggest that you try it with four wires.
Simple cheap and flexible.

Pictures of my half hearted sleeved dipole rescued from the junk pile.



JimB

 

[Joe Sar]

Thanks. It sounds like you are agreeing that it might be dangerous to put anything in the top center above the GPS module, and that I should rather put it on the leg.
Yes, I will try to bend the tube and install along the leg, or fold the braid back whichever is easier. In the interim, my son was trying to help remove some plastic and cut it.
I ordered 2 new antennas and will finish this project when I get the new antennas.
Thank you very much for your insight.

 

[Joe Sar]

Thanks for you help. I ended up wiring a jack on the side of the copter so I can have the flexibility of trying out different antennas, and am using a full antenna now. As far as the engineering point, one poster on another forum suggested that even if the folded tube is removed, a dipole pattern will be maintained as follows: The coax goes horizontal till a certain point. At the point you bend it to vertical, you leave half of the coax (1/4 wave) vertical with the braid, and half (1/4 wave) the radiating center conductor. He felt that even though the bottom quarter wave is the entire coax, it is sort of forced into being a dipole. I really don't think this is true, and I don't have a SWR meter at these frequencies. I opted to suffer the loss of another connector and keep the antenna intact.
Thanks so much for your help. This project worked out well for me, and I will post surely my next projects when I get to them. You have been a great help.

 

[JimB]

It is good to hear that you have had success, thanks for the feedback.

JimB