## Where do the antenna actually begin?

Discuss commercial and home made antennas.
JEL
Posts: 182
Joined: Wed Feb 07, 2018 2:08 am

### Where do the antenna actually begin?

Hey all

Do I measure the antenna-length (The brown wire) from point A or point B ?

Point A [Correction: it is point B] is where the wire is connected to the connector.

Point B [Correction: it is point A] is where the connector's 'shield' ends.

I ask because I am not sure if/how the shield interferes with antenna resonance.

The antenna I'm trying to make here is an ADS-B antenna for 1090 MHz, so the length of the connector's 'shield' is significant if it has an impact on the wire's resonance.

I hope my question makes sense
Thank you.
jacob.
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Last edited by JEL on Tue Feb 27, 2018 4:10 am, edited 1 time in total.

Aussie Susan
Posts: 51
Joined: Sun Jul 31, 2016 1:55 am

### Re: Where do the antenna actually begin?

Depending on the frequency, I would say that it probably makes no difference.
Given the frequency you can work out the wavelength (I'd multiply it by something like 0.8 as this is wire and not free space). If the wavelength is more than a cm or two then the result will be the same no matter where you measure it.
If that does matter then I'd go for point B.
What do you mean by the shield interfering with the antenna's resonance?
What is the underlying problem you are trying to sort out?
Susan

JEL
Posts: 182
Joined: Wed Feb 07, 2018 2:08 am

### Re: Where do the antenna actually begin?

Hi Susan. Thank you very much for your reply

I asked the same question on the 'radioreference' forum and got this reply:
(Note that I mixed up point A and B, which is reflected in this answer, so when he says A he means B and vice-versa)
The antenna would typically start below point A where the metal screw holder exits the white plastic. However, you have bent the antenna wire down parallel and close to some grounded metal and under a clamp and that will act as a transmission line to some extent.

So its impossible to say where your antenna starts without using some test equipment to measure its resonant frequency. At 1090MHz even small changes in length of a few mm will change the resonant frequency. The screw at point A is large enough to change the resonant frequency because its part of the antenna.

Leaving the wire insulation on or taking it off will change the resonant frequency a little, so without a way of measuring the resonant frequency you will never know. If your antenan was for a much lower frequency like 120MHz then these problems would have much less effect.
prcguy
So it seems like it's not quite a trivial matter to resolve

dlritter
Posts: 46
Joined: Mon Nov 23, 2015 1:08 am

### Re: Where do the antenna actually begin?

and for the sake of discussion, I will say the antenna starts from the ground plane reference which in this case is the metal fixture holding the insulator. However, at 1090 that hole is so big the rf probably does not know it's there.
Just make a 1090 co-linear from rg6 and you will have gain to spare.
N6DL

Aussie Susan
Posts: 51
Joined: Sun Jul 31, 2016 1:55 am

### Re: Where do the antenna actually begin?

Another answer could be that neither of these are where the antenna begins because you are using shielded wire and to it is only carrying the signal to where the antenna really is!
And yes I know that the cable does have an effect.
However at 1.06GHz, the wavelength is about 270mm in free space and so something a bit over 220mm in the cable.
While prcguy is right that a 1mm change would make about 1% difference, the real question is: would that make a significant difference in your application?
Susan VK3ANZ

JEL
Posts: 182
Joined: Wed Feb 07, 2018 2:08 am

### Re: Where do the antenna actually begin?

I ditched the monopole-design I was attempting here and went for a simple dipole instead.
Less rabbit-holes to fall into

Sitting where it sits in the picture (Lowest floor in an urban apartment, with 'Low-E' windows, so really sub-optimal conditions) I get a max range of about 220 km (137 US miles, 120 nautical miles) at 40,000 feet altitude (~12 km)

It's very directional though, but I guess the wire-mesh rack (Causing some Yagi-like effect perhaps?) it sits on is the cause of that (I tried it in the window, but had much worse reception there)

I will attempt a collinear at some point and see how that goes.
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snn47
Posts: 239
Joined: Tue Dec 27, 2016 11:00 pm

### antenna begins at the feedpoint = point B directly at the coax

The antenna begins at the feedpoint = point B directly at the coax in your case.

The length of the connector's 'shield' is insignificant and has no impact on the wire's resonance if it has the length of 1/4 and is either mounted on a metal shield as ground plane/counterpoise or you can use 3 or more wires, also with 1/4 wavelength length. The actual length will vary with wire diameter and objects close by that incluence the resonance. You can find more info in previous posts.

If the antenna is not resonant it is not matched to the coax, and then the coax will become part of the radiating structure.
To match it antenna and cable impedance have to match therefore the cable should be 50 ohm. However since you don't want to transmitt also 60 to 70 ohm coax works, but it will not provide a perfect match.

And no the antenna does not "starts from the ground plane reference which in this case is the metal fixture holding the insulator. " it starts for a monopole like this at the feedpoint = end of the transmission line, here a coax cable, if the antenna is matched.

Any length of wire connected to the receiver will provide a signal, as any antenna will. Unless you follow the correct measurements length/diameter of a reference design by someone who know RF-design, you will never even can come close to the theoretical gain most poster claim their design will have. A wet noodle cooked in very salty water or a glas tube filled with the correct length of salt water can be more efficient. than most of those posts claim to have.

A 1/4 or quarterwave antenna is therefore imho the safest way to built yourself a working antenna, that proides some gain.

@Jel The reflection from the rack and mesh should shield the antenna and provide directivity. Also the distance to the rack and reflecting surface will influence your antenna pattern. Which program/website did you use for LOS diagramms?The wir

JEL
Posts: 182
Joined: Wed Feb 07, 2018 2:08 am

### Re: Where do the antenna actually begin?

@snn47;

Thank you for a lot of good info there

I am still experimenting with various designs and positions and cables and combinations.
Takes a lot of time, but more knowledge is gained after each step

I used "Virtual Radar Server" (Reading the SDR's feed via dump1090. No internet-connection, but just the local reception)