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How does this antenna work?

 
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slk23
Antenna Theory Regular


Joined: 11 Sep 2013
Posts: 13

PostPosted: Mon Sep 16, 2013 4:30 pm    Post subject: How does this antenna work? Reply with quote

This antenna was provided to me for use with a 978 MHz receiver. Can anyone explain the purpose of the 'wires' and connections near the feed point? I assume it's a balun but I don't understand how it works. I'm also puzzled by the overall length: 8" compared to the 12" wavelength of 978 Mhz. However, it's 6" (a half wavelength) from the end of the short parallel stub to the end of the antenna so perhaps those are the relevant dimensions?

I field tested this antenna and it seems to work very well.

Front side:


Back side. What's the purpose of this L-shaped wire?
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Mon Sep 16, 2013 5:57 pm    Post subject: Reply with quote

Ok here's the deal - you want a half-wavelength total length for your antenna (or more). But a half-wavelength is the key.

in your topside view, the center pin is your feed. The two outer pins are ground, and the backside view everything is ground. Now at RF, signal and ground are two sides of the same coin - it is just the two connections from a coaxial cable (inner conductor and outer conductor).

From the top view, they take the signal wire, run it straight for a bit and then connect it straight back to ground in parallel with the antenna (branching to the left). This is a tuning mechanism - that represents a shunt inductance to the feed.

On the ground side, they have a weird copper trace that goes over the small arm on the top side. This is a shunt capacitance - again this is just for tuning and doesn't do much/anything for radiation.

The design is a bit overly complicated if it is just a single band antenna. Typically some of these weird designs are used if they are needing to get real creative for multiband antennas.

One more thing - the length of the cable that you plug into will have an impact on the antenna. What you have there is essentially just a dipole antenna - and the other have of the dipole is formed by the feed coaxial cable that plugs into it.
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slk23
Antenna Theory Regular


Joined: 11 Sep 2013
Posts: 13

PostPosted: Mon Sep 16, 2013 6:20 pm    Post subject: Reply with quote

Interesting.... I did some poking around on the web and this appears to be a J-Pole antenna. The overall length matches but the parallel wire is a bit shorter than 1/4 lambda on my antenna. And as you said the feedline is going to be part of the antenna since it lacks a balun.

It's likely that my antenna wasn't made specifically for 978 MHz. I suspect it was chosen from commercial sources as a 'good enough' choice for this frequency.

For comparison I built a coaxial dipole with 0.46 lambda length. It worked fine but the J-Pole seemed a bit better. I'm disappointed since I was hoping my DIY efforts would outperform something I could buy.
I wish I had access to equipment that would allow me to fine tune my dipole Sad

BTW, the application is an ADS-B receiver for aircraft use.

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