Coil in dipole antenna

What is the lumped structure halfway along my GSM antenna? The antenna is approximately 25cm in length. One variation has a coil, or spiral, halfway along the length of the antenna. What is the purpose of the coil or spiral ?

Attachment gsm1 shows the spiral, attachment gsm2 shows the lumped structure.

How do these components affect tx and rx of the signal? What are the design criteria for placing these structures where they are on the antenna?

It is phasing coil to make collinear array

yes, it's to create a phased collinear antenna as AndreyG said

it's purpose/effect on TX and RX is to increase the gain of the antenna but still keep the antenna omni-directional


Dave

http://martybugs.net/wireless/collinear.cgi

Yes, it is a beauty!

Thank you for clearing that up. I have used the terminology to Google the topic to bits. WarDriving was another new word too

From your own experience ,and although the gain for the collinear is very good in the plane of reception, is there an advantage to using these antennas in a closed environment where the received signal can be very complex and not always ideal, e.g., an office space with a 3G signal leaking in from the exterior. The signal will be all over the place and noise will be high.

to bobilgner:
yes if you can live with increased physical dimensions.
When you say: "signal can be very complex" you assume multipath I believe. Collinear array by offering narrower main lobe reduces multipath to some degree. Hence your signal will be "less complex". To some degree - the difference with the dipole is not that dramatic.