Antenna Reference / Loop Antenna Question

[fringe]

Hi guys. As a general question, I was wondering if anyone had located and good antenna theory/design references on the web. I've done a lot of googling, but *good* stuff can sometime be difficult to find.

On a more specific note, does anyone know why you would connect the feed line to a loop antenna mid-loop rather than at the gap?

i.e.:

|-*---|
|
|-* --|

Where the *'s are the feed line connection points.

Is this an effort at impedance matching? The coaxial feed line is connected directly to the antenna with no line balancing methods in place. The antenna is supposedly designed for the 400 MHz range.

 

[Comcokid]

The ends of a loop (the gap) or the ends of a dipole antenna are effectively a very high impedance. Some loop antenna diagrams show either a impedance matching or a movable tap method where the coax attaches - for impedance matching to the coax. I believe a good source for some information on this will be a ARRL Handbook, or a VHF/UHF experimenters book (Amateur radio books). Might be able to find a copy at the local public library.

 

[VEBill]

As mentioned by Comcokid, the coaxial cable can be attached where the impedance (the ratio of V/I) on the elements matches the cable.

This is a bit of a simplification because, in most cases, the two leads need to be spread apart to reach the two points, and this complicates things a bit.

There may also be some reactance that needs tuning out, and thus you'll often see a series capacitor of various constructions.

In addition to the ARRL books, there are also the RSGB antenna books. These are both national ham radio organizations, USA and UK respectively.

 

[fringe]

Thanks. I figured the placement was for impedance matching.

It still seems like it would be a good idea to connect the coax through a balun or other balancing method, though. VE1BLL is correct, the leads do need to be spread out in order to reach both sides of the loop and with nothing nothing in place to isolate the coax from the antenna the feed line essentially becomes part of the antenna.

 

[VEBill]

The topic of baluns and their necessity (or not) has been endlessly debated and discussed in the various amateur radio journals over the past decades. The informed consensus seems to be that, in most cases, baluns make no positive difference to the real world performance of antenna systems.

In short - coax cable is more balanced than it might appear.

The currents inside the coax are forced to be equal and opposite by physics. And so long as the coax in the immediate vicinity of the antenna is dressed away at right angles (symmetrically), then there should no reason for significant common mode currents to be imposed on the outside of the shield.

 

[biff44]

most of the stuff tends to be sub microwave, but this is a very good site:

http://www.antennex.com/

Lots of antenna design archives.