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Monopole-type antenna

 
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wolberine
Antenna Theory Regular


Joined: 01 Mar 2011
Posts: 35

PostPosted: Fri Jul 08, 2011 2:12 pm    Post subject: Monopole-type antenna Reply with quote

I've been having issues with radiating efficiency while prototyping a long wire/monopole antenna in a compact form at 434MHz. I've got about 7.5'' of wire wrapped in a U-shape horizontally about .5'' above a PCB. Looking down on the board, the feed starts on the right side of the board, halfway down from the top, and the antenna continues down, around a battery, and up the other side to the top of the plastics. The two corners of the U are right-angled turns.

I'm used to living with a link budget with inefficient antennas ~-9dBd. However, this antenna is around -14dBd. I can't get decent gain until I extend the wire completely straight.

I've done this type of antenna before in a circular shape and gotten results of ~-8dBd.

One interesting clue is the match is a bit funny. When matched, the --6dB RL is ~50MHz. The impedance looks like the left half of the number "8", which I've never seen before at 434MHz. Typically I get nice round circles.

Does anyone have any tips for improving the radiating efficiency? i.e. Should I get higher off the board? Maybe start the feed low and continue the rest of the antenna steadily higher? I'm really limited in my space because of the plastic casing form factor.
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Schubert
Antenna Wizard


Joined: 08 Apr 2009
Posts: 161

PostPosted: Sat Jul 09, 2011 6:48 pm    Post subject: Reply with quote

When you say " When matched, the --6dB RL is ~50MHz.", what do you mean? The antenna is resonate at 50MHz? Not sure I understand.

It's hard to know exactly what's going on without a picture. But the only advice I can offer is that if you have a small volume (relative to a wavelength), maximize the volume of the antenna in any way possible. So yes, getting higher off the board should help, as well as increasing lengths where you can. Matching only removes mismatch loss; hence if you put in lossy components it is possible that the matching won't help much. It depends what the before and after VSWR is.
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