Electronics > RF, Microwave, Ham Radio

434 MHz - Educate me on Dipole Shortening

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Benta:
I'm looking at how to receive 434 MHz (70 cm) signals in the smallest space without significant loss of sensitivity.
A 1/2-wave dipole would be around 33 cm length, but I only have 25 cm of space.
An idea would be coil-shortening the dipole halves to around 2/3 length.
Ideal would be something like a 3 mm brass or alu half with a "cork-screw" section for the inductor.

However, I've no idea whether this is feasible, and the web pages I've found deal with 40 m or 80 m band antennas, which is no help.
Also, how much would this impair sensitivity?

Do anyone have ideas or knowledge about this?
Or references, web pages etc.?

TIA.

Andy Chee:
Is the antenna for transmit & receive? or receive only?

Benta:
Receive only. I'm just trying to get the best results in a limited space.

Andy Chee:
For receiving purposes, you don't need to be elaborate with your inductor.  Brass "corkscrew" is definitely overkill for receiving.  PCB mount inductors are more than adequate (e.g. the ones that look like fat green resistors)

HOWEVER

The "corkscrew" inductor is usually constructed like so, because it can be bent into the same material as the antenna.  Your decision will be whether to locate the inductor at the base, the middle, or at the end of the antenna.

Efficiency wise, you want the inductor at the ends of the dipole (though not at the extreme ends).  But this adds weight to the ends causing the antenna to sag and droop down, which may or may not be relevant to your situation if you can arrange alternative mechanical support.

A capacitance hat combined with inductor at the ends, will improve efficiency even more.  Again, the weight and geometry of the hat may preclude its mechanical inclusion in your design.

Inductor at the center/base of the dipole is the absolute worst for antenna efficiency, but is the most practical option due to mechanical simplicity.

Andy Chee:
Incidentally, you could try some antenna modelling yourself.

https://eznec.com/

Basically it's the RF antenna equivalent of LTSpice and other electronics simulation software.

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