434 MHz - Educate me on Dipole Shortening - Page 1
Forum section: electronics / rf-microwave

author: Benta
author_rank: Super Contributor
timestamp: September 13, 2024, 11:38:42 pm
content: 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.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 14, 2024, 12:26:01 am
content: Is the antenna for transmit & receive? or receive only?

author: Benta
author_rank: Super Contributor
timestamp: September 14, 2024, 12:31:26 am
content: Receive only. I'm just trying to get the best results in a limited space.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 14, 2024, 01:37:56 am
content: 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)HOWEVERThe "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.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 14, 2024, 03:35:28 am
content: 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.

author: RFDx
author_rank: Regular Contributor
timestamp: September 14, 2024, 11:15:11 pm
content: A half size folded dipole would fit in the available space. The resistive component of the antenna impedance would be similar to that of a full size dipole. The inductive component is easy to compensate with capacitors.

author: ftg
author_rank: Frequent Contributor
timestamp: September 16, 2024, 08:17:01 am
content: Before going to folded dipoles and matching transformers, I'd just try folding the tips of the dipole back with a decently large radius and measuring the result.

author: A.Z.
author_rank: Super Contributor
timestamp: September 16, 2024, 12:20:24 pm
content: shortening an antenna, by various means, results in a resonant antenna with reduced efficiency, that being said, if omnidirectional pattern isn't important, the so called "oblong loop" will offer more gain than the dipole and better performances; basically, calculate a full wave, divide it by 1/3 and 2/3, divide the results by 2 and you'll have the sizes for short and long sides of a rectangular looplong side vertical, short side horizontal, feedpoint at mid of bottom side, impedance 50 Ohm

author: A.Z.
author_rank: Super Contributor
timestamp: September 16, 2024, 01:46:23 pm
content: Just to be even more clear, the attached pic shows an "oblong loop" for 434MHz, the image also carries the sides length, but then it's easy to recalculate them, as I already wrote, gain is around 9dBi and impedance match to 50 Ohm is almost flat from 433 to 435 Mhz (and over), the size may possibly fit your need, the only "problem" is that such an antenna is bi-directional so it has to be aimed toward the desired signal source/destination

author: radiolistener
author_rank: Super Contributor
timestamp: September 17, 2024, 04:39:45 am
content: You can shorten antenna as much as you wish, just add proper matching circuit. But note that signal level depends on antenna dimensions and noise floor is fixed, so when you reduce antenna dimensions it leads to less sensitivity, because weak signals drops below noise floor.

author: A.Z.
author_rank: Super Contributor
timestamp: September 17, 2024, 01:33:49 pm
content: Another way to shorten a dipole is to set it up in "inverted U" configuration; pick a 1/2 wave horizontal dipole, take a single arm lenght, calculate 60% of it, that will be the "horizontal portion", the remaining 40% will drop down more or less vertically, gain will drop of about 1dB but the overall lenght will be reduced

author: LM21
author_rank: Regular Contributor
timestamp: September 17, 2024, 08:46:03 pm
content: We had ISM transmitters in mobile equipment. Transmitter had 1/4 lambda ground plane antennas. They were just a piece of wire stashed in a small box together with the rest of electronics. For receiver antenna we bought a VHF whip which we meant to shorten for UHF.Luckily we tested the system first, and it worked well. So, before you measure and tune anything, check if it works well enough without all fuss

author: Benta
author_rank: Super Contributor
timestamp: September 18, 2024, 11:14:24 pm
content: Thanks for your suggestions.My apologies for being silent for a couple of days, my HDD crashed and I've been picking up the pieces. Back online now.The "oblong loop" sounded interesting, and led me on to a simple loop antenna. Specifically a full-wavelength one.It would have a circumference of around 70 cm in my case, meaning a diameter of around 22 cm.That's perfect.I also read that the broadside gain is higher than a dipole, which is even better.I'll be looking into this in "Rothammel", but if anyone has further links to information I'd be grateful.Thanks.

author: A.Z.
author_rank: Super Contributor
timestamp: September 19, 2024, 06:59:31 am
content: A note, the rectangular loop I proposed presents a 50 Ohm impedance at the feedpoint, the circular one you're considering won't, so in this latter case you will need some impedance matching

author: Benta
author_rank: Super Contributor
timestamp: September 19, 2024, 09:37:32 pm
content: From what I've found, the circular type has 100 ohms feed impedance. I can work with that.thx.

author: A.Z.
author_rank: Super Contributor
timestamp: September 20, 2024, 07:25:40 am
content: Quote from: Benta on September 19, 2024, 09:37:32 pmYou may consider using an OTS 2:1 transformer like this, for examplehttps://www.coilcraft.com/en-us/products/transformers/wideband-rf-transformers/smt/wb_smt/wb2-1-2wsl/HTH