Need a highly directional SMALL antenna for 219MHz - suggestions? - Page 1
Forum section: electronics / projects

author: cvanc
author_rank: Frequent Contributor
timestamp: October 05, 2015, 10:17:22 am
content: Don't need a lot of gain, need small size and directionality. For mobile radio direction finding (RDF), on foot and under difficult terrain conditions. What I have now is both too large and too loose pattern-wise.Any and all suggestions welcome; thank you.

author: KJDS
author_rank: Super Contributor
timestamp: October 05, 2015, 10:40:38 am
content: A rubber duck with a slightly longer bar a quarter wave behind it should give a sharp null that you can use to set the direction

author: VK5RC
author_rank: Unknown
timestamp: October 05, 2015, 11:16:48 am
content: I would have a look at some of the Ham ARDF sites if you haven't alreadyThe classic yagi but built with steel measuring tapes so it is flexible and can be got through small spaceshttp://www.qsl.net/dj2qv/style/tapeyagi.pdfSmall loops, the Null is very sharphttp://www.dl3bc.de/ardf-enUse science and electrically rotate the "Antenna" (but you actually have 4) and use doppler shift to find the directionhttp://www.n5dux.com/ham/files/pdf/A%20Doppler%20Radio%20Direction%20Finder.pdfEdit ; found the doppler kit i have builthttp://www.ramseyelectronics.com/Ramsey-Doppler-Direction-Finder-Kit/dp/B0002T5SU6

author: GreyWoolfe
author_rank: Unknown
timestamp: October 05, 2015, 03:35:59 pm
content: The steel measuring tape yagi antennas are popular with the fox hunting crowd in my ham radio club. most of them use a 2 element beam so it is quite portable yet still gives decent front/back and side rejection. It isn't 9 dB of gain like the link VK5RC posted but it should work. When done, you can fold the elements in on themselves for easy transportation. A quick calculation has the reflector at 27.5", the driven element at 26" and the spacing is about 10.25. I don't know the dimensions of your antenna, but this seems reasonable

author: hagster
author_rank: Frequent Contributor
timestamp: October 05, 2015, 04:48:42 pm
content: Using the null as the DF vector is the way to go. Possibly with a small loop.You won't get anything with good directional gain with anything other than a large antenna though. Basically as your antenna shrinks towards a point source, it becomes more omni directional.

author: TheSteve
author_rank: Unknown
timestamp: October 05, 2015, 10:42:01 pm
content: 3 element tape measure yagi and build yourself an offset attenuator. The offset attenuator is basically an adjustable mixer. It will give you the attenuation needed to zero in on the transmitter when the signal is strong enough to swamp your receiver. I use this one here with great results -http://www.homingin.com/joek0ov/offatten.htmlI can find a hidden transmitter within a foot using the antenna and attenuator.

author: GreyWoolfe
author_rank: Unknown
timestamp: October 06, 2015, 12:37:20 pm
content: +1 for TheSteve. I'm not a fox hunter and forgot about the attenuator. I helped a fellow ham a few years ago build his own. It can be done on a very small pcb and integrated into a handle for the beam. His was powered by a 9V battery and basically lasted over a year before needing replacement.

author: @rt
author_rank: Super Contributor
timestamp: October 06, 2015, 02:23:54 pm
content: Hi, Since you know the frequency, you might be able to tune the nearest harmonic on the same radio,and the directional antenna become a different cut, and you receive an apparently weaker signal with no extra hardware.At least one step of attenuation for free.

author: TheSteve
author_rank: Unknown
timestamp: October 07, 2015, 06:38:13 am
content: If you do need a *really* small antenna you can use a loop antenna such as this one. It requires more skill to use and has no front to back ratio so if you have no idea of the direction you will need to use your body to shield the signal which takes practice. You also need an offset attenuator for sure. The antenna is non-resonant on purpose(and it needs to be like this). I have used this antenna on 2 meter(144 to 148 MHz) fox hunts and it actually works quite well.

author: rs20
author_rank: Super Contributor
timestamp: October 07, 2015, 06:47:11 am
content: Aren't "highly directional" and "small" directly conflicting requirements? Parabolic dishes, diffraction limiting, etc?

author: Howardlong
author_rank: Super Contributor
timestamp: October 07, 2015, 09:34:07 am
content: Quote from: rs20 on October 07, 2015, 06:47:11 amIndeed, that's why antenna nulls are frequently used for direction finding rather than maxima.

author: TheSteve
author_rank: Unknown
timestamp: October 07, 2015, 04:42:58 pm
content: Indeed - with a small loop antenna you are using the signal nulls for direction finding instead of the signal peaks.