868 Mhz omnidirectional antenna design help please

[imk]

Hello
I am using a pair of ZULU radio serial modems to chat between a PC base station and a autonomous wheeled robot.
After much experimenting I have found a simple 8cm vertical whip made from a length of 1mm dia copper is working most reliably.
Problem is, I still get the odd drop out when the robot is driving by large metal objects like radiators etc.
Any ideas please for omnidirectional antenna design that is better than this simple whip design please?
Many thanks in advance imk
 

[metrologist]

So it seems you made a 1/4 wave radiator. You could look at 5/8 wave or a LoRa colinear.



Here's some's notes on various experiments: http://ph2lb.nl/20160914.pdf

This one seems neat: https://www.thethingsnetwork.org/forum/t/diy-external-antenna-for-gateway/3011

[TheMG]

The issue is likely due to multipath interference. This happens when a combination of the incident signal, and the same signal reflected from a nearby object out of phase, arrive at the receiving antenna. This causes localized signal fading when the signal arrives about 180 degrees out of phase, but also depending on the modulation/coding scheme used can also cause difficulties in decoding the received data.

It's for this reason that many modern devices, such as WiFi and cellular, incorporate multiple antennas to create diversity, as well as modulation and coding schemes that are more tolerant to this phenomenon.

You could switch to a directional antenna which would improve thing however since your antenna is on a moving object that is not practical. Does your quarter-wavelength antenna have an appropriate ground plane? The lack of a suitable ground plane can hurt the performance and result in rather unpredictable radioation pattern (in other words, your omnidirectional antenna can end up having undesired directionality!). An effective antenna can be made simply by folding back the shield of coax over itself the right amount, and using the center conductor as the radiating element. This creates a dipole antenna of sorts.  Lastly, if you have this radio module on your own PCB do make sure you have properly designed impedance-controlled traces from the module to the antenna.

If none of this helps you may have to look at a different radio transceiver, perhaps one more suitable for use on a radio controlled vehicle.

[imk]

Hello and many thanks for the replies.
Further reading and i am wondering if something like and upside down Groundplane would work.
Where i just had three or four radiators/legs coming from the antenna pad on the radio module?
imk 

[imk]

Learned a lot from this https://youtu.be/oiBi9RbNBUY
And explains why my rubber duck design was so poor :-(
Will be trying a 5/8 mono then maybe  a 1/4 ground wave.
Also one problem maybe the dupont  patch gnd/0v wire which loops close to the mono.. bit more experimenting to do..

[imk]

5/8 Monopole results very encouraging, thanks for the heads up..

[rfclown]

If you want dirt cheap and simple, use a monopole (piece of wire). It works. If I want dirt cheap and simple without a wire sticking up, I'll do an antenna structure on a PCB (squiggle of line or PIFA). If I want an omni that works well and isn't so susceptible to variation due to proximity to other objects (body, etc) I use a sleeve dipole like this:
 https://www.mouser.com/ProductDetail/Linx-Technologies/ANT-868-CW-HW-SMA?qs=PKuFCuYbGOemNMmf6j2o7g%3D%3D&mgh=1&gclid=EAIaIQobChMI7vuehvy_-AIV4w59Ch2lpwmJEAQYAiABEgLMvvD_BwE

it has a balun built into the structure, which makes it MUCH more stable. Wave your hand around a monopole while looking at it on a network analyzer compared to a sleeve dipole, and you'll wonder why they call a monopole an antenna. You can make a sleeve dipole out of coax, wire, tubing (brass, whatever), copper tape, whatever... but I usually just buy them. For an education, buy one and cut it open. They usually coil the top wire (one side of dipole) just to make it shorter. The other side of the dipole is a sleeve of metal over the coax insulation. There is another sleeve of metal connected to the coax at one end which serves as a balun. Clever design.

[imk]

If you want dirt cheap and simple, use a monopole (piece of wire). It works. If I want dirt cheap and simple without a wire sticking up, I'll do an antenna structure on a PCB (squiggle of line or PIFA). If I want an omni that works well and isn't so susceptible to variation due to proximity to other objects (body, etc) I use a sleeve dipole like this:
 https://www.mouser.com/ProductDetail/Linx-Technologies/ANT-868-CW-HW-SMA?qs=PKuFCuYbGOemNMmf6j2o7g%3D%3D&mgh=1&gclid=EAIaIQobChMI7vuehvy_-AIV4w59Ch2lpwmJEAQYAiABEgLMvvD_BwE

it has a balun built into the structure, which makes it MUCH more stable. Wave your hand around a monopole while looking at it on a network analyzer compared to a sleeve dipole, and you'll wonder why they call a monopole an antenna. You can make a sleeve dipole out of coax, wire, tubing (brass, whatever), copper tape, whatever... but I usually just buy them. For an education, buy one and cut it open. They usually coil the top wire (one side of dipole) just to make it shorter. The other side of the dipole is a sleeve of metal over the coax insulation. There is another sleeve of metal connected to the coax at one end which serves as a balun. Clever design.

Hello rfclown and many thanks for the post, question please.
As my monopole antenna  rises vertically from the PCB adjacent to the ZULU-2 radio modem i don't have any need for a length of coax.
So can i just basically put a  1/4 wave length of grounded tube over the current 1/2 wave whip to covert it to a sleeved dipole?
This arrangement looks in principal to be the same as the sleeved dipole images i have googled, also would ally tube work as i don't have any narrow copper tube?
Mant thanks imk

[rfclown]

Yes, you can put a 1/4 wave sleeve connected to the shield at the feed point, and have 1/4 wave of wire sticking up. The coax runs up the sleeve. Picture shows a bought one cut open. They coiled the top wire to make it shorter, and put a brass sleeve over the coax going the other way. I don't think the type of metal matters too much as long as it is a good conductor. I added the short piece of wire soldered to the top to center this one better at 868 since it was a 915 MHz antenna. You can buy either frequency, but I have a pile of 915 antennas (former employer dumped a load of them). The other picture is one that I made. I used brass and plastic tubing from a hobby store. Top tube connected to center wire of coax, middle tube connected to ground of coax at the feed point. The third tube is also 1/4 wave soldered at one end to the coax braid to act as a choke. Made it for a project that I never finished. You'll see people putting ferrite beads on the feeding coax to keep rf currents from running on the outside of the shield. The choke is for that same purpose.