Very simple homemade outdoor 868Mhz antenna (groundplane)


#1

Hamradio is all about experimenting, so why not build another antenna ?

One of the most common antenna's is the ground plane antenna which is basicly a 1/4 lambda driver element and 3 or 4 reflector elements (creating a artificial ground). It's a simple antenna to build with a good bandwidth, it doesn't have any gain over a dipole because it's dipole. Ok for the record it has a +2.15dBi gain (so you 3$ ebay 5 dBi antenna only has a 2.85dB gain over a dipole) :wink:

More info at : Electronic Design

The design

Note : length adjusted for best SWR (see SWR measurement below).


Materials used

aprx 50cm of 2mm diameter brass or copper wire cut to 5 pieces of 10 cm.
4 x cable ends M4 / Blue
1 x N-style chassis part
4 x M4 10mm cylinder screws
4 x M4 lock nut.

Step 1

Remove the blue plastic parts.


Step 2

Solder the cable ends to one end of the tubes.


Step 3

Solder the tube to the center pin of the chassis part.


Step 4

Mount the ground plane tubes to the chassis part with the M4 10mm screws and the M4 lock nut. And trim the tubes to the correct length (see : design).


Step 5

Bend the ground plane wires to a 45 degr. angle.


You're almost done. When you want to use the groundplane antenna outdoor. Please apply a bit of silicone sealing around the top of the chassis part (so no water is collected on top).

SWR measurment result

After the inital building I took the antenna to my local Hamradio club (Veron/VRZA Twente) to have it tested with a RigExpert AA-600 (which when connected to the computer can work up to 1.4Ghz) from a fellow Ham. Some minor tweaking (I made it for a lower frequency so I could trim it for the 868Mhz) the SWR dip was right on the 868Mhz.


Installing

This groundplane antenna can be easily installed on a 32 mm PVC tube.The connected cable will pull the groundplane antenna to his place.


At this moment I can't tell anything about the true signal response. I have to clime the roof to swap the antenna's.

Note : I have written a LAB about it which you can find it on this link.


The BIG and SMALL ANTENNA topic part 1
The BIG and SMALL ANTENNA topic part 2
Gateway queries & ranges
#2

Excelent tutorial, thanks for sharing!


#3

5 pices cut and all too short :wink: :grin:


#4

found some cheap parts :smile:

but the copper rods are 3 mm .. do you have to recalculate then ?

€1.79 5 pcs Diameter 3mm Pure Copper Cu Metal Rod Cylinder Length 100mm
$ 1.05 20pcs Ring Ground Insulated Wire Connector Electrical Crimp Terminal 14-16AWG M6
$1.46 N type female jack RF COAX connector 4-hole panel mount with solder cup chassis

finishing touch ? :

fyi the dimension are exactly the same as the Aurel GP 868 - so they work


#5

@ursm :blush: you got me there. Indeed they should be longer aprx 10 cm.

@BoRRoZ The 3mm rods shouldn't be a big problem. It probably will be a bit more broadband, but they won't fit in the insulated wire terminals. But you could bend loops on the rods for mounting.
Please don't use SO239 UHF chassis parts but use a N style chassis part (better for the 868Mhz).


#6

according to this list 16 AWG = 3 mm so they should fit

and off course you are right, n connectors are better for higher frequencies.


#7

@BoRRoZ Hmmm I thought that 14 AWG aprx 1.6 mm diameter. I think the 3.5 mm in the chart is the outer diameter of the wire (incl insulation).

When I look at : http://www.engineeringtoolbox.com/awg-wire-gauge-d_731.html

AWG 14 =
Diameter (inches) 0.0641
Diameter (mm) 1.63
Cross Sectional Area (mm2) 2.08

So when you look for 3 mm diameter you would have to look for AWG 8 (3.26 mm diameter for the wire).


#8

well.. I think you're right again and the Chinese is wrong :laughing:

bought some wire the other day and this list is indeed different


#9

hi @lex_ph2lb :
1.) do you think this SMA could also work? With them it would be possible to use an u.fl cable to connect it.
2.) would you recommend that for a node too or only for a gateway?
thanks


#10

@ursm : SMA is very good for high frequencies. And when you want to make a GP to directly attach to a node (without cable) that chassis part (male) is perfect. But when you want to extend it with a cable you need a Male / Female SMA cable. If you have them it's no problem, but normally female SMA chassis part are used so a male / male cable can be used.

It will be small and a bit fragile but good enough for experiments. Remember that the drawing is based on a larger N style connector so the ground plane wires should be aprx 8 mm longer when using the SMA style chassis part.

The antenna can be used for a node and also for the gateway. But when the node is portable I wouldn't recommend it (the wires can bend easily changing the resonance point). For stationary nodes it could work.


#11


N chasis female vs SMA chasis


#12

@lex_ph2lb Thanks for your explanation in detail. :smiley:
yes, I plan testing the antenna for the tth_zh heartbeat, which should be received by many gateways in town.
The I prefer the male SMA because I've u.fl to female RP SMA which I use to connect to the (male) antenna
My problem at the monet is, that I couldn't find cable ends with holes of 2.5/3mm; so I've probably to solder the ground plate wires directly on the SMA.


#13

@BoRRoZ Indeed size does matter :smile:

@ursm Soldering direct to the SMA is not a problem. But try not to melt the dielectric.


#14

a while back I bought this set, thinking off converting it to 868 gp

but I thought the connector is pl259.. which it isn't :wink:
but the quality is ok.. maybe you have an idea ?


center is 4 mm


#15

First try on a sim of a 868Mhz GP antenne with 4NEC2.
http://www.ph2lb.nl/blog/index.php?page=simulation

Think I have to read the manual first :wink:


#17

I have tried to simulated a 868Mhz groundplane in freespace, and 0.25m, 0.5m, 1m and 5m above average ground.

The results and the NEC files can be found on my 868 Mhz groundplane simulation page : http://www.ph2lb.nl/blog/index.php?page=simulation

GP in Free space

GP 0.25m above ground

GP 0.5m above ground

GP 1m above ground

GP 5m above ground

Conclusion : GP preform better above ground and height does matter. :wink:

Fancy 3D pictures on my page.


Antenna length for 868 and 433 MHz
(Andrea-Patrizia) #18

This is my Antenne.

1/4 Lambda 900Mhz GSM Cellular Car Antenna.


#19

Thank you for the design! Here's my version, let's see how it does.


#20

Looks good. Keep us posted on you're experience with it.


#21

Results are very encouraging. I stuck it to the drain pipe with three ground planes instead of four and two meters of coax cable to a single channel (Jaap Braam repo) wemos. Before the antenna was a vertical 868MHz whip behind the glass of the window to the right.
The range seems to have doubled but needs more testing :slight_smile:
https://ttnmapper.org/?gateway=A020A6F42F21FAC2&type=radar