The project requires me to make a monopole antenna for the HC12 Module set to a 435.8Mhz Frequency using an UFL to SMA connector. Calculating the Quarter-Wave Antenna for 435.8 MHz gives a 17.19cm lenght.
If I am correct, I need to cut off the insulation and its braid of 17.19cm in lenght, and I need to expose the dielectric insulator and center core outside of the casing? However, the Module is deep inside of the project's casing, specifically 9.8cm deep.
My question is, do I need exactly the UFL to SMA connector cut down to 17.19cm and its center core exposed? Or what can I do is to buy a 30cm lenght UFL to SMA connector, leave 9.8cm inside, and expose 17.19cm outside? I am inexperienced in making antennas in general, I do not know much about coaxial cables.
I couldn't find any sources showcasing how it works, but here are similar projects i found
The attached image is also the UFL to SMA Connector that I bought
The radiating part is the center of the coax with NO shield around it. So make the coax as long as you need and then strip off the shield braid to make the radiating part with the given length.
I had a similar problem with an ESP-07 device (which has a small coax connector for the WiFi antenna).
I found this YouTube video that helped me along and now I can build the needed antennas quite qickly.
My exposed antenna length is 29 mm for the WiFi band so in your case adjust according to the frequency you use.
Maybe this can help even if the frequency is a bit lower than the 2.4 GHz of WiFi.
Note that the video describes how one should pull back the braid over the cable insulation to make it a dipole.
Also:
I use shrink tubing over the antenna to protect it from being misformed.
A straight piece of wire soldered to the connector center pin will work fine. The length should be a little over 17 cm, but the length is not as critical as you think.
The dipole made from coaxial cable will work better (higher antenna gain), but the total length must be at least 34 cm.
Most all low cost radio modules do not use precision components in the frequency dependent matching circuits between the RF IC and the antenna.
Thus you will find, if you try it, that trimming the length of the antenna away from the theoretical 1/4 wave can improve the transmitted power output, of an individual module, by 3dBm or so.
So do not worry that the antenna length is not an exact 1\4 wave.
Okay! Most of what we have read here will apply to a receiving antenna. A transmitting antenna is different because it is the load applied to the output of the transmitter final amplifier. In order to accept the full available output power from the transmitter, the antenna must very closely match the transmitter output impedance. The antenna is a series resonant LC circuit! It needs to have wire length (L) and capacitance to its environment (C) so it is resonant at the frequency fed to it. The higher the frequency, the more critical the length of the wire. [But, it is necessary to experiment to find that length because one does not know the capacitance that it presents.] If the antenna is not well designed, it will not accept all the energy fed to it and that energy will have to be dissipated in the transmitter and that could cause early failure of the output amplifier. Also, a 1/4 wave wire is only half of the antenna and it needs a ground plane to work against. [Think of the 1/4 wave whip on a vehicle where the vehicle body is used as the ground plane or a radio station antenna that uses a system of radial wires for the ground plane.]
Agreed that impedance matching on an RX antenna may well be less critical since in general low cost radio modules do not seem to use significant impedance matching components on the RX antenna path.
But if a module is used for RX then somewhere there is another matching module that is is the TX.
True, but you won't find that matching in low cost hobby radio modules, so don't worry about it. As @srnet pointed out, experimentally trim the antenna to get slightly better performance.
My favourite DIY aerial is a sleeve aerial.
Strip the outer plastic about 25cm and push/fold back the braid over the remaining cable. Then cut the inner part to ~17cm and the braid to~17cm.
Now you have a proper dipole of 34cm.
Finish off with 35cm of heatshrink, to keep it all in place.
Leo..
The one I am making is the transmitting antenna. The receiving antenna I made is a Yagi Antenna, with a straight dipole and a 50ohm coax (I think it is RG58). It is hooked up to my laptop using an UFL to SMA connector.
The connection is as follows
Yagi Antenna >>> SMA to IPX >>>>> HC12's IPX Slot >>>>> USB to TTL >>> Laptop
I tested the Yagi Antenna (Receiver) with the HC12's helical antenna that came with it (Transmitter), at our longest street of where I live (400m). I was successfully receiving data, and I feel confident that I couldve received even further if it wasn't for our street's length.
However, during the testing, there were times that the data would cut off. There were a lot of trees and cars on the street. Maybe it was also due to my arm tiring out carrying the Yagi and the Yagi pointing away from the direction of the transmitter.
I was wondering if making a monopole antenna would solve these issues.
Is the 17cm braid also outside of the casing alongside the 17cm inner part? If so, the SMA to UFL I bought is too short (30cm), which I wont have enough room for the braid.
If not, from what I understand, I must heatshrink the folded braid part?
Google only knows "sleeve antenna" This is one hit, with image.
Lots of high frequency wireless devices (modems etc.) use this design.
Diesn't matter if you use the braid or some sort of metal tubing.
RG58U coax is good for constructing this aerial.
Leo..
Typical sleeve aerial. The thin half is the center of the coax, the thick half is the sleeve that acts as the other half of the dipole and a balun.
I think it's described in the famous aerial book of Karl Rothammel.
Leo..
As the author of that web page points out, in theory, and given test equipment, one can experimentally change the radial droop angle to match any transmitter output impedance in the range of about 37 to 73 Ohms.
