As I have started using the UHF 433 systems for range, I have been experimenting with various TX and RX Antennae.
I came across this link http://www.fpvmanuals.com/category/manuals/equipment-manauls/antennas/ which explains how to build what they call a turn stile antenna. Could anyone try and explain why this antenna would work as I had really poor performance with it. I ended up using a simple cross dipole on the RX side with much better results.
Someone I know in the electronics field suggested that the two V's could in fact partially cancel out and in doing so, produce unusual impedance plots at the required resonant frequency. Anyway... any comments would be useful...
Joe or anybody else, can someone explain (again) how to build a good dipole (433Mhz band) with a ferrite balun or another easy to diy build balun ?
I have build this dipole with no balun out of a RG316 coax cable and servo wires. I think I am well tuned on the frequency resonance (length of the dipole branches were cut to 150 mm each to account for a 0,91 velocity factor due to the plastic tube surrounding the servo wires). But it is 72 ohm probably so I'd like to bring its reactance down to 50 ohm. How to ?
PS : i sandwiched two balsa pieces with hot glue on the feed point for mechanical resistance.
Hi Hugues, do you have any means of measuring the SWR at all? Is your required diole for use on ground or in the aircraft? Do you require horizontal or vertical polarisation ( if the latter a sleeve dipole may be more suited). At 433MHz a Ferrite choke balun will work fine, no resonances. For airborne applications the ferrite do add a little extra weight though.
Re the 50ohm/70ohm issue - do not stress to much on that - the SWR is around 1.4:1 and if you TX power is 100milliwatt, around 97milliwatts are radiated, and a mere 3 milliwatts lost...not important. It is more important to preserve the pattern by reducing the RF currents on the coax.
Let me know - I am happy to make one, measure it up, send results and cutting/build detail for you to try.
I have no tool to measure SWR. I'd like one (where can I get one ?).
I made two such dipoles : one to install on my taranis with a DTF UHF tx and the other one airborne on a fixed wing UAV (XUAV Talon). I was planning to install them vertically on both ends (vertical polarisation).
I will have to bend the coax feed to make it vertical though (or I should get a right angle SMA adapter).
I'd love you making measurements and guide me optimize these things. I have lots of material and connectors to make many more lol...
Here some picture of the antenna on Taranis. Obviously I will bend the coax feed to make the dipole vertical (can I use a metal spring around the coax to maintain a bend shape ? Or will it impact the electric field ?)
On one of the picture I installed a ferrite choke at the feed point, but have no idea if this is a good idea or not.
Hi Hugues. Can you give me a photo or sketch showing how you wish to install it on the Talon? I would recommend vertical polarization , as you have chosen, but would like to suggest using a sleeve dipole. It is no larger than the dipole you have made, requires no 90deg bends, and eliminates the currents flowing on the coax that would flow in your construction. It will be mechanically easier to implement and fit.
I would like to get an idea o the material you may have or may be able to obtain, so I know not to design one based on items you cannot get.
I assume from what you have done that you have some RG316 coax? Do you perhaps have any semi-rigid 50ohm coax , such as UT-95 or UT-114 - a solid copper tube outer shield rather than a braided one. Do you have any solid teflon rod, say about 10mm diameter , not more than 20mm long would be needed. If not, we can use wooden dowel rod which you should be able to easily obtain. Last, most reasonable hobby shops sell brass tubing, about 300mm long sections, of various diameters - we need a length of 6mm and maybe one of 8mm diameter.
Let me know what you can get, and I will do one and send you the info, measurements, photos, etc, so you can see how to duplicate it. Once properly tuned it will be easy for you to duplicate.
Here are some pictures.
I have kept what looks like brass sleeves from dismantled duck antennas : can I reuse those ? Dimensions are shown on pictures. Otherwise, no problem to get brass tubing.
I can get RG316 cable, no problem (I also have coax RG59 50 Ohms)
I don't have a solid teflon bar but wood is easy enough to get. (where would you get a solid teflon bar ?)
I also have straight and right angle SMA connectors to solder.
Hugues, I have posted a blog with a sleeve dipole and a V style dipole with design ideas and measurements - take a look and lets discuss further how I can help.
Thank you ! that's of great help. I will build this.
OK Hugues, I will see what I can do.
What type is the connector that connects your dipole antenna to your RCS Radio ( you called it "Taranis'?).
The sleeve dipole is close to 350mm long, plus some length for the coax, will that fit ok vertically on your plane?
The salvaged tubes you show are way too small - for 433MHz the required tube looks like it will be around 10 to 11mm diameter, and around 160mm long, over RG316 coax - that is what my measurements and tests are showing so far.
So you can start looking for some 10 or 11mm (OD) brass tubing...
Teflon rod, in fact all plastics, polypropylene, etc, you would get from an engineering plastics supplier - most will sell in meter lengths. But we do not really need any of that for 433MHz..
Watch this space..
Regarding a device to measure SWR -
for 433MHz you could probably obtain a reasonable quality UHF SWR meter from a good Ham Radio Equipment supplier.Maybe makes such as Daiwa, Diamond, etc. The problem is that with the inexpensive meters that are not of the 'read and hold' electronic variety, you need a continuous source of RF energy at the input in order to generate a constant reading on the meter or display. I would guess that the transmitter/telemetry modules you are using do not have a constant RF output, but transmit in bursts. The meter would not register and maintain the display with these bursts. So you would need a constant carrier transmitter on the correct frequency to provide a continuous wave RF carrier. I believe there are some 433MHz video transmitters available cheaply? that would work well as the RF source.
I think the problem you're having with this antenna may be due to the description of it on fpvmanuals. It says "the turnstile antenna is dual polarized and will help mitigate issues with polarization". Not only is that not true, that's not even a thing. The turnstyle is circularly-polarized. To get the best reception, the antenna at the other end must also be circularly polarized with the same rotation. If the other end is linearly polarized you will have reduced performance and if it is circularly polarized in the opposite direction you will have terrible performance.
That is probably why things got better when you went to a simple dipole.
For 433 MHz, your best bet would be a dipole with a ferrite choke balun. Even without the ferrite it's still very good.
If you still need more range, the Yagi-Uda antenna is a good bet. It's just a dipole with some carefully-sized wires placed near it, they're not even electrically connected to each other.
I'm not sure that anything Joe wrote has anything to do with your question, and I would get second opinions before I spent a lot of time following his advice. I don't know if you noticed this, but the antenna he includes pictures of is not actually a turnstile. A real turnstile uses a carefully-sized length of coax between the two dipoles to delay one of the signals by 90°, so the peaks of the waveform arrive in order at each wire in a perfectly-timed clockwise or counter-clockwise order, hence, circular polarization.
The antenna Joe shows is not something you would show to a beginner, and frankly even if you could do all the math required and built it correctly (which he hasn't) it's still a low-efficiency abomination.
Hope that helps.