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Forum Index : Microcontroller and PC projects : Help with antenna matching
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OA47 Guru  Joined: 11/04/2012 Location: AustraliaPosts: 1013 |
Would like a more knowledgeable 'shedder to help me understand using a 4:1 coax balun to match RG174 coax to a 433Mhz folded dipole. From what I understand the coax balun should be half lambda so that each end attached to the dipole is a maximum level. The balun should give 4:1 impedance ratio that is folded dipole starts at 300 ohm impedance and divided by 4 gives 75 ohm resultant. I would like to connect the antenna using SMA connector and I am thinking that the impedance of RG174 to SMA cables I have are 50 ohm. Can I use the co-ax balun as they look very simple to achieve the connection or am I barking up the wrong tree again.  OA47 |
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lew247 Guru Joined: 23/12/2015 Location: United KingdomPosts: 1702 |
A folded dipole's impedence is appx 70 ohms What are you using to transmit? ie: what are you transmitting If it's just one of the devices for the MM just use satellite coax directly to the dipole if it's a long distance away, Satellite coax because it's the cheapest low loss and it's 75 ohm The mismatch won't in reality matter and you won't notice any difference in range or anything else If it's a high powered radio transmitter and it's meant to have 50 ohm again you can connect directly to the dipole it won't make a huge difference The only time you need to worry about the balun is if your using over 100Watts transmit power or are using a HF aerial |
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crez Senior Member  Joined: 24/10/2012 Location: AustraliaPosts: 152 |
The difference between 75 and 50 ohm will make little difference in a receive situation. The 87mm on the picture is a bit off. I calculate 277mm for a coax with foam dielectric or 229mm for a solid dielectric. |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 1013 |
Thanks lew247 and crez. I am looking to use these antenna on low power 433Mhz radio telemetry units that are most likely transmitting 100mW or so. Most of the units are working fine with a stick antenna on an aluminium ground plane but a couple are scratching for a little bit more signal and I thought a 3-4 element yagi might be the answer. @crez the diagram I used was for a 1200Mhz system that's why it was 87mm. OA47 |
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| Bill7300 Senior Member Joined: 05/08/2014 Location: AustraliaPosts: 159 |
@Lew247 No, the characteristic impedance of a folded dipole is 300 ohms, so OA97's balun thinking holds up, as does his thought of a 3-4 element Yagi improving gain. Bill |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 816 |
Bill is correct, folded dipole is 300 ohms |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 1013 |
Do you think that I can get away with the (Chinese)RG174 with SMA connectors that I can remove from the stick antenna? OA47 |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 1013 |
@Bill7300 I think you have given me an upgrade from OA47-OA97  |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 816 |
Loss* RG-174 RG-58 RG-213 400MHz 17.3 B 11.2dB 4.8dB (loss per 30m length) I would avoid it unless it is a REALLY short length (30cm or less) If its 10 metres or more, I would go with 213, only problem is its thickness (its about a cm thick), low loss rg58 would be the other alternative (its only 5mm in diameter) Approximate Yagi-Uda antenna Gain levels elements Approx anticipated gain dB over dipole 3 7.5 4 8.5 5 9.5 As you can see, if you are using a 3 element yagi (gain of about 7.5db) if you use a 10m length of 174, you literally have gained almost nothing! (losses of 5.7db) |
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| lew247 Guru Joined: 23/12/2015 Location: United KingdomPosts: 1702 |
deleted |
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| Bill7300 Senior Member Joined: 05/08/2014 Location: AustraliaPosts: 159 |
@OA47 Whoops, sorry about that - although a real upgrade might have been to a Schottky type? @Boppa Agree that the 3 element would give marginal improvement with that cable loss but I guess the advantage of working at this frequency is that physical size of the antenna is not great and more elements can be added to the design to improve gain further with relative ease. Bill |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 816 |
One question, are you building these yagis yourself or are you using commercial units? Just wondering because most commercial yagis already have the balun inbuilt and are available from under $20. Although the balun may not be intended for tx use, at 100mW many receive only ones could be used too as long as they use a transformer style balun (A hint for cheap low power tx baluns- the old style 75 ohm to 300 ohm tv baluns can be used up to 5w and (depending on manufacturer) will usually work down to about 5mz and up to about 700mHz)- I have used these myself on UHF cb yagis (4w) and they work well despite being 75 ohm and the cb 50 ohm I used one of these with the screw terminals attache to holes drilled in the aluminium element on my home made uhf cb antenna (modified uhf tv antenna), put a belling lee on the end of the rg58 coax and then put the whole lot in a jiffy box with a couple of slots filed out of the lid for the folded dipole to come out (as those balums weren't waterproof obviously)  These days I'd just use of of these instead (cut the excess 300 ohm wires off as much as possible)  Another 'tv antenna' that is useful for transmitting through is a 'log periodic' antenna, they are broadband on both rx and tx, and although dont have as much gain as a yagi with the same number of elements, their extremely broad frequency response means they can be used without fear of too high a swer once you move off the design frequency (unlike a yagi)- in fact for a long time I used the one antenna (through a switch of course ) for a scanner, 2m, 70cm and uhf cb... they can be told by their 'twin boom' design  |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 1013 |
I was thinking of manufacturing one myself as the prices I had seen were around $150. Would like a link to the ones around $20. The SMA cable length is around 500mm so not a massive loss. Anyway it would still be the same amount of cable loss for the current stick antenna. OA47 |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 816 |
alibaba Factory price amplified yagi antenna 6 elements UHF 433mhz yagi antenna another eta I have had a lot of good results from alibaba, admittedly anything coming from China can be slow, sometimes as long as four weeks, but sometimes less than a week too (and if the tracking is to be believed, most of that delay is our fault- I have sat there watching 'Brisbane...Brisbane...Brisbane- for three weeks...) |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 1013 |
@ Boppa. I have not had any experience dealing with Alibaba. Would you have any idea of the shipping costs on these units? They weigh around 2kg and the shipping quote only seems to work for US. OA47 |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 1013 |
@Bill A schottky type insinuates that it is fast acting.....definitely not me ..... happy to stay germanium.  OA47 |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 816 |
Couldn't get TNT to work with it (their website often glitches) but I sent them a message asking for you (got an account there on alibaba, lol- although its affiliated with aliexpress, alibaba is aimed more at people wanting bulk orders) |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 816 |
Ouch Unit price of sample is US$15.52/pcs. shipping cost to Australia is US$53 by DHL,shipping time:5-7day |
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