matchstick Posted January 8, 2015 Posted January 8, 2015 i went and bought a bunch of 433mhz long range transmitter/receiver pairs (cheap as chips, or rather, a 1 person serving of fish and chips in the most literal sense), to use 1 or two and the rest to sell.They claim to work up to 3km but i have only been able to test up to 1km, and at that point the antennas direction it points affects whether it gets the signal to the receiver or not.Anywho, as a concept, or the use, is such a transmitter at all suspicious or considered overkill?Like what do you think when i describe it as it is?
Col Posted January 8, 2015 Posted January 8, 2015 why do you need 3km range or even 1km range for that matter? If your getting 1km from a cheap as chips transmitter i can only imagine the spurious harmonics its dishing out Good thing you dont live near me cos i`d open up with 400w (legally) and take out the front end of your receiver
matchstick Posted January 8, 2015 Author Posted January 8, 2015 Well, initially I figured given the cost and the fact Chinese 433mhz receiver transmitter pairs rarely even work to half the listed range, ide have something that works to at least 500m or line of sight. So, ide at least have a few hundred meters with very reliable reception
Col Posted January 9, 2015 Posted January 9, 2015 In the uk, assuming its a legal import, it wont have more than 10mW RF output regardless of what range they put on the box The receiver is more important than how much output the transmitter has. A cheap receiver will have crap sensitivity (deaf) and selectivity (sorting out the wanted signal from the unwanted).
Bobosan Posted January 9, 2015 Posted January 9, 2015 The small 433mhz xmtr/rcvr modules are designed and used for close proximity work 30-95m away. Many of the chinese made remote fire pyro boxes use these RF chipsets.
Ubehage Posted January 9, 2015 Posted January 9, 2015 If you ever find yourself in a situation, where you would have to explain your motives - then a 3km igniter might not help you
schroedinger Posted January 9, 2015 Posted January 9, 2015 Like you allready said they allready have problem at 1km, sk why not use it? A guaranted connection on 500 m isn't overkill.Only make sure that they don't interfer with other transmitters.I had the problem with my pocket firing system containing two wireless realays. The problem was that the chinese coded all their relays and transmitters to the same signal. 1
Col Posted January 9, 2015 Posted January 9, 2015 (edited) The reason why consumer RF devices are low power/short range affairs is to ensure they cant interfere with other users It takes specialised kit and knowledge to test a transmitter for spurious emmisions and harmonics, so for most people buying souped up systems there`s no way they can be sure it wont cause interference. The smart way to increase the range of a line of sight low power system is to attach a 4-9 element yagi (tv antenna size) to the transmitter and one on the receiver. Cable losses are high at uhf frequencies so you`ll need a very low loss coax even for short runs, 1/2" heliax is the best. If the regs have a clause that puts a limit on the ERP (effective radiated power) then you wont be able to use a non standard antenna. Edited January 9, 2015 by Col
Bobosan Posted January 10, 2015 Posted January 10, 2015 I would be more concerned about the receivers than transmitters. Spurious emmisions from any device may activate a receiver unless encoding is used.
Col Posted January 10, 2015 Posted January 10, 2015 Harmonics are the issue, cheap transmitters typically dont have top quality bandpass filtering due to cost. Without adequate harmonic suppression you can tx on 433mhz and cause interference on 866mhz and 1290mhz (2nd & 3rd harmonic).Primary users of the 70cm band will complain if you cause them grief. In the uk thats the ministry of defence Hams have the band on a secondary basis, they can complain but not against the primary user. Unlicenced users cant complain regardless of who causes them interference.
Bobosan Posted January 10, 2015 Posted January 10, 2015 Harmonics are the issue, cheap transmitters typically dont have top quality bandpass filtering due to cost. Without adequate harmonic suppression you can tx on 433mhz and cause interference on 866mhz and 1290mhz (2nd & 3rd harmonic). aye. Receivers are no different unless the front end is properly filtered.
Col Posted January 10, 2015 Posted January 10, 2015 Receivers cost more to make than transmitters so i wouldnt expect much from a budget version. To some extent you can improve performance by using a decent antenna. The typical whips on most units will radiate poorly in all directions, given that the power is limited to begin with.. you dont want to be sending any in the wrong direction.Gain is relatively easy to acquire at uhf, swapping out the whip for a decent 5-element yagi will increase the effective radiated power upto 10 fold, (your standard 10mW will now perform just as well as 100mW on the naff whip). The deaf receiver will perform much better with the same mod. Unwanted signals/noise arriving on the back and sides of the beam are attenuated, making it easier to hear the wanted signal coming in on the front. If you need to use multiple receivers, the best option is use a vertical colinear on the transmitter to provide omnidirectional coverage with some gain. Best leave the standard whips on the receivers if you go that route cos upgrading those would just drag in more noise.
Arthur Posted January 10, 2015 Posted January 10, 2015 3Km in a remote area with no competing rf to cause interference may be possible in a cheap device, 3KM in urban conditions would be highly unlikely as other RF spectrum uses would swamp the low level of RF that we are calling signal. The usual Chinese transmitters are coded, some are fixed codes, some are switchable codes and some are code learning. The devices with switchable codes ALL come with the same code set at manufacture so will interfere until the code of the Tx and Rx are reset to something different.
Col Posted January 11, 2015 Posted January 11, 2015 With repeater coverage of hams (and the mod) you`ll be hard pushed to find a clear spot even in remote rural areas. Checked up on the uk regs and legal devices are allowed 10mW "ERP" which puts paid to any antenna modifications.
Arthur Posted January 11, 2015 Posted January 11, 2015 Transmitter antennae must be as design, but receiver antennae can be improved upon. A good directional aerial will lose a lot of mush from "behind" improving s/n ratios.
Col Posted January 11, 2015 Posted January 11, 2015 (edited) Aye, that would be legal and a worthwhile improvement. A decent beam will have a polar plot that looks like a blimp in 3d, which reduces unwanted signals and noise from the back and sides.Make sure the rx antenna uses vertical polarity or you`ll lose upto 30db from cross polarisation. The vertical pattern of a beam is not as clean or sharp as the horizontal due to ground reflections but its infinitely better than a standard rx whip. 5 element yagi, vertical and horizontal patterns: (antenna is 2ft long x 1ft high) Edited January 11, 2015 by Col
Arthur Posted January 11, 2015 Posted January 11, 2015 IIRC 433 is somewhere near the old TV frequencies so would a TV aerial do on the receiver -could be 18 - 30 dB of forward gain? All the same who needs 3Km range when 500m would fire most of the safe things I've ever seen (big shells for example)
Col Posted January 11, 2015 Posted January 11, 2015 TV frequencies are too high, ch 21: 471mhz. ch 68: 850mhz. The tv antenna reflector ( longest element) will be too short in length so it would act as a director, the polar plot would be a messy bi-directional affair at best.Be aware that front to back and front to side rejection, forward gain and swr (impedance match between tx and antenna) can all be optimised but you cant have max gain, max FB/FS rejection and perfect match at one frequency so there are some trade offs involved. The uk 70cm amateur band runs from 430-440mhz, most commercial yagi`s will cover the entire band with reasonable performance, a 5-element will have 8-10dbi forward gain. Four 19 -element tonna yagi`s (10ft long) stacked and bayed in a 4ft square provides around 20-22dbi. On a good day 400w can bounce the signal off the moon and back to earth. Here`s a map of the 70cm ham repeater locations in the uk, its quite extensive and new ones come online regularly. The M.O.D has even more so the chances of finding a quiet area is pretty slim.
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