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Forum Index : Microcontroller and PC projects : 458MHz amplified splitter?
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Does anyone know where I can get an amplified splitter for 458MHz or so? Receive only, no transmit path used or required. I have a roof-mounted antenna, tuned for 458MHz, currently feeding one singular receiver unit. I want to use the same antenna to receive signals for THAT system, and also to pick up signals transmitted on an adjacent channel frequency. My first thought was a passive TV splitter, as they are supposed to be broadband up to about 2GHz, but the only problem with a passive split, is that you lose 3dB(half) of your input signal across the split, and that makes that approach unattractive, as taking 3dB off the receive sensitivity could result in failure of one receiver to see the signals from far away nodes. The next thing I thought of was perhaps a simple TV booster amplifier such as this thing here from Jaycar, which has variable VHF and UHF controls, so set VHF to minimum, and UHF to minimum also - or around 3dB-ish gain, and put a passive split after that. Does anyone see any issue with that idea, or perhaps do any of the members here know of a special type of active splitter concept I can use in this situation? Smoke makes things work. When the smoke gets out, it stops! |
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| Boppa Guru  Joined: 08/11/2016 Location: AustraliaPosts: 814 |
Jaycar used to have an amplified tv splitter similar to the above but with dual outputs, I got 3 of them here atm, Ill take a pic of the model number Or just use a masthead amp with dual outputs? (depends where you want the split to go to) |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
These are the ones, had a lot of good use out of them over the years, I got some through jaycar, others through bunnings   Latest one I bought was about 18 months ago for the tv I used in the 4x4 for camping, cost around $40 Cant find them on either of their online cattledogs, ebay has one http://www.ebay.com.au/itm/DIGITAL-TV-SIGNAL-BOOSTER-AMPLIFIER-KINGRAY-SA162F/200782447030?epid=17003301376&hash=item2eb f9101b6:g:U5cAAOSwC8FZcBJV I can see other brands out there, just I had a lot of issues where I was living with lightning, lost lots of masthead amps to thunderstorms, found these to be quite immune to them compared to other brands |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Yes, the good 'ole Kingray SA162 - I know it well.  It does not have any kind of gain control though, so my thoughts were that it might overload things more then it helps. Not sure though. I will have a look for one of those, as I am sure I have at least a couple myself. Smoke makes things work. When the smoke gets out, it stops! |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
A remarkably good lil unit arent they, I havent had one failure in years, and I put a lot of them in Actually I tell a lie, I did have one failure, due to cat urine LOL- pussycat was in the habit of sleeping on the warm vcr and had an accident, vcr failed, amp splitter failed and tv failed all within 6 months... |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
Hmm the 164 (4 way outputs) version has a gain control adjustable by a small screwdriver |
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| skyv Newbie  Joined: 14/03/2015 Location: AustraliaPosts: 14 |
Radio Part have a few offerings http://www.radioparts.com.au/category/tv-antenna-systems/tv-antenna-access/splitter-amps skyv |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1667 |
This one is 435 Meg. Wonder if it's re-tunable by changing a few components. Might be others that are more suitable. Phil. Edit:- Seems from the description it can be reconfigured... IE,  |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Thanks skyv and Phil.  I have enquired with the Aliexpress seller if their amp can be adjusted to 458MHz. That would be an ideal solution, coupled into a standard tw-way passive splitter, with each output port feeding a receiver. With a amp gain of 3dB + whatever insertion loss of the amp itself, each receiver should see the same signal level as the original antenna and receiver. I like the Aliexpress unit, as being band-pass, it will have nice attenuation on all other frequencies - a channelised amplifier, as I used to call them. A broadband amp would probably work I expect, but being broadband, the amp could also pass through a lot of other spectrum noise which might swamp the receiver if you see where I am coming from. RF is fun, isn't it!  (rhetorical!)Smoke makes things work. When the smoke gets out, it stops! |
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| kg4pid Regular Member  Joined: 08/03/2015 Location: United StatesPosts: 50 |
Have you tried using the splitter without any additional amplification? In the grand scheme of things 3db isn't much. If the splitter does affect the signal, then it was marginal to begin with. Your coax could easily have that much loss and more at that frequency depending on type and length. A TV splitter will be 75 ohm, what is your antenna and coax rated for? Mixing 50 ohm and 75 ohm will also cause minor losses. Max |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Good points! The ant and receiver are 50-ohm, but any TV splitter will be 75-ohm. Just HOW much can I expect that to alter things? I do indeed plan to do some site tests before anything else. Smoke makes things work. When the smoke gets out, it stops! |
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palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 1993 |
Hi Grogs, what about a scanner antenna splitter, it would be 50 ohm. 458MHz is just below the scanner frequencies that I think start about 460 MHz. Paul. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
For a receiver only, the mismatch is almost immeasurable, even on transmitters it usually only slightly raises the vswr slightly Same with passive splitters, any impedance mismatch will be unmeasurable (those scanner splitters are simply wideband splitters commonly used in sat systems for splitting to 2 set top boxes (most are marked 30mhz to 3ghz if you look at the specs on the side) I even used lowloss 75 ohm coax on my s/w rig for a while (as it was built into the wall and being a rented house wasnt allowed to drill holes for coax, so I just used the tv antenna coax) and it barely made a difference even on tx, swr was still around 1.2 so I think you will find that it will make no difference at all to your receiver |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
@ Paul - that sounds ideal. Do you have any links? @ Boppa - Cool, thanks. That is reassuring. :) Perhaps I am being over-cautious to some extent. Antenna and coax match is important if you are transmitting with any kind of power on 100% duty-cycle(FM transmitter), but for receiving, perhaps not QUITE so critical. Smoke makes things work. When the smoke gets out, it stops! |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
I dont know, I was putting 10w into a mobile antenna that I couldnt get under 1.7:1 with fm with no issues, but then it was a tr9000, and they were built like a tank Nice little radio, until somebody stole it out of my car My other baby  Still got it (in storage atm) and still got that homemade PSU, an old atx psu, put a mains switch in the hole where all the cables came out, fit a 12v 7w tail light bulb to the 5v and gnd wires and bingo instant 12vdc 20a psu for next to nothing, and with very little to no noise coming through the transceiver surprisingly, no extra filter caps fitted at all |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
I do agree that the SWR/VWSR is more important to get right if the final of whatever is not that robust. Some sensitive stuff would just cook their final if the output match was not pretty much spot-on - give or take a tiny bit. Designs have changed now I guess, and even the FM transmitters out of China - SOME of them are very clean and strong. Others are......well....enough said. Smoke makes things work. When the smoke gets out, it stops! |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
I think there have always been good and bad rigs, but I do think that impedance mismatches simply arent that critical, esp for receivers In the early days of lans, I installed literally hundreds of lans with the old 50 ohm coax (10mbs- wow!!!) and you had to be ultra critical with those- capping unused points with a capping plug (resister in a plug) and even using a 75 ohm bnc instead of a 50 ohm bnc plug or socket would seriously slow a lans speed right down- that was a situation that impedance mismatches were critical, but antenna/coax combos are so open to frequency/manufacturing tolerances that I just cant see that its quite that critical, even air humidity has a (admittedly very small) effect on the vswr of an antenna system |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
I REMEMBER THOSE OLD RG58 COAX NETWORKS!!!! NetBEUI was the protocol I seem to remember, and it was not routable, but was really easy to get going. Yeah, I used to have a whole heap of those 50-ohm BNC terminators. @ Boppa - You seem to know your RF, I am looking for a good add-on LPF for an FM transmitter. Only needs to be able to handle 10W or so. Do you - or any other member reading this - know where I can get such a thing? The FM transmitter I am playing with on the bench is a very nice unit at the MCU control and LCD side of things, but the harmonics and spurious emissions are nasty, nasty, nasty, so I would like to try an add-on LPF to see if that could clean it up and make it compliant. Smoke makes things work. When the smoke gets out, it stops! |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 1993 |
@ Grogster Here is one on EBay splitter But if you Google "Scanner Antenna Splitter" there is heaps of info. Paul. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
palcal- those are the ones I was talking about earlier- except he has bought a 30mhz-3ghz splitter (list price about 6 bucks- bulk wholesale off alibarba under 3 bucks), added two leads(4 bucks each) and an adapter($1.50) and charged a shedload more for it(fifty bucks) Hey not knocking him for it, all the more for making some dosh for himself, but its not a `scanner splitter' anymore than a BNC plug is a `scanner plug' Those splitters are commonly used as tv splitters, tv/fm splitter for home tuners, basically anywhere yo need to feed 2 receivers from a single antenna and have the approx 3db loss mentioned before (hence the opening post about an amp before them to recover the loss Tell the truth I am not sure (despite having used hundreds of them) if they are actually 75 ohm, 50 ohm or simply dont care ohms.... |
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