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Boppa
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Posted: 29 November 2017 at 7:48pm | IP Logged Quote Boppa

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
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Posted: 30 November 2017 at 10:12am | IP Logged Quote Grogster

@ 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.

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Boppa
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Posted: 30 November 2017 at 10:43am | IP Logged Quote Boppa

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
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Posted: 30 November 2017 at 10:51am | IP Logged Quote Grogster

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.

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Boppa
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Posted: 30 November 2017 at 11:43am | IP Logged Quote Boppa

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

Edited by Boppa on 30 November 2017 at 2:22pm
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Grogster
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Posted: 30 November 2017 at 3:26pm | IP Logged Quote Grogster

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.

Edited by Grogster on 30 November 2017 at 3:28pm


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Posted: 30 November 2017 at 3:39pm | IP Logged Quote palcal

@ Grogster
Here is one on EBay splitter

But if you Google "Scanner Antenna Splitter" there is heaps of info.
Paul.

Edited by palcal on 30 November 2017 at 3:40pm


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Boppa
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Posted: 30 November 2017 at 4:49pm | IP Logged Quote Boppa

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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Boppa
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Posted: 30 November 2017 at 4:59pm | IP Logged Quote Boppa

Grogster wrote:
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.


There are LPF for fm radio broadcasts, amateur radios, most are home built simply because you really want it to apply to a particular set of frequencies...- what particular frequency are you looking at?
plus if you are having harmonics issues with a tx unit, you want a LPF that has its cutout above your actual transmitting frequency (you really need the filter curve figures because you dont want the filter cutting off your tx power!

usually you have your filter set halfway between your tx frequency and your first harmonic frequency thats detectable, but it doesnt have to be a particular exact frequency, they simply arent that sharp a cutout for most, and a few Mhz (or even a few 10's of mhz at uhf)
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Phil23
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Posted: 30 November 2017 at 7:27pm | IP Logged Quote Phil23

Grogster wrote:
I have enquired with the Aliexpress seller if their amp can be adjusted to 458MHz.....


What would be really nice to know is what is the chippy in the middle...
Noticed tons of various ones available, but seeing a data sheet for whatever that one is would help a lot.

Quote:
A broadband amp would probably work I expect, but being broadband, the amp could also pass through a lot of other spectrum noise.....


My TV distribution was fine here, until a while ago; 7 outlets to 5 TV's & 2 "Boxes"...

Telstra's 850Meg then started swamping everything's receivers front end.

Fixed that, filter between the antenna & dist amp. Easy peasey.

Then Optus stands a 4G tower with 700Meg among others 180m down the road...
This one gets in not thru the aerial, but via the old run of RG-59 from the lounge room to the bedroom, about 10-15m.

Only bit that's left that's RG59 & not RG6, AND a real sh*t of a crawl under the house to replace.

Cheers

Phil.
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Posted: 30 November 2017 at 7:57pm | IP Logged Quote Phil23

Boppa wrote:
In the early days of lans, I installed literally hundreds of lans with the old 50 ohm coax (10mbs- wow!!!).....


Lantastic ring any bells?

Cut my teeth on the 2Mbps adaptors before stepping up to the 10Mbps big boys...

Oh, and did play with the 115200kbps one that was software only & used RS-232 ports.


Phil.
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Posted: 30 November 2017 at 8:06pm | IP Logged Quote Phil23

Grogster wrote:
NetBEUI was the protocol I seem to remember, and it was not routable, but was really easy to get going.....


Dead right; and at the really scary end of things was TCP/IP....

And those stacks & Winsockets....
I just didn't want to go there if it could be avoided.
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