LMR400 to RG6

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wqwx352

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I have RG6 Cable already ran throughout my house like most people do and I just cancelled Cable so I want to know if anyone has used RG6 instead LMR400 for their base stations?

I bought LMR400 cable and have it connected to my Antenna 30ft above the ground on my roof but it's just coming through a vent as a temporary solution and I have to move my radio to my office which already has RG6 ran.

Can I just use a LMR400 to RG6 Adapter without much signal loss or should I buy more LMR400 cable and couple them, then drill another hole to my office with a wall plate which kinda seems redundant.
 

W9BU

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Can I just use a LMR400 to RG6 Adapter without much signal loss...
No. LMR400 is a 50 ohm cable. RG6 is a 75 ohm cable. At any plain junction between a 50 ohm cable and a 75 ohm cable, there will be a 1.5:1 SWR. As a result, some signal, on both receive and transmit, will be lost at the junction.
 

prcguy

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But wait, there's more! If you consider a simple resistive load like placing a 75 ohm dummy load on a 50 ohm transmitter, you will have an expected 1.5:1 VSWR and predictable loss. However, when you have random lengths of 50 ohm coax connected to random lengths of 75 ohm coax and toss in a 50 ohm antenna on the 75 ohm coax, you have a mess. Specific lengths of coax that are different impedance than the source and load will make an impedance transformer and you can end up with worse than a 1.5:1 VSWR or better, its a crap shoot.

Why not just do it right and use LMR400 or other low loss 50 ohm coax with a 50 ohm rated radio and 50 ohm antenna?



QUOTE=W9BU;2999428]No. LMR400 is a 50 ohm cable. RG6 is a 75 ohm cable. At any plain junction between a 50 ohm cable and a 75 ohm cable, there will be a 1.5:1 SWR. As a result, some signal, on both receive and transmit, will be lost at the junction.[/QUOTE]
 
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WQWX, when you vector in velocity factors, cable lengths and such- "A crap shoot" is a good way to put it. It *could* be a perfect 1:1 match. :)
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I think a lot depends on what your antenna is supposed to do. If it is just to hit local repeaters and close-in simplex (I am making the assumption by your callsign that this is for UHF, --- GMRS, No?)-- it might work fine, - mystery SWR's and all. If its for something more weak signal'd, maybe not such a good choice. But what's to lose?... you may be pleasantly surprised (and not need to punch a hole in your wall.)
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If it were me, I change out the LMR cable and replace it with a length of RG/6... an easy splice; but then that's me.
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Happy experimenting !
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Lauri :)
 
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Golay

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75 ohm coax and a 50 ohm antenna

I always like the topic "Can I use 75 ohm to transmit". We had a long running discussion about this at our ham club for a couple months. The point we were pondering was basically:

If I have a 50 ohm coax, and a 50 ohm antenna, and the match is 1.5 to 1, I can tune the length of the antenna to get to 1:1.
- so -
If my match with the 75 ohm coax is 1.5 to 1, can I tune the 50 ohm antenna so that the match is 1:1?

It's been discussed also here on RR a time or two.

I will agree with those who say it's not quite a good idea to splice 75 feedline to 50 feedline. I actually tried this with trying to use a 3-element 2 meter beam for a TV antenna. I connected the 75 ohm from the boob tube to the 50 ohm coming in from the beam and got a terrible picture. The 2 meter beam was outside and high (vertical and pointed in the right direction), yet the indoor TV antenna got a better picture. I couldn't quite understand that one, but oh well.
 
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50 Ohms v.s 75 Ohm feed line to a 75 Ohm or 50 Ohm antenna is not going to be an earth shaking mismatch. And if your a purist, wanting that 1:1 SWR, the antenna can be tuned to the new impedance- this may mess up its radiation pattern slightly, or not be easy depending on the way the antenna is constructed. I wouldn't get up a sweat over it, especially since a 1.5 to 1 match is only a 4-5% reflection- unless the transmitter is very very Very finicky, the world will continue to spin very nicely about its axis.
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(If my opinion matters; I did/do this all the time, both professionally and as a ham, and I've never had any issues.
My lab had six, seven, eight ?, one thousand foot spools of Belden 8213, 75 ohm coax- these were left from the previous "administration." The bean counters admonished me to use them up first before I order'd more....... so years later when I finally left there was still plenty of that cable. We used it for all general things RF; 50 or 75 Ohms didn't matter. )
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Oh, and BTW-- I think using a vertically polarized 2 metre beam as a VHF television antenna had more to do with the poor performance than a slightly mis-matched antenna. :)
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Lauri :)
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a PS- if the coax is a perfect half wave length- the velocity factor must be included - it doesn't matter what the impedance of the coax is;--- if the transmitter is 50 Ohms, the antenna is 50 Ohms- the SWR will be 1:1.
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Magic !, No ?
 
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jlove1974

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No. LMR400 is a 50 ohm cable. RG6 is a 75 ohm cable. At any plain junction between a 50 ohm cable and a 75 ohm cable, there will be a 1.5:1 SWR. As a result, some signal, on both receive and transmit, will be lost at the junction.
winner winner. Just pull the RG6 and run some LMR400UF
 
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