Coax size between repeater and duplexer

[Defender92]

I’m thinking I’m going to build a repeater. I’m making a shopping list. I want to use two Kenwood tk-8180 radios and a mobile Celwave 633-6a-2 duplexer. 
 

I keep seeing images of people using what looks like rg400 coax from the radios to the duplexer. I used lmr400uf ultra flex from DX Engineering to connect my base station and I have some left over. But the lmr400uf is still stiff for such a small distance from radio to duplexer. 
 

If I use a smaller coax like the rg400 vs the lmr400uf will it make that much of a difference?

 

thank you!

[SteveShannon]

47 minutes ago, Defender92 said:

I’m thinking I’m going to build a repeater. I’m making a shopping list. I want to use two Kenwood tk-8180 radios and a mobile Celwave 633-6a-2 duplexer. 
 

I keep seeing images of people using what looks like rg400 coax from the radios to the duplexer. I used lmr400uf ultra flex from DX Engineering to connect my base station and I have some left over. But the lmr400uf is still stiff for such a small distance from radio to duplexer. 
 

If I use a smaller coax like the rg400 vs the lmr400uf will it make that much of a difference?

 

thank you!

Use one of the many coax loss calculators to see if the loss for that length is acceptable to you. 

[WRYZ926]

Even small coax like RG8X, RG316, and RG58 will work just fine for short cables. RG316 is popular to connect repeaters to duplexers. They are generally 3 foot or shorter so loss is not an issue.

 

[WRUE951]

40 minutes ago, WRYZ926 said:

Even small coax like RG8X, RG316, and RG58 will work just fine for short cables. RG316 is popular to connect repeaters to duplexers. They are generally 3 foot or shorter so loss is not an issue.

 

Yup,,  pretty much what you see used for Repeaters with Duplexers built into them. . Keep the lengths as short as possible, defiantly no longer than 3' 

[WRYZ926]

We run separate duplexers with all of our repeaters. The repeaters and duplexers are all mounted in a server rack so no coax going between the repeaters and duplexers are not longer than 2 feet, most are closer to 12-18 inches in length. 

If the cables are going to longer than 3 feet then definitely go with coax that has less loss since you are going to always lose some power after the duplexers.

[Defender92]

Ok that’s what I was hoping. The size doesn’t matter too much on such a short wire. I was going to get some rg400 and connectors from DX Engineering and crimp / solder it. But these Uxcell pre-made jumper cables are very tempting at $15 each. Seems like good reviews. I’m probably going to try them. I’ll check it with my SWR meter. But I’m not sure the SWR meter will tell me how good or bad they are?

 

 

[WRKC935]

RG400 is silver plated conductors, double shield and Teflon dielectric. 

It's what Motorola specifically uses for all their commercial radio jumpers up to 800 Mhz.

If you are not familiar with putting connectors on cables, it's a better move to just buy premade jumpers that are pretested and verified to be good. 

The bit of extra money is worth the piece of mind you get in knowing they will work and are assembled correctly.

For 50 or 100 watts, its fine, the losses are not that significant in the 24 inches of cable that going to a different cable would really make a difference in system performance.  And the cable is flexible enough that you don't have to fight it like you would LMR400 or any other .429 diameter RG8 class cable. 

If you REALLY can't be convinced that it's the right stuff however and want that extra diameter, the go with RG393 jumpers.  Same construction as RG400 / RG142 (142 is a solid center conductor) but it's the .429 diameter.  Your looking at about 15 bucks a FOOT for 393 if it brand name like Belden. 

And if you ARE going to build your own cables, buy brand name connectors and not the crap on Amazon.  There are good connectors on Amazon, but there is some really cheap stuff too that for a repeater, I just wouldn't bother.  Remember that repeaters are a fixed station.  Meaning once you install it at a location, it's just gonna be there.  They don't get moved around like from place to place.  Better 'stuff' being used to build it means less screwing with it and less problems down the road.

 

[WRUE951]

On 7/2/2025 at 10:56 AM, Defender92 said:

Ok that’s what I was hoping. The size doesn’t matter too much on such a short wire. I was going to get some rg400 and connectors from DX Engineering and crimp / solder it. But these Uxcell pre-made jumper cables are very tempting at $15 each. Seems like good reviews. I’m probably going to try them. I’ll check it with my SWR meter. But I’m not sure the SWR meter will tell me how good or bad they are?

 

 

Those will work just fine.