Effects of Series Connecting Two Different Types of 50 Ohm Coax Cable

[N3IYG]

Is there any problem in series connecting two different types of 50 ohm coax cable?
As an example I have 25 feet of RG-8\U. I need to extend the length to an antenna
75 feet away. I want to use 50 feet of LMR-400 connected in series with the RG-8\U
to reach the antenna. Both are 50 ohm but they are constructed differently and have
different velocity factors, etc. Would this cause reflected energy and a higher SWR at the
connection point? Or any other problems.

My current application is 10 meter 28MHz. However I may want to use VHF 145MHz
and UHF 440 MHz dual band in the future. Money is a factor so I want to utilize as
much of what I have on hand.

 

[mmckenna]

No, not really.

In industry, we do this all the time. I've got a site where I have a 200 foot run of 1 5/8" Heliax from the equipment room to the top of the tower.
At the transmitter, I'm using 3/8" helix super flex. From the transmit combiner to the 1 5/8" Heliax, I'm using 1/2" heliax superflex.

At the top of the tower, the 1 5/8" stops and switches to 1/2" heliax again to connect to the antenna.

Pretty common to have different types of cable along the run. Usually the very low loss stuff used in the higher frequencies, 800MHz, Cellular, etc. is not very flexible. Using short lengths of a more flexible cable at the ends is pretty much standard.

There will probably be a small impedance "bump" at the connection, but it shouldn't cause you any issues.

For the heck of it, why don't you price out a 75 foot run of LMR-400, though? While it won't make a huge difference, it does remove a possible failure/water intrusion point, and will probably only cost a few more bucks.

 

[cmdrwill]

mmckenna has a good point. I would go with a new run of genuine LMR400 or Davis buryflex. Be sure to get the correct connectors for the LMR series cable.

You can use the RG 8 cable for another project.

 

[jonwienke]

The fewer adapters and conversions you have in your coax, the better. The RG-8/U is fine for CB, but it's pretty lossy in UHF. I'd go LMR-400 or similar for the entire run. There's not much difference in the CB freqs, but in UHF the difference can be over 6dB.

 

[KD8NIV]

I'm running a 120' of LMR 400UF coax and used a barrel connector, and added 50' of RG 213 coax to my G5RV to reach the shack, so far no problems with it, been using this about 6 months so far........my tuner tunes the dipole on 10-80m, no problem...

 

[jonwienke]

At 30MHz and below, the type of coax matters much less. But when you're talking UHF, the differences are huge. At 445MHz, 100' of RG-8X has over 8dB of loss, while LMR-400 has less than 3dB of loss. At 30MHz, the losses are 1.66dB and 0.677dB respectively.

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