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I'm going to be putting up a Comet GP6 UHF/VHF Ground Plane Antenna. It will be chimney mounted to a mast, and the total height is around 40 ft. The upper half of the chimney starts at the first story of the house and stands around 25-30 ft above the roof adding to the extra length of the coax. I already own 80 ft of Tram RG213/U Mil-C-17G and was considering using that (they are split so I would have to use use connectors on them so they can be used together). I also own a 82ft Belden RF400.

I'm not sure what to use or if I should buy something else. I'm also considering installing a feedline choke at the antenna side of the feed line similar to an HF antenna I'm setting up. I know the RF400 would be a better feed line but I was hoping to use it on my HF setup 160m-6m end fed Balun 9:1 unun setup that's 100-120 ft away from my ham shack. Same coax I'd like to use for my G5RV. What would be the best type of feedline to use for the VHF/UHF antenna in this situation?

I do know there are a lot of variables. I will be using both digital & analog. The average distance to repeaters in my area is around 25-30 miles. I'm currently able to hit them with my 50W mobile rig without issue. I'm willing to buy different coax if needed. Leaving out the difficult to determine variables like obstacles (there is only 1 to my north but it has not hindered my contact to repeaters on my mobile in the driveway obstacles don't appear to be an issue). My rig will be a Yaesu FT-991a which runs 50W FTM UHF/VHF. I'm just trying to see if there is a preferable coax for my setup.

Thanks!

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    $\begingroup$ Welcome to SE. The amount of feedline loss you can tolerate depends on the "link budget": analog or digital system, how much power you're running, distance to the "receiver," intervening obstacles, etc. Since the number, value and variety of variables can make it difficult to calculate the tolerable loss, it may be easiest to simply try both feedlines and see what happens. $\endgroup$
    – Brian K1LI
    Commented May 2, 2019 at 13:31
  • $\begingroup$ I do know there are a lot of variables that you mentioned above Brian. I will be using both digital &analog. The avg distance to repeaters in my area is around 25-30 miles. I'm currently able to hit them with my 50W mobile rig without issue. I'm willing to buy different coax if needed. Leaving out the difficult to determine variables like obstacles (there is only 1 to my north but it has not hindered my contact to repeaters on my mobile in the drivew). My rig will be a Yaesu FT-991a which runs 50W FTM UHF/VHF. I'm just trying to see if there is a preferable coax for my setup $\endgroup$
    – Mike
    Commented May 2, 2019 at 14:38

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LMR-1700 (*)

I tend to shy away from questions of the form "what is the best ...?" because the answers tend to be subjective; they have to be, there are too many individual circumstances. But this is a really good question because low-loss coax cable has some folklore associated with it and this is an opportunity to address some of it.

Assuming you are restricting to 50 Ohm coaxial cable, the only parameter you are concerned with is loss, and there are no other considerations, I believe LMR-1700 is the cable you are looking for. But it is expensive.

If cost is a consideration and you are still restricting to 50 Ohm coaxial cable, check out the Times Microwave catalog. They have multiple grades of low-loss cable and publish easy-to-understand specifications, including the equations used to compute loss at different frequencies. If there are other considerations (say, ease of use or availability of connectors, for example), that is there too. I suspect, after reading the spec sheets and checking prices, you will agree with @BrianK1LI that the RF400 you have on hand is "good enough".

(*) There are good reasons to down-grade this answer. If your reason is along the lines of "there is a different cable with lower loss" please add it in the comments. I do not wish to spread misinformation.

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    $\begingroup$ There exist all manner of rigid and semi-rigid coax cables with lower loss than LMR-1700. For example, compare 0.6 dB/100ft at 450 MHz for LMR-1700, versus 0.439 for AVA7-50, a 1-5/8 inch Heliax cable. I'm sure even lower loss example exist, if cost and size are no concern. $\endgroup$
    – Phil Frost - W8II
    Commented May 3, 2019 at 17:02
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Belden datasheets indicate RF400 has 2.7dB attenuation per 100ft at 450MHz; RG213 has 4.1dB at 400MHz. Given the lengths of the two cables you have on hand, it appears that the difference in attenuation between the two cables is on the order of 1dB, which is ordinarily insignificant given all the other variables involved.

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I had to answer back to myself because after gaining quite a bit of experience and also moving with my antenna systems I've learned a lot. #1 What is the best coax for UHF and VHF? The best is Andrews Heliax (7/8").

Why?

  • Low loss over long distances (long runs of coax)
  • Durability is the major differentiator. LMR uses an aluminum foil shield rubbing against a raw copper braid. I'm going to cite an excellent write-up on it rather than paraphrasing the well-written page. There is a lot more to consider in a repeater build but the question was not about a repeater system. I will be updating all my antenna systems next year with 7/8 Heliax. I'm not a fan of aluminum foil-shielded coax and corrugated copper is far superior.

In the beginning I used what I had available but when presented with choices, after deep dive research and speaking with ELMERS this was my conclusion. I'm in no way saying everyone must buy Heliax but in my application, weighing all the things I want out of it this was my ultimate conclusion. Don't take my word for it simply read the online reviews of people who've used Heliax applications in their setup.

Heliax and LMR

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